Does autofocus work in. How does autofocus work in a smartphone? DSLR auto focus modes

The phase detection autofocus system has been around for a long time. Many photographers complain about the autofocus performance of certain camera models, but in fact, the problem is not with the cameras, but with the focusing system itself. If you read the old camera reviews of the 2000s, you can see that autofocus problems have been from the very beginning of the phase detection autofocus system to this day. To find out what the problem is, you need to understand how autofocus works. This will be discussed in the article.

How DSLR Cameras Work

To understand the details of focusing, you must first understand the DSLR device.

1. Light flow
2. Main mirror
3. Secondary mirror
4. Camera shutter and sensor
5. Disk for setting up the primary mirror
6. Disk for setting up a secondary mirror
7. Phase sensor
8. Viewfinder pentaprism
9. Viewfinder

Light passes through the lens and hits the translucent main mirror. It reflects light into pentaprism. Some light passes through the primary mirror and hits the secondary mirror, which reflects the light onto the phase sensor. The sensor itself contains sensors. Two sensors are used to detect one AF point. The camera compares the signals received from the sensors. If the signals do not match, the autofocus adjusts the focus and the comparison is made again.

The problem with phase detection autofocus is that the sensor adjusts the focus so that it gets the optimal image, but the main sensor of the camera on which the image is recorded is the matrix, and it is located in a different place. In order for autofocus to create an ideal image that will be recorded by the camera sensor, the distance from the lens mount to the phase sensor and to the sensor must be exactly the same. A millimeter shift will cause autofocus to malfunction. Also, autofocus performance depends on the position of the mirrors.

The principle of operation of the phase sensor

Light entering the sensor passes through the lenses and hits the light-sensitive sensors. When focusing is correct, light from the edges of the lens converges at the very center of each sensor. If the image is the same on both sensors, it means that the focus is correct. If focusing is not correct, the light will converge not in the center, but in other parts of the sensor.

Focus: 1 - very close, 2 - wrong, 3 - very far, 4 - too far

Knowing where the light is focused in the sensor, you can calculate in which direction and by what value you need to correct the position of the objective lenses.

After the sensor detects if the subject is in focus, it makes a focus correction if the answer is no. Focus correction is performed with the objective lens as many times as necessary to achieve normal focus. The system works very quickly, so all actions take a split second. When the system is focused, the camera will beep. then you can press the shutter button.

We examined the principle of operation of one AF sensor (point), but there are many of them in modern cameras. It's not hard to find cameras now that have 41 or even 61 AF points. The reliability and accuracy of the sensors is increasing. There are more stable cross-type AF points. Modern cameras can easily not only focus quickly, but also track moving objects.

Disadvantages of phase detection autofocus

The main problem is inaccuracy in the assembly of the camera at the factory. If the slightest failure occurred during the production process and the sensor or one of the elements affecting its operation were not installed accurately, then the system will work with an error. Manufacturers are aware of this problem and therefore a system has been developed to fine tune the focusing system. During testing, cameras that have problems are identified and additional settings are made.

The calibration process checks each AF point individually. Each point is accurately calibrated and all changes are recorded in the camera program. This eliminates autofocus problems in a production environment.

This article provides information about autofocus in digital SLR cameras, what type it is, how to use it, and what to do with its problems (and how to determine them).

What is camera autofocus

The autofocus of a camera is called a device for automatically focusing the lens on the subject. To denote autofocus, the international abbreviation AF is used, which can be found in the names of autofocus lenses (about them below) and in the camera settings menu.

How autofocus works

Everything is very simple: depending on the type of focusing, the camera reads in a certain way the data on what is seen through the lens. This data is then analyzed and, if necessary, the camera sends commands to the autofocus motor located in the lens. This motor moves the lens unit and focuses with the correction required by the camera. This process is repeated until the camera decides that focus has been achieved.

The same "screwdriver" on the Nikon camera mount

Note: some Nikon cameras have a focusing motor located in the camera itself. The common people call these cameras "carcasses with motors", and the mechanism of such focusing itself is called a "screwdriver" because of their external resemblance. The advantages of the "screwdriver" are that the autofocus of the camera will work with old optics of the Soviet era, despite the absence of a focusing motor in the old lenses. However, keep in mind that old optics require improvements in order to Nikon camera was able to focus to infinity, which should not be done on Canon cameras (since the distance between the rear lens of the lens and the Canon sensor is closer to this value on old SLR cameras).

Contrast autofocus

Contrast autofocus is the most simple form autofocus in modern SLR cameras.

This type autofocus is pretty reliable and is based on the simplest running of the lens unit back and forth, and then determining the position when the image on the focusing sensor was the most contrasting (that is, in most cases, sharp). An example of the algorithm for the operation of this afthofocus method is shown in the animation.

The disadvantages of this method include its extremely low speed. Also, this method works only if the image from the lens comes directly to the camera matrix, that is, the camera mirror is raised (this usually happens when the Live View preview screen is on).

Phase detection autofocus

Phase detection autofocus is a more complex mechanism that requires a little understanding of the DSLR camera first. Let's consider the camera operation scheme shown below.

Here the image falls on the mirror, from where it is redirected upward and enters the viewfinder with the help of re-reflection from the pentaprism (it is not in this diagram above). But the whole point is that the camera mirror is a little unusual, it is designed in such a way that in a certain area in the center of the mirror, light penetrates through it, where this light is already waiting for another small mirror, which directs the central part of the image downward, directly to the phase sensors. autofocus. If you switch the camera to Live View mode and raise the mirror in this way, you can see a recess under the place where the mirror was - this is where these sensors are located.

Phase-detection autofocus sensors work as follows: light from different areas of the image hits the beam splitters (see the diagram), from where a pair of beams through the lens enters the sensor. The sensor is a pair of light-sensitive sensors. The bottom line is that when the image is in focus (that is, on the separator, the rays from the image converge to one point), on a pair of sensor sensors, these rays fall exactly in the center. The advantage of this method is that you can see in advance in which direction you need to correct the focus so that the rays hit the center of the sensor, and moreover, how large the current error is.

From here advantage of phase detection autofocus: high operating speed.

However, for accurate operation of such autofocus, the distance from the lens to the sensor must be exactly the same as from the lens to the beam splitter. Therefore, there is also disadvantages: the mechanism requires fine tuning of focus sensors and / or a reflective mirror, the tilt angle of which can greatly affect the operation of this mechanism. Improper adjustment may result in defective pictures due to persistent inaccurate focusing.

Hybrid autofocus

In modern advanced DSLRs and mirrorless cameras, there is a hybrid autofocus that combines all the advantages of the previous focusing methods. In this case, focusing is performed on the matrix, which itself already has phase detection autofocus sensors at the focusing points. Thereby phase detection autofocus speed and contrast accuracy are achieved, which in this case slightly "brings" the focus to the most accurate value after the phase sensors have worked.

In this case, the camera can analyze the image from the matrix and focus in intelligent modes on the most significant parts of the frame for the scene (for example, on the faces of people in the frame). For the availability of such autofocus, see the description of a specific camera model that you want to purchase or already have in stock.

Autofocus problems

The process of correcting lens and / or camera problems when autofocus is not performing well is called alignment. Alignment (from the German justieren "measure") is the process of aligning structural elements along an axis, in a narrower sense it is applicable to photography. fine tuning lens or camera mechanisms.

Lens alignment

Quite often, a problem with the lens is the cause of inaccurate autofocus on a DSLR. To make sure that the matter is in the lens, you should check the autofocus operation with a different lens: if the camera focuses accurately with a different lens, then the problem is most likely in the lens and it must be brought to a service center for adjustment.

Camera alignment

Sometimes the source of the problem is the camera itself, or rather, the phase autofocus sensors. As mentioned earlier, this mechanism is prone to breakage at the slightest inaccuracy (the distance from the rear objective lens to the phase sensors should coincide as closely as possible with the distance from the rear objective lens to the matrix).

To check the camera, you should again take another lens and check it for accuracy. If the autofocus of the camera constantly “smears” with another lens, then you can safely carry the camera for adjustment. Or both lenses, hehe. No, well, if the second lens, which you borrowed, for example, from friends, works fine on their camera, then the problem is in your camera.

You should be aware that the service center can adjust your camera and lenses for each other, so you can bring a camera with all your optics. Usually, a good service center stores a perfectly adjusted reference camera, and the same lens, the actual lenses, are aligned on such a camera, and cameras - on such a reference lens. Or you can adjust your camera for your own lens, but this is not very reliable if you are considering expanding your lens fleet.

It should also be remembered that in advanced SLR cameras there is an option to adjust the phase detection autofocus for each of the autofocus lenses. That is, you programmatically ask the camera a command that, they say, "with this lens, make an adjustment for autofocus closer, and with this - farther away." How to determine the correctness of the amendment that you have asked - below.

Identifying Autofocus Problems: Front Focus and Back Focus

To determine the correction in the camera options (and just to diagnose autofocus problems), you should know that wrong job autofocus is divided into two types: "undershoot" and "overshoot" focus (front focus and back focus, respectively). To determine this defect, you can use special AF alignment targets, of which there are plenty on the Internet. They should be printed out and focused on the center of such a target. Before that, do not forget to completely defocus the camera somewhere far or near, so that it does not have "indulgences" when trying to focus on the target.

Personally, from my own experience, I will say that the autofocus of my camera was almost equally “undershot” to the focusing target on both lenses, which clearly indicated the problem of front focus with phase detection autofocus sensors. I had to carry it for adjustment.

Observe the focus point in use. Usually this is the center point, however, in the settings it can be changed to any other point / group of points, and then the camera may, for example, try to focus on the left side of the frame, while you placed main object shooting in the center or on the right.

Look for contrasting objects in the focus area. The fact is that any autofocus will not understand what you want from it if you focus on an ideal (white / black / any other color) object without any texture or detail.

In the absence of suitable contrasting objects, aim at an equidistant contrasting object, and then frame the frame (move the camera gaze to where you originally wanted). This method is very helpful in difficult situations, for example, sometimes it is easier to focus not on the perfectly blue sky in the center of the frame, but on the edge of the cloud on the left, and then transfer the ideal blue background to the center of the frame again. At close distances, use this method carefully, since, for example, the distance from the camera to the face of a nearby person is very different from the distance from the camera to their feet.

For very fast action scenes, use AF tracking mode(Canon calls it servo focus). In this mode, the camera will periodically send focus signals to the camera, thus increasing your chances of getting a focused shot on your subject. For example, I actively used this option when taking macro photography of working bumblebees that do not stop in one place for more than one second. Although any autofocus is bad for macro shooting (more on that below), and I got very few non-rejected frames, but for shooting, say, a cyclist who is going towards you - this is the very thing!

Intelligent autofocus mode will choose the focus points for you. According to my observations, my camera in this mode selects the closest lightest object and activates the focus points that it covers. This mode is intended for those who do not want to bathe with autofocus at all =).

Autofocus is contraindicated for macro shooting with high magnification., since the depth of field is usually so shallow that the camera cannot catch it. Thus, the lens begins to travel back and forth in search of focus. You yourself will understand that it is more convenient to shoot macro with your hands, although not easy. Moreover, for a large macro shot, it is easier to aim not by focusing, but simply by moving the camera closer and further from the subject.

With SUCH magnification, you can forget about autofocus. About handheld shooting without a flash, by the way, too.

Expanding the autofocus capabilities of Canon cameras

In order to expand the autofocus capabilities (and not only), I advise you to install the Magic Lantern firmware. During installation, you may need to update the firmware of your camera model to the latest version by downloading it from the official Canon website. Then follow the instructions to install Magic Lantern.

I must say right away that there are similar firmwares for owners of Nikon or other brands of cameras, you can find a list of them.

Magic Lantern provides software scenarios for using autofocus, such as:

• autofocus trap (focus trap): the camera automatically takes a picture when it enters the field of focus of any moving object, such as a bird;
• focus point patterns: now you can select not only single AF points or all at once, but also individual groups (all top, bottom, right, left, etc.);
• follow focus: manual control automatic focus with a constant speed, it can be useful when video with a subject moving towards you / from you is dependent on;
• move focus (rack focus): the same, but in full automated process, choose the initial focusing distance, the final - and go!
• focus stacking: an extremely useful option for macro photography, allows you to take several shots with a shift in the focusing distance, then you can combine these photos in any popular photo editor into one photo with a huge depth of field and detail for macro photography.

Conclusion

Autofocus of a camera is a difficult topic, which cannot be well understood "with one kick", especially if you have any problems. If you get blurry photos on a DSLR, then before carrying the camera to the service I advise. If the tips given in it helped you to achieve high-definition images when manually focusing or focusing on the screen (contrast), and automatic focusing through the viewfinder (phase) continues to miss, feel free to take the camera to the service center!

How the autofocus system works.

Focusing is a sore point for most amateur photographers (and professionals too). Believe it or check: any photography forum will convince you, and camera tests necessarily contain a section dedicated exclusively to autofocus operation.

Discussions of autofocus on photographic forums most often end with mutual accusations of ignorance or virtual grabbing at the lapels of a jacket with shouts “Who are you ?!”. I thought to do self-education and figure out - at the everyday level, how autofocus works in modern digital cameras. It turned out that there are very few materials on the Internet, and even less understandable to a person without special education. The search results and compilation of information (thanks to LenzRentals!) Are set out below.

Modern digital cameras use two autofocus systems: contrast autofocus and phase detection autofocus. Let's start with a simpler (and less common in DSLRs) autofocus system: contrast autofocus.

Contrast autofocus

Contrast autofocus works as follows: the processor evaluates the histogram received from the camera's matrix, moves the objective lens slightly - shifting the focus point, then re-evaluates to see if the contrast has increased or decreased. If the contrast has increased, the camera continues to shift the focus point in the selected direction until the image is the most contrasting. If contrast has decreased, the lens is instructed to shift the focus point to the other side. The process is repeated until maximum contrast is achieved (which essentially means moving the focus point slightly beyond the maximum contrast position and returning to the point after which the contrast began to decrease). An image “in focus” by contrast-detection AF is the image with maximum contrast.

If your camera shows a histogram in Live View, you can manually focus on contrast.

With contrast autofocus, an image is evaluated from a small area of ​​the matrix - used as a sensor and coinciding with the focus point chosen by the photographer. This allows you to select the subject you want to focus on, and saves the camera processor from having to evaluate the contrast of the entire image - only the contrast at the selected AF points is evaluated.

Disadvantages of contrast autofocus

The main disadvantage of contrast autofocus is its slowness. The multi-step process of "shift the focus point / lens lens - estimate - shift - estimate" takes time, and the camera can start by moving the focus point in the wrong direction - then it will have to go back. Due to the extremely low speed and the impossibility of tracking focusing, contrast autofocus is not very suitable for dynamic scenes. The sluggishness makes it difficult even to shoot stationary subjects. Contrast autofocus is much more than phase dependent on good lighting, and - obviously - requires good contrast of the subject on which the focus is made.

Benefits of contrast autofocus

The contrast autofocus also has advantages, thanks to which it is not only still used in cameras, but also increases its presence. First, the contrast AF system is simpler. It does not require additional sensors and microcircuits, which are needed for phase detection autofocus. Simplicity reduces cost and (for many, price is more important than speed) is the main reason for using contrast autofocus in compact digital cameras. (Another reason is that the depth of field in compact cameras initially more and the requirements for the accuracy of autofocus are significantly lower).

The simplicity of the contrast AF system reduces its size. For example, recently introduced mirrorless digital cameras with interchangeable optics tend to be miniaturized, and the contrast autofocus system does not require to “divert” the image away from the camera matrix: therefore, no need for prisms, mirrors and lenses required for the phase detection autofocus system. Miniature is one of the most important advantages of interchangeable-lens mirrorless cameras - they all use contrast autofocus.

The second advantage is that the contrast AF system uses the camera sensor. There is no need to “divert” the light beam through special prisms and mirrors to additional sensors, which can be unbalanced with respect to the camera matrix. In contrast autofocusing, the real image on the camera's matrix is ​​assessed, and not a separate image, which should be (and “should” does not mean that it is) precisely calibrated for compliance with the matrix.

It is for this reason that contrast detection autofocus provides more accurate autofocus than phase detection autofocus. I emphasize: "when using a matrix for contrast focusing." Olympus and Sony SLR cameras use an additional, smaller sensor for contrast AF in Live View mode, which means - as with any system that requires alignment - there is still the possibility of incorrect alignment.

Overall, the contrast AF system is simpler, cheaper, smaller, and theoretically more accurate than phase detection autofocus. But it is much slower. Manufacturers are working hard to speed up contrast autofocus, there are successes, but it will remain slower in the near future.

Phase detection autofocus

Basic principles

Phase matching (also known as phase matching) was introduced by Honeywell in the 1970s; it was first used commercially in a Minolta Maxxum 7000 camera. Honeywell sued Minolta for patent infringement and won; so manufacturers had to pay Honeywell to use phase-detection autofocus.

Phase detection autofocus is based on the principle that, outgoing / reflected from a point in focus, beams will equally illuminate opposite sides of the lens ("will be in phase"). If the lens is focused in front of or behind this point, these light rays pass through the edges of the lens differently (“out of phase”).

Most existing phase detection autofocus systems use mirrors, lenses, or prisms (beam splitters) to split beams passing through opposite ends of the lens into two beams; and a secondary lens system to refocus those beams on the autofocus sensor (typically a CCD). This sensor detects where light beams are falling through opposite edges of the lens. If the point is in focus, the beams hit the sensor at a certain distance from each other. If the lens is focused closer or farther than the desired point, the distance between these beams will be less or more. Lots of words, let's try to look at the graphical display of the process - (Fig. 1).

Rice. 1 How phase detection autofocus works

I'll make a reservation right away: the description and figure give a very simplified explanation of the principle of operation of phase detection autofocus - in order to get an idea of ​​how it works. Physics and mechanics of the process, the description of which would take more than one page full of formulas, numbers and other incomprehensibility, remained behind the scenes.

The figure clearly shows that the camera processor in the phase detection autofocus system immediately determines whether the lens is focused too close or too far from the subject, so one of the disadvantages of contrast autofocus (the camera does not know which way to move the focusing point) is initially absent - instead of moving forward and back and determining in which direction the high contrast lies, in phase detection autofocus the processor immediately sees in which direction to shift the focusing point.

And then the process goes on. Each autofocus lens is equipped with a microprocessor that informs the camera about its presence and state, for example, "I am a 50 / 1.4 lens and my focusing element is 20% closer than infinity" - or something similar. When you press the shutter button halfway, the following happens:

The camera reads the data from the autofocus sensor, checks against the data array containing information about the properties of the autofocus lenses of this manufacturer, does some calculations and says to the lens something like "Move the autofocus point this way to infinity."

The lens has sensors and microcircuits that measure either the amount of current supplied to the focusing motor, or how much the focusing element has moved. The lens shifts the focusing element and sends a signal to the camera "near the target".

The camera checks the data from the autofocus sensors, and sends a signal to the lens for a more accurate adjustment; the fine focusing process can be repeated several times until the lens is focused “right on target”. If something goes wrong, the infamous "yaw" of the lens occurs.

After focusing, the camera orders the lens to lock focus, and informs the photographer (with sound and indicator in the viewfinder). The whole process takes a fraction of a second. Very fast.

Phase detection autofocus circuit

The autofocus sensor cannot be in front of the sensor, so manufacturers use partially transparent areas in the mirror, allowing light to pass to the secondary mirror, from which it is reflected back to the autofocus sensor (Fig. 2).

Rice. 2 Phase detection autofocus circuit

Typically, the AF sensor is located under the main mirror (Fig. 3) along with the metering sensors. The red arrow shows the Canon EOS 5D's autofocus sensor. Image via Canon USA

Rice. 3 Location of the AF sensor

Phase detection detection sensor types

Each sensor is capable of evaluating only a small part of the image. Horizontal sensors work more accurately with vertical parts. In most images, vertical detail is predominant, so there are more horizontal sensors. There are also vertical sensors, as a rule, located crosswise with horizontal ones (Fig. 4). Some cameras are even equipped with diagonal phase detection autofocus sensors.

Some AF sensors (almost always located in the center), using different lenses and the size of the sensor itself, achieve greater AF accuracy, especially when using fast lenses. Most of the time, they only come into play when using lenses with f / 2.8 or brighter aperture. Figure 4, for example, shows that when using an f / 2.8 lens, a cross-type sensor will be used, while for darker lenses, only one less accurate AF sensor will be used.

Rice. 4 Cross-type AF sensor

Early phase detection detection systems (and some modern medium format cameras) had only one sensor in the center of the image. As computing power and engineering prowess increased, more and more sensors were added. Now most cameras have from seven / nine to 52. You can - depending on the requirements of the scene being filmed - choose one, all, or a group of sensors. You can tell the camera which sensor / sensors to use.

Numerous phase detection autofocus sensors, together with the camera's processor, are capable of wonderful things. By determining in which sensors the moving object is in focus and how it changes - by measuring the movement of the object and reading the readings at the shortest time intervals - the camera can predict where the moving object will be after a certain period of time. The tracking autofocus works on this.

Effect of lens aperture

Regardless of the type of sensor, autofocus will be more accurate when using fast lenses. In the process of focusing, the camera opens the lens as much as possible, closing the aperture to the value you have chosen only at the moment the shutters open. The wider the angle of the light beams, the more accurate the phase detection autofocus. In the above diagram, the angle of rays taken from an f / 2.8 lens (blue lines) will be greater than from an f / 4 lens (red lines), which in turn will be larger than from an f / 5.6 lens (yellow lines). When using a lens with a maximum aperture of f / 8, only the most accurate sensors are capable of working, but focusing will be slower and less accurate. It is for this reason that f / 5.6 lenses stop autofocusing when we try to use a teleconverter that reduces their maximum aperture to f / 8 or f / 11.

Benefits of phase detection autofocus

We have already mentioned the main advantages of phase detection autofocus:

It's much faster than contrast ratio - fast enough for moving subjects.

The camera is capable of using a group of sensors to estimate the movement of an object, which gives us tracking / predictive autofocus.

There are also less obvious benefits. Groups of phase detection autofocus sensors can be used for "electronic depth of field" - a preliminary estimate of the depth of field. Some cameras (although there are few of them) are equipped with a trap autofocus function - they take a picture at the moment when something hits the active focus point. If the sensors detect movement in a static scene, they can report inappropriate camera movement. But - the main thing - speed and tracking autofocus

Disadvantages of phase detection autofocus

Firstly, phase detection autofocus system requires physical alignment... The path of the light to the camera sensor must match the path of the light to the AF sensor so that the subject in focus on the AF sensor is in focus and on the sensor. Each lens must contain a microcircuit that provides feedback to the camera and informs it about the exact position of the focusing element, about how far the element moves when a certain current is applied to the autofocus motor. All this must be precisely coordinated and verified in such a way that the lens moves the focusing point exactly where the camera pointed to it, and the camera knows the exact position of this point. The slightest inconsistency leads to inaccurate focusing.

Secondly, the system requires software configuration... Each camera and lens is programmed by the manufacturer, a large amount of data is entered into memory. This data ensures that the camera and lens work in perfect harmony, and AF accuracy can sometimes be improved through firmware updates. Such updates are often released following the introduction of new lenses.

Manufacturers hide the algorithms of their phase detection autofocus systems. Third-party lens manufacturers are forced to experimentally read and decode the signals exchanged between the camera and the lens and, based on this data, develop their own microprocessors and their own algorithms. As a result, autofocus accuracy may be lower when using third-party lenses. Changes in algorithms by camera manufacturers lead to the fact that autofocus on third-party lenses refuses to work (they need to be reprogrammed, as recently happened with the Sigma AF 120-300 / 2.8 and Nikon D3X).

As already mentioned, lens aperture affects the accuracy of phase detection autofocus. High-aperture lenses able to focus in more difficult conditions. Usually, aperture dependence is not a problem because dark lenses have a greater depth of field. However, there are maximum apertures (usually f / 5.6 or f / 8) where phase detection autofocus simply refuses to work. (Remember, we are talking about the maximum aperture of the lens - the camera automatically opens the lens aperture fully during focusing, so the set value does not affect autofocus if the maximum aperture of the lens matches the capabilities of the camera).

Since light hits the AF sensors only when the mirror is lowered, they stop working at the moment the picture is taken, and do not start working until the mirror returns to its original position. This is why phase detection autofocus does not work in Live View, and tracking autofocus can fail with continuous shooting.

There are other problems that we do not notice. Linear polarizing filters interfere with phase detection autofocus. There are not many linear polarics left now, but it happens that having bought it "cheaply" the owner is then surprised by the inaccuracy of autofocus. Phase autofocus can simply "blown away" on some subjects (such as a checkerboard or lattice), and contrast easily copes with them.

Live View:

I highlighted Live View because it is what keeps manufacturers working to improve contrast autofocus and create hybrid systems. As mentioned, contrast autofocus has certain advantages, and overcoming its limitations will benefit everyone who photographs.

Olympus and Sony have already created systems that split the light beam, sending some to the viewfinder and some to the secondary image sensor. This system allows phase detection detection to be used even in Live View. But the risk of inaccurate focusing also increases, because not a matrix is ​​used, but an auxiliary sensor.

Canon has described a system that uses phase detection autofocus on initial stage and then fine-tunes the focus using contrast AF.

Nikon seems to have applied for a patent on the principle that certain pixels of the camera's sensor will be used as phase detection autofocus sensors. This - in my opinion - will be just a revolution.

FujiFilm has already launched a line of compact digital cameras with a hybrid autofocus system.

Wait and see. But it is clear that for the first time in recent years, changes to autofocus systems may be revolutionary, not evolutionary. That - you must agree - conceals a lot of interesting and exciting things for photographers.

#5

The article is very useful! Thank you!

#6

And again, thank you very much for your kind words and feedback! I am very glad if the material seemed useful and interesting.

#7

#8

Can you ask a question?
Is the sensor sensitive to the spectral content of light, and how does this affect focusing accuracy?
Thanks.

#9

written in the heading "JUST about autofocus", where is the pancake just? of course it is written accessible, but in sooooo complicated language, not a single gram of simplification

All digital cameras, whether it's a cheap soap dish or an expensive DSLR, have automatic focusing. Why then do you need a manual one if the system itself does a good job? This is a logical question, and the answer to it is also logical: often automatic focusing does not work correctly, sometimes very slowly and inaccurately. Therefore, knowledge of how focusing works and how to set it up greatly expands the creativity of any photographer and, who would have thought, ensures correct focusing in different situations.

Manual focusing allows you to highlight the main subject in the frame or, conversely, draw the viewer's attention to the details of the picture. This is exactly what we're going to talk about today - focus and situations where it is most appropriate to use manual focus.

Focus points

The focus is a certain point where all the rays reflected from the image converge. Therefore, for the image to be "in focus", the focus point must be on the camera sensor. Focusing allows you to prioritize the image by riveting the viewer's attention to the main objects, rather than minor details.

The focal point is a point in space, and it is there that the subject is located. The image of this object appears clear on the matrix. The choice of point is determined by the decision of the photographer to choose the most important detail in the frame and direct all the viewer's attention to it.

In expensive SLR cameras (even in mirrorless cameras), the user is offered a choice between specific focus points. It is also possible to select everything at once. If you choose focusing by one point, then the picture will be sharp only in that part of the image that coincides with the selected point in the viewfinder. If you choose focusing on all points at once, then the automation itself will select the focus in accordance with its own intelligent algorithms. Automation often misses the mark and spoils the picture.

According to the class of the camera, there may be many or not focus points. Inexpensive entry-level DSLRs are equipped with 11-point focusing systems. A great example is a camera Canon EOS 700D.

Expensive professional cameras have 61 focus points. Example - camera Canon EOS-1D X.

Autofocus miss

Do not think that such a phenomenon is rare. Even in expensive DSLRs, autofocus miss is not excluded. This is due to the fact that the focus is selected automatically, and often the camera corny "does not know" the goals of the photographer and, therefore, focuses on the wrong object that the photographer intended. Sometimes the system cannot determine a specific focus point, and then the focus buzzes, trying to "aim" at an unknown target. Intelligent focusing algorithms are often used to focus irregular objects in the foreground or middle ground.

Still, autofocus is important. It is impossible to always manually aim at objects and delve into the settings, especially if the events in the frame are fast and do not allow you to wait. Therefore, in sequential shooting, automatic focusing at all points is always useful.

Interesting: in compact inexpensive soap dishes, the focusing problem is practically absent. Soap boxes usually focus at infinity, resulting in every object being in focus — the foreground, the horizon, and the middle ground. However, this severely limits creative potential camera and the photographer himself - you cannot select a specific object and direct all attention to it. Therefore, to a greater extent, this is a disadvantage, not an advantage.

Focus application for beginners

Professional photographers, when communicating with beginners, recommend using only one central focus point instead of autofocus on all points. It is much easier for a beginner to determine the center of the frame at the time of shooting through the optical viewfinder of a DSLR.

In this case, the technique for taking a picture will be as follows: first, choose the main subject of shooting - it should be sharp in the frame. Then, in the viewfinder, place it in the very center of the frame, aligning it with the focus point (which is in our center). Next, hold down the shutter button halfway to lock the autofocus. Most cameras emit a characteristic squeak at the same time. Then the release button is clamped all the way.

The result is a photograph with a sharp subject right in the center of the frame, other details out of focus, less sharp. Attention to them is riveted weakly. From an artistic point of view, such pictures look very good. In this case, it was about using the center focus point, but it could easily be the right or left point. Then the main subject would have to be placed on the right or left side of the frame to be in focus.

Auto focus modes

Each company that produces photographic equipment uses its own intelligent focusing algorithms. They give approximately the same result. The modes in which these algorithms are used can be called differently. For example, in camerasCanon has a modeOneShot- it is intended for photographing stationary and stationary objects. For objects in motion Canon has provided a modeAlServo- slightly different algorithms for determining the main objects and focusing will already be used here.

HaveNikon calls these modes differently:SingleServo - for motionless stationary objects,ContinuousServo - for moving people. Also, both brands have predictive focus modes, when the system tries to predict the further movement of the object in order to ensure correct focus.

However, despite the development of technology and improvement of algorithms, autofocus often misses. Here are some examples of photos with focus errors:

Quite often photographers have to deal with autofocus miss, and the reasons are often the same: shooting conditions that "deceive" the focusing system.

When is it appropriate to use manual focus?

The very first and most common moment when it is best to manually focus on the subject is low light. Often, in low light, the camera lens starts to rotate back and forth and cannot specifically identify the correct object. However, this problem is partly solved thanks to the autofocus illumination (the foreground is highlighted, which allows you to ensure correct focus), although not completely.

Macro is another mode in the camera when it's best to manually focus on the subject. The problem is that macro photography works in very narrow ranges of sharpness, therefore, any inaccuracy will ruin the picture.

Portrait shooting. Usually there are no problems here, because in this mode the system focuses on the eyes of the model. However, professional photographers sometimes use manual focus to highlight the lip line or other part of the face. However, this is a completely different story.

The most common complaints I hear from most photographers of all levels are “my images are not sharp” and “I can't get focus.” Many people blame their equipment and, indeed, there are quite a few cases in which the equipment is to blame. However, I found that the vast majority of cases are simple mistakes users. They often boil down to a lack of understanding of how the autofocus (AF) system works. This article will give you a better understanding of focus and sharpness, and hopefully help you get the pictures you enjoy!

1. Adjust your diopters

You ask mine what? Your diopters - or your eyepieces. You will never know if the camera is well focused on a subject if you yourself cannot see what is in sharp focus through the viewfinder eyepiece. There is a small wheel on the side of the eyepiece (as shown below) to adjust the focus of the eyepiece according to your vision.

You can set the eyepiece to a fairly large offset, but if you need even more correction, then for DSLR / SLR cameras from many major manufacturers, interchangeable diopters ranging from -5 to +4 are available. No, this will not help your autofocus perform better, but it will help you understand when it misses and correct the situation with manual focus.

2. Understand your viewfinder

What the hell is all this stuff inside? At this point, you might want to get your hands on a manual (remember that paper book that came with your camera?). Most conventional DSLR cameras have 9-11 focus points. The best of the best, professional cameras can have from 45 to 51 points (although in fact you can only choose 11-19 points, the rest of the points are additional).

There are two types of focus points: co-plane and intersection points. Single plane points only work on contrast lines that are directly perpendicular (90 °) to their orientation. So, if you look through your viewfinder, similar to the one shown above, you can see that most of the points are rectangular, some are oriented horizontally, some are oriented vertically.

Points on the same plane will only work perpendicular to their orientation. So - let's say you were shooting a tree - a vertically oriented focus point could not find the edge of a tree trunk, but a horizontal one could. You can use this to your advantage when choosing a focal point that locks onto the line you want and ignores those you don't need to focus on.

Cross point focus points will work with contrast lines in any way. Most cameras have a single focal point of intersection in the center, surrounded by points on the same plane. Latest cameras today contain intersection points at all focal points.

Each focal point also has a specific sensitivity. Most require a lens with an aperture limit of at least f5.6 to at least use autofocus. On most cameras, the surrounding AF points are of this sensitivity, and the center point provides increased sensitivity if you are using a lens with an aperture limit of at least f2.8.

So if you are working in low light conditions, you can achieve better autofocus using the center focusing point. Even if you are not using an f2.8 lens, or better yet, there are no light restrictions to favor the center point, it can still give a more accurate result since it is the crossover point.

When we look at the focusing point rectangles, it is worth remembering that the real area of ​​the sensor is 2-3 times larger than what is displayed. Keep this in mind as you focus. If you fix the focus on someone's bridge of the nose, remember that the person's eyes will also fall into the effective area of ​​the sensor. Autofocus locks onto the eye rather than the bridge of the nose because the eye has more contrast around the edges than flat lighting on the nose. This may often not matter, but if you are working at a very shallow depth of field, you will feel the difference in which area of ​​the image will be the sharpest.

3. Reach out a helping hand for your lens

Most autofocus systems have a certain amount of errors or deviations and may indeed miss the optimum focal point due to the mechanics and inertia of the lens movement. You can minimize this effect by manually focusing the lens close to the desired focus, and then letting the autofocus system complete aiming. Or, if this seems a bit overwhelming to you, at least let the autofocus make two attempts to aim correctly. Press the shutter button halfway to adjust the approximate focus, and then press again to fine-tune it.

The advantage of lenses is more high level in that they allow full manual control even when autofocus is locked. Cheaper lenses do not allow manual focus adjustment after locking, although this good way make sure the focus is as perfect as it can get.

4. Find a good line

Autofocus systems operate on lines of contrast, so they can have difficulty on subjects that are not in contrast (for example, on the cheek or forehead, on a white dress or black tuxedo, sand, monochrome walls, etc.). On areas like these, autofocus can be focused all day and never lock. The approach is to find the "best line" - it could be the eyes, the lines between a contrasting shirt and suit, between heaven and earth, a doorway. Anything with contrast will help autofocus work better and faster.

Weak focus area

Best area to focus

5. Do not use all-point focusing mode

Unless you're in a terribly fast-paced situation that requires incredibly responsive aiming, it's best to avoid All-Point Focus. This mode does not know what you want to focus on and usually locks onto what is closer to the camera. There are situations in which this is exactly what is needed, but they are few and far between.

6. Focus and change the composition - but do it right

I'm used to focusing and changing composition, using the center focusing point all the time. I fix the focus and then re-compose the frame. I once read a few articles that argued that you shouldn't do this - that you should use the point closest to the area you want to focus on. Theoretically, this is justified by the fact that during the transfer of the camera, due to the movement of the lenses and the change in the angle value, the distance between the subject and the lens also changes.

If you use the focusing point directly on the subject and do not recompose the frame, then there will be no change in the distance between the subject and the lens, and therefore, erroneous focusing. So, I decided to take some pictures to show you that it is - and it is not.

There was absolutely no advantage in using the nearest off-center point without changing the composition. In fact, focusing using the center point and rearranging was, indeed, more accurate in all cases, except for one - macro. I took shots at all focal lengths from 17mm to 200mm and checked all macro distances of 10 meters - with the same results.

Each test using the center focus point and recomposition of the frame resulted in a sharper picture. More sharpness from using the center point and re-composition, less from using the external focus point directly on the subject. To clarify - the above theory is correct, in that you lose sharpness by moving the frame away from the fixed center point. What's not true is that using the outer point directly on the object will return sharpness - not.

I think there is a high probability that it is on my camera that the center focus point is three times more sensitive than any of the other eight points, and it is the most accurate. But this is the case with many cameras, with the exception of high-tech new models such as the Canon 1Ds Mark III or Nikon D3X. Other possible the reason is that most lenses are sharper in the center and lose clarity towards the edges.

Here are three of the most typical examples of what I shot with two different cameras. An inset is an inscription at a scale of 100%.

Center focus point, no composition changes. Lens 50mmf 1.8.

Lens 50mmf1.8. Center focusing point. The camera is moved to the left after fixing the focus.

50mm f 1.8 lens. Left focusing point. Focus on the subject.

What can I say - to switch the focus point or not, in my opinion, is a matter of time. But try it yourself, your result may differ.

A small note on macro - such shots should always be shot with a tripod and manual focusing, due to the extremely shallow depth of field and the proximity of the lens to the subject.

7. Use Correct Focus Mode

Most DSLR cameras have at least two similar focusing modes. The first is “One Shot” (Canon) or “Single Servo” (Nikon). In this mode, it is assumed that the object is stationary. Focus locks, you get lighting confirmation on the internal display, and then you release the shutter. You cannot release the shutter if the focus is not locked.

The second type is “AI Servo” (Canon) and “Continuous Servo” (Nikon). This mode is for shooting subjects in motion, including sports, wildlife, etc. The camera locates the subject using the focusing point, and the focus will change constantly to keep up with the subject, but will never lock. The shutter release button will operate even if focus is not complete.

Some cameras also have other modes, such as Canon's “AI Focus”, which are fine if the subject is static but may start to move, as is the case with small children. Autofocus will lock on the subject, but if the subject moves, the camera will enter AI Servo mode to track it.

The third option, pre-focus, is designed for objects that move towards or away from you. The camera will try to predict movement and provide you with an acceptable focus.

8. Don't change depth of field for good focus.

While using a larger depth of field with a smaller aperture can increase the "apparent" sharpness of an image, remember one thing: no matter what the depth of field is, there is only one focal point. So always practice good focusing techniques regardless of the depth of field used.

9. Use a tripod or find a support

When we take a photograph, we are all unconsciously rocking back and forth - in particular, leaning towards an object with a heavy set of camera and lens. It `s naturally. In all, this manifests itself to one degree or another. And if you're shooting with a very shallow depth of field, that small distance you move can seriously affect the sharpness and clarity you want. If you are using a depth of field of 4 inches, a deviation of 2 can be disastrous. So use a tripod.

Now I must add, since I use a tripod myself - I hate them... They affect the way I work and most of the time the way I shoot. So if you prefer to avoid using a tripod, at least take the time to practice the position. good photographer... One leg in front of the other, the legs are slightly bent, the arms are pressed to the sides, do not dangle in space (the remotes can come in handy here remote control), and the body weight is centered above the legs.

10. If still unsuccessful - use manual focus

I hear a deep sigh every time I offer this to photographers. They regularly state, "I only shoot manual, never automatic." But offer them one more time tame focusing and they will look at you as if you just asked them to sell your children. Manual focusing in most cases (assuming your diopter settings are correct) will help you achieve the best and most accurate result. Especially in this digital age, when it is so easy to view an image at 100% or even 200% magnification on our monitors.

In fact, if you look at the unofficial autofocus specifications, you will notice that they are not very accurate. Here is the specification for "sharpness": An image is considered sharp if it appears sharp in print 6x9 from a distance of 10 inches. Yeah, like that. No 100% zooms, no 20x30 prints. Just this.

Today, a number of new cameras are being manufactured with built-in Live View. This can be a useful tool when focusing manually. Turn on your Live View, zoom in on the subject / focal point and check the sharpness on the display. This does not work very well in my case, since I am almost always in bright conditions: deserts, beaches, etc. - but for some it works great.

One note about the image above. I usually use it to show the manual focus switch, but another switch is also of interest: 1.2m to infinity and 3m to infinity. This switch is related to what I talked about earlier: don't force your lens to hunt for focus across as much space as possible. If you know you will not be focusing on anything closer than three meters, move the lever to the appropriate position and the lens will not be forced to wander in search of focus. This may result in a more accurate primary autofocus.

11. What should I focus on?

For close-up portraits, there is generally a consensus: the eyes. For other types of portraits, it is still a face, unless you want to focus on some other part of the body on purpose. Bring precise focus where you want to direct the viewer's gaze.

Landscapes are not always that simple, but you should still stick to the rule mentioned above. Don't bet that "this is a landscape with a wide viewing angle, focus at infinity." If you have a subject in the foreground, focus on it and let your depth of field bring the image to the background. If the subject in the foreground is not in sharp focus, it is confusing, since we naturally see more sharply near objects rather than distant ones in our natural environment.

I can now work with "Hyper Focal Distance" focusing, but that is beyond the scope of this tutorial. If you are interested in this topic, which is likely, use a quick Google search.

12. The subject is in focus, but is it sharp?

Focus and sharpness are two different things. Explaining the concept of sharpness may take another separate lesson, so I'll just point out a few useful points.

If the image is out of focus, you cannot bring it back into focus by sharpening it. You just get a very sharp out of focus picture. Most RAW images need some kind of sharpening. Whether you use Smart Sharpen, Un-sharp mask or high pass filtering techniques, most RAW images benefit from sharpness. However, as I gradually started using higher quality cameras, I saw less and less need for sharpening, and now only use this tool in about 25% of my shots.

Remember also that sharpness depends on the final product. You will not sharpen the same image that is sent to the Internet and for a 16x20 print. And with that in mind, if you intend to sell your photo through a stock agency, you shouldn't sharpen it at all. Most will tell you not to do this because you cannot predict what the image will be used for and at what size.

On the left are images directly from the camera, unedited (English "Straight Out of the Camera"), on the right - with added sharpness.

13. Consider an excerpt

Exposure is another parameter that can lead to a lack of sharpness. Each person has a limit to the shutter speed at which they can shoot handheld with different lenses. focal lengths... Some people are more stable than others, but if you shoot at a shutter speed that can't compensate for the movement (shake) of your hands, your image will be blurry. When it comes to standard and wide-angle lenses, most people can consistently shoot handheld for about 1 / 30-1 / 60 of a second.

Larger telephoto lenses require much faster shutter speeds. General rule where people start: "1 to the focal length of the lens." So, if you have a 200mm lens, shoot at 1 / 200th of a second, and start from this level to understand your future possibilities. Personally, I'm shaking like California in worst days so I usually shoot at faster shutter speeds. It also depends on how far away you are from the subject, as the further you are, the more noticeable the movement.

If the subject is moving, holding the camera steady or on a tripod will not help — you will still need to select a satisfactory shutter speed to stop the action. Most start at 1/250, but this depends on how fast the object is moving. Requirements also vary depending on whether you are shooting in a static position or following your subject. If the latter, you can choose a slower shutter speed and also get some interesting effects. This will allow you to show movement in the background, but will freeze the object itself.

Image stabilization systems on the lenses allow you to shoot handheld at faster shutter speeds (up to 3 stops), but will not "freeze" the movement any better than a lens without a stabilization system. You can only capture the action with shutter speed (or high-speed flash).

Image not out of focus. The shutter speed is too long.

14. Choose the correct exposure

Correct exposure and good lighting (the essence of the whole photo) is the key to good focus and sharpness. While sharpness is determined by the line of contrast, if you underexposed or shot in dim lighting, the image will not be sharp, even if all other parameters for good focus are met.

15. I did it all. I'm still out of focus!

There is a small chance that something is really wrong with your hardware. Lenses from third-party companies are sometimes not well-designed, and therefore will not always function perfectly with branded cameras. Some work great, others don't. But sometimes even branded lenses aren't perfect.

Cameras such as the Canon 50D and 1D / Ds Mark III are perfectly matched in front and back focus for over 20 different lenses, so if you know that one of the lenses is perfectly focused in front of your subject all the time, you can adjust the camera. to fix this. If this option is not available, you need to determine if your lens, or both the lens and camera, need repair.

Here's a test you can do at home to see if it's you or the cell. Find a ruler and place it on the table away from the camera. Place the camera on a tripod and open the aperture as much as possible. Take a picture of the ruler at a 45 degree angle, focusing on a specific mark - in this example, a six.

If this is the mark that you will see as clearly as possible when you open the image, then everything is in order with your equipment - go back to work on the technique! If the sharpest point is in front of or behind this mark, then you will know that this is a hardware problem and should be taken to the service.

16. Conclusion

I've covered a ton of questions in this tutorial - great you read to the end! It seems to me that good focus and sharpness are two of the most important technical parameters to be saved in the image. This can make a difference between what looks professional and what looks like amateur work (and we all want to look professional - whether we are or not).

Please feel free to leave your opinion in the comments - have you ever had problems with focus and sharpness?