Autofocus in a smartphone. Contrast and phase detection autofocus, which is better? AF area modes

The camera's autofocus system adjusts the lens to focus on the subject and can provide the difference between a sharp shot and a missed opportunity. Despite the seemingly obvious task of "clarity at the focal point", hidden work required for focusing is, unfortunately, not so simple. This chapter is designed to improve the quality of your shots by providing an understanding of how autofocus works so that you can get the most out of its capabilities and avoid its disadvantages.

Note: Autofocus (AF) works either using in-camera contrast sensors ( passive AF), or by sending a signal to illuminate or estimate the distance to the object ( active AF). Passive AF can be carried out by methods contrasting or phase detector, but both rely on contrast to achieve accurate autofocus; therefore, from the point of view of this chapter, they are considered qualitatively identical. Unless otherwise noted, this chapter deals with passive autofocus. We'll also look at the Active AF Auxiliary Beam method towards the end.

Concept: autofocus sensors

The camera's autofocus sensor (s) are located in different parts of the image field of view and are the whole system behind achieving sharp focus. Each sensor measures relative focus by contrast changes in the corresponding area of ​​the image, and the maximum contrast is considered to correspond to the maximum sharpness.

Focus change:		Blur	Semi-focus	Sharpness
	400%	Sensor histogram

The basics of image contrast are covered in the chapter on image histograms.
Note: Many compact digital cameras use the image sensor itself as a contrast sensor (using a technique called contrast AF) and are optionally equipped with multiple discrete AF sensors (which are more common with phase-detection AF). The diagram above illustrates the contrast AF method; the phase detector method differs from it, but also relies on contrast as an autofocus criterion.

Focusing process in general outline works like this:

1. The autofocus processor (AFP) changes the focusing distance slightly.
2. The AFP reads the AF sensor and estimates how and how much focus has changed.
3. Using the information from the previous step, AFP adjusts the lens to a new focusing distance.
4. AFP repeats the previous steps in sequence until satisfactory focus is achieved.

The whole process usually takes a split second. In difficult cases, the camera may not achieve satisfactory focus and will start repeating the above process, which means autofocus fails. This is a terrible case of "focus hunting", where the camera constantly moves focus back and forth, without achieving focus. However, this does not mean that focusing on the selected subject is impossible. The next section discusses the cases and causes of autofocus failure.

Factors affecting autofocus

Your subject can have a huge impact on how successful autofocus is, often even more than differences between camera models, lenses, or focus settings. The three most important factors affecting autofocus are the amount of light, the contrast of the subject, and the movement of the camera or subject.

An example illustrating the quality of the various focal points is shown on the left; hover over the image to see the advantages and disadvantages of each of the focal points.

Note that all of these factors are interrelated; in other words, autofocus is achievable even on a dimly lit subject if it has a high contrast, and vice versa. This has important implications for your choice of AF point: choosing a focus point that is on a clear border or pronounced texture will help you achieve better AF, all other things being equal.

The example on the left compares favorably in that the points of the best autofocus coincide with the position of the subject. The next example is more problematic because autofocus works better in the background than on the subject. Hover over the image below to mark areas of good and bad AF performance.

In the image on the right, if you focus on fast moving light sources behind your subject, the subject itself may be out of focus if the depth of field is shallow (as is usually the case when shooting in low light conditions such as the one shown).

Otherwise, focusing on an external illumination of the subject might be the best approach, minus the fact that this illumination rapidly changes position and intensity depending on the position of moving light sources.

If the camera fails to focus on the ambient light, a less contrasting (but more static and reasonably well lit) focal point can be selected the legs of the model or leaves on the ground at the same distance with the model.

However, the above choice is complicated by the fact that it often needs to be made within a fraction of a second. Additional specific AF techniques for stationary and moving subjects will be discussed in the respective sections towards the end of this chapter.

Number and type of AF points

The stability and flexibility of autofocus is primarily a result of the number, position and type of AF points that are available with a given camera model. SLR cameras top class have 45 AF points or more, while other cameras may even have only one center point. Two examples of AF sensor locations are shown below:

The examples on the left and right show the Canon 1D MkII and Canon 50D / 500D cameras, respectively.
For these cameras, autofocus is not possible at apertures smaller than f / 8.0 and f / 5.6.

Note: The "vertical" sensor is called only because it detects contrast
along the vertical line. The irony is that such a sensor, as a result,
the best way detects horizontal lines.

For DSLR cameras, the number and accuracy of AF points may also vary depending on the maximum aperture of the lens used, as shown above. This is an important factor when choosing a lens: even if you don't plan on using the lens's maximum aperture, it can still help your camera achieve more high precision autofocus. Further, since the center AF sensor is almost always the most accurate, for off-center subjects it is often best to use the center AF sensor first for focusing (before changing the composition).

Multiple AF sensors can be operated simultaneously for increased reliability, or separately for increased distinctiveness, depending on the selected camera setup options. Some cameras also have AutoGRIP, an option for group photos that ensures that all points in the focus cluster are in an acceptable degree of focus.

AF modes: tracking (AI SERVO) or one-shot (ONE SHOT)

The most widely supported camera focus mode is Single, which is best for still images. This mode is prone to focusing errors for fast-moving subjects because it is not designed for movement, and it can make it difficult to track moving subjects with the viewfinder. Focusing once requires focusing before a picture can be taken.

Many cameras also support an autofocus mode, which continuously adjusts the focusing distance for moving subjects. Canon cameras call this "AI Servo" mode, and Nikon cameras call this "continuous" focusing. Tracking mode works on the basis of an assumption about the location of the object at the next moment in time based on the calculation of the speed of the object from the data of previous focusing. The camera then focuses on the predicted distance ahead of time to account for the shutter speed (the delay between the shutter release is pressed and the start of the exposure). This greatly increases the likelihood of correct focusing on moving subjects.

Examples of maximum tracking speeds are shown for various Canon cameras below:

Values ​​are valid for ideal contrast and illumination when using the lens.
Canon 300mm f / 2.8 IS L.

The above graph can be used to roughly calculate the capabilities of other cameras. Actual tracking speed limits also depend on how uneven the movement of the subject is, the contrast and illumination of the subject, the type of lens, and the number of AF sensors used for tracking. Also keep in mind that using focus tracking can dramatically reduce your camera's battery life, so use it only when necessary.

AF-assist beam

Many cameras are equipped with an AF-assist beam, visible or infrared, which is used in the active autofocus method. This can be very useful in situations where the subject is poorly lit or has insufficient contrast for autofocus, although using the assist beam also has its drawbacks as autofocus is much slower in this case.

Most compact cameras use a built-in infrared light source for AF operation, while digital SLRs often use a built-in or external flash to illuminate a subject. When using an auxiliary flash, it may be difficult to achieve autofocus if the subject moves noticeably between flashes. Therefore, the use of Assisted Illumination is only recommended for stationary objects.

In Practice: Capturing Motion

Autofocus will almost always work best when capturing motion in AI servo or continuous mode. Focusing efficiency can be greatly improved provided that the lens does not need to search over a wide focusing range.

Perhaps the most versatile way to achieve this is pre-focus the camera on the area in which you expect a moving object to appear... In the example of a cyclist, the pre-focus can be carried out on the side of the road, since the cyclist is likely to appear close to it.

Some lenses for SLR cameras have a switch for the minimum focusing distance, setting it to the maximum possible distance (closer to which the subject will never be) will also increase efficiency.

Note, however, that pictures can be taken in continuous AF mode even if accurate focus has not yet been achieved.

In Practice: Portraits and Other Stills

Still pictures are best taken in single focus mode, which ensures that accurate focus is obtained before exposure begins. The usual focus point requirements in terms of contrast and illumination apply here, but a little movement of the subject is also required.

For portraits, the eye is the best focal point because it is the standard and because it provides good contrast. Although the center AF sensor is usually the most sensitive, the most accurate focusing for off-center subjects is achieved by using off-center focus points. If you use the center focus point to lock the focus (and then change the composition), the focusing distance will always be slightly less than the actual one, and this error increases with the approach of the subject. Accurate focusing is especially important for portraits as they tend to have a shallow depth of field.

Since the most commonly used AF sensors are vertical, it may be appropriate to worry about whether the contrast is dominant at the focus point, vertical or horizontal. In low light conditions, autofocus can sometimes be achieved only by rotating the camera 90 ° while focusing.

In the example to the left, the rungs are predominantly horizontal lines. If you focus on the farthest of the front steps (in order to obtain a hyperfocal distance), to avoid autofocus failure, you can orient the camera to the landscape position during focusing. After focusing, you can rotate the camera to a portrait position if desired.

Note that this chapter discusses how focus instead of on what focus. See the chapters on depth of field and hyperfocal distance for more information on this subject.

Many of my readers have complained about the poor performance of the autofocus in the camera. Let's take a quick look at how the autofocus system works in modern SLR cameras and, in general, the methods of focusing in difficult cases.

If you understand the logic of this system, then you will know how to "treat" such problems.

Currently, there are mainly two types of passive autofocus used in cameras. Contrast and Phase. More recently, their combinations have also appeared, when coarse focusing is done using the phase method (the fastest), and super-accurate using the contrast method.

Therefore, it will be nice to cover both methods, and at the same time we will figure out why LiveView you can adjust the focus perfectly even when we get a stable focus error in the viewfinder and autofocus also works with an error (front / back autofocus).

Firstly, almost all mirrorless cameras use contrast autofocus. Again, some of them have recently begun to be equipped with a faster phase method for determining focus.

The essence of the contrast method is related to its name, i.e. The camera determines whether an image is in focus by the position of the objective lens at which the maximum image contrast is achieved. In this case, the contrast is determined by the final image on the camera matrix or its sections (central, for example).
(What are these areas outside of our "depth" of the article)

LiveView mode

The picture shows a DSLR camera in LiveView mode, with the mirror up as we adjust focus across the screen. The same thing happens on a mirrorless camera, only in automatic mode.

On the one hand, since we adjust the focus according to the final image on the camera matrix, the accuracy is ideal, but on the other hand, in order to understand in which direction the image contrast increases when the lens is moved, and in which direction it falls, to us (the camera ) you have to move the objective lenses and compare the resulting images.

1 - lens
2 - the main mirror (in this case, in the raised position)
3 - camera shutter
4 - camera sensor

What does contrast autofocus look like?

The camera opens the shutter and takes a picture. From the picture, the camera cannot tell in which direction to move the lenses in order to obtain a more contrasting image, and, accordingly, a more accurate focus. Therefore, the camera simply moves the lenses in a certain direction, for example, forward. After that, it again reads the image and compares the contrast value of the image with the original one. If the contrast has dropped, then we are moving the lenses in the wrong direction. And the camera shifts the lenses in the opposite direction, further than they were at the very beginning at a certain distance (determined by the camera firmware). Again compares the picture - overshoot or undershoot?

There is a certain method of how to get to the right place, into focus with the help of the minimum number of such "shots". But we will not go deeper, since we do not need this on this moment... Anyone who wants to - can look for himself, I no longer remember the name of the method.

The sequence of steps in the contrast method for determining the correct focus is different for different manufacturers cameras. You can make large jumps and gradually decrease the range, catching maximum contrast (reminiscent of the dog's search technique), or you can walk through the entire focusing range in successive small steps until you cross the threshold beyond which the contrast begins to drop.

I suggest moving the sliders on this animation, courtesy of Stanford University

Unfortunately, you do not have a flash player installed.

But DSLRs mainly rely on the phase focusing method, which is much faster, so we'll move on to that.

The phase-detection autofocus method differs from the contrast method in that it allows, in one single measurement, to draw a conclusion where to move the objective lenses to achieve optimal focus.

Below is a diagram of phase detection autofocus. Many have seen the main mirror of the camera, which rises at the time of shooting and emits a popping sound, but does everyone know about the additional mirror, which ensures the operation of phase detection autofocus in SLR cameras?

What looks like a small match in the diagram attached to the middle of a large match (main mirror) is actually small mirror, which works due to a translucent window in the main mirror.

Where is this window located? Let's see.

In the sequel, you will learn how to adjust autofocus, what you can and shouldn't do.

(continued on next page)

Autofocus is one of the most rewarding achievements of modern photography. Majority modern systems video surveillance is impossible to imagine without autofocus. Learning to control this technology is one of the most important skills of any photographer.

What is autofocus?

For a start, it would be nice to answer another question. What is focus? In photography, this concept is central, it refers to an image with high definition, originality, some small details... Achieving accurate focus is what photographers usually strive for.

Having a camera in our hands as a system with perfect vision, we see the object of our interest - a display with perfect detail. Just as with poor vision, poor focus makes the world appear blurry. Fortunately, unlike our eyes, the focus of the lens can be adjusted to get the sharpness you want, however, this is not easy and not always possible. This is where autofocus comes to the rescue.

At its core, autofocus is any technology that automatically (without the intervention of the photographer) changes the focal length of the lens. This feature can be more accurate than eye control and manual focus, and can be used to improve focus on moving objects that our eyes and reflexes struggle to track.

Using autofocus

Most people are already familiar with autofocus. It exists on nearly all modern cameras, from cutting edge Hasselblads formats to regular smartphones, and is almost always set to focus by default. Simply put, no autofocus - no certainty about what you are doing.

Don't you find it odd that after buying a fancy DSLR, autofocus seems less flexible than it does on a phone? With smartphones, everything is simple, you press the button with your finger, you get a nice little picture, and everything that gets into the frame can be seen very clearly. What a nice trick.

This is the viewfinder screen of the D3100, which has an 11-point AF system. More advanced cameras are now working all the way up to the 61st AF point.

Looking at a DSLR, you think, what a hassle, being limited by the number of dots in the viewfinder! Without going into unnecessary details, let's say, DSLRs use a different method of autofocusing than digital cameras and smartphones, for which you don't particularly need to process what the lens sees.

This may seem like a disadvantage at first glance, but this AF mode is faster and more accurate. In this article, we will focus on Special attention autofocus system on digital SLR cameras instead of smartphones (who wanted to read about an iPhone, google it).

Now that we know we rely on fixed points, it's time to learn about two key issues. How do we pick the right moment and what happens if the focus doesn't stop on the object we want?

Autofocus vs. Manual focus

First, we need to see what mode is selected in the menu. Most of the modes belong to the so-called - "auto-scene modes", where the camera settings change depending on the type of shooting you choose. Naturally, these modes involve autofocus (there are, of course, exceptions, such as the macro mode).

For example, for a DSLR camera, the main mode is autofocus. When you press the shutter button, you kind of give a signal to highlight certain points on the video finder. These focus points are a reflection of how the camera sees the subject. If this is not what you were trying to film, then you are out of luck.

In order to independently control autofocus on a DSLR, you need to use one of the "manual" modes (P, A / AV, S / Tv or M). In these modes, the focus point can be manually selected. Focusing accuracy varies from model to model. But usually DSLRs are similar in this. Manual mode will help you if you want to take full control of your shooting.

Of course, you can do it differently, but most photographers follow this method. To focus on objects in the center, you need to be careful. This is the easiest way to get an image in focus and can be achieved in three steps.

Step 1.

Focus mode - One shot... Set the focus point to the center of the viewfinder. The AF midpoint will align with it and the image will come out much sharper.

Step 2.

The point should be directly on your subject, press the shutter button halfway for so-called pre-focus. Once this is done, your camera's AF LOCK will clearly "see" what you want to shoot, what the focal length is to the subject, and will remember this even if you move the camera.

Step 3.

We decided on the focal length, now you have complete freedom in the frame. Usually stationary subjects are pretty boring for a photo, but when you're happy with the composition, press the shutter button all the way.

We use the center button on AF to focus on the subject for the first time, then, after AF lock, we can create freely. This is called pre-focusing.

Select the focus point manually.

It is very rare that the focal point is exactly where you want it, even with the new 51-point systems. So if we have the ability to change the composition after pre-focusing, what's the point in additional points?

The first reason is that there may be times when it is physically impossible to change the composition. While the "focus and create" method is great for most situations, there are times when you need the most accurate focusing and no amount of "eye" is appropriate.

In such situations, having a flexible multi-point AF system becomes very useful.

The main purpose of such systems, however, is not at all to save time. Rather, the goal is to enable the photographer to capture moving subjects. This is especially important for wildlife photography and sports photographers, for them the ability to use autofocus correctly decides, as for everyone who shoots dynamic objects.

Let's say you want to take a photo of a child running. By the time you get the focus, the child will be long gone (forget about trying to change the composition after focusing first in this case).

Even with the very fast autofocus of modern systems, there is no way to take more than one frame at a time without changing the focal paradigm. How can you use a high frame rate to then select one of the sequential shots?

Most DSLR cameras support, in addition to the aforementioned one-shot autofocus function, a very powerful continuous autofocus function (AF-C in Nikon and AL Servo in Canon).

How does it work in general, that immediately after the system has been focused on the first frame, the movement of the object will be tracked, and moreover, automatic focus will be selected almost immediately!

This will continue as long as the shutter button is pressed halfway and held. During use, the camera will adjust the lens to maintain focus on the subject, predicting how the subject will use its speed.

This way, you can take a series of photos in quick succession without worrying about focus and maximize the likelihood of getting the best shot.

The tip that was most important to me when I was learning to shoot with autofocus. Since autofocus is done with sensors that detect it, it only works well when the focus point is with some kind of contrast!

For example, when I set the AF point to the edge of the subject, the focus is instantaneous and very precise. But if I try to point it towards the middle of an object where hue and color are consistent, the sensor cannot tell how sharply it sees it.

Think about it, the sensor only has information at its disposal to determine focus. It’s as if you were looking through a straw and trying to determine whether you have perfect vision or not. This is only possible when you can see the edges of objects, and not when there is only a white wall around.

To reuse the previously selected focus, you can see what happened when I tried to focus on two different points directly. The left image will be more accurate as there is a sharp contrast between the flash drive and the background. The right one will not be as accurate as the contrast is not so strong. (In general, the camera will not let you take a picture until the sensors are sure that focus has been found.)

Most DSLR cameras have an AF illuminator and can be turned on on some models. This helps to focus in the dark. If everything is black around, the camera faces the same problem as in tip # 1, the sensor has no idea what is in focus and what is not. Note, however, that you cannot use this mode in areas where flash photography is prohibited.

As it may seem, this is the solution to most problems, gave money - got an easy way to improve autofocus. Fast - that is, having a maximum aperture (lower f-number, for example, f / 1 / .8), that is, the lens has a larger aperture.

When the camera tries to autofocus, it always opens the aperture as much as possible to let in as much light as possible, according to the settings, of course. The greater the lens' maximum aperture potential, the easier it will be to autofocus.

Indeed, when using DSLRs low level with small apertures, such as f / 5.6 lenses, usually whale, autofocus will not work at any points other than the center, even pro-class cameras can only cope with lenses of great maximum aperture potential.

In the first decades of photography, cameras were large and represented a simple but cumbersome "accordion" structure connecting the lens and the cassette part to the photographic plate. Before shooting, in place of the photographic plate was inserted frosted glass(focusing screen), and the photographer manually moved the lens (usually a single lens) to focus the image, covered with a dark blanket to increase brightness and contrast. This process was not fast, but there was no particular hurry: the photosensitivity of photographic plates at that time was low, the exposure was minutes, so they shot mostly static scenes - landscapes, still lifes and portraits of people who had to sit still for this.

Handmade

By the beginning of the 20th century, the sensitivity of photographic materials increased, the format decreased, cameras became much more compact and more convenient, but it became difficult to focus the lens according to the image on a small focusing screen, even with a magnifying glass. This problem could be solved in several ways. First, focus the lens at hyperfocal distance, so that most of the objects in the frame are sharply displayed. Secondly, mark the distance scale on the lens and focus by setting desired values"approximately". And, thirdly, it was possible to apply a fundamentally new solution by equipping the cameras with a device for measuring distance - a rangefinder. This simple optical device consisted of a beam-splitting prism and a rotating mirror, separated by a certain distance (base). The photographer, looking through the window of the rangefinder, turned the mirror until the images were aligned. With the help of triangulation, based on the angle of rotation and the base, it was possible to find the distance to the subject and set this distance on the lens (manually). Cameras began to be equipped with such devices from the beginning of the 20th century, and in 1916, in the 3A Autographic Kodak Special, designers for the first time mechanically combined distance measurement with simultaneous focusing of the lens. This device gained real popularity thanks to the Leica company, which began to supply its cameras with rangefinders starting with the Leica I (1925) - in fact, such cameras came to be called rangefinders.

Remove bifurcation

At Photokina in 1976, Leica introduced a camera with Correfot (which it has been developing since 1960), the world's first autofocus system. According to one of the legends, despite the interest of the public, the company refused to release it, "because customers already know how to focus the lens correctly." In fact, the system was simply too power hungry (a set of six batteries lasted less than an hour of filming) and overall "raw". Therefore, the first mass-produced autofocus camera was in 1977 the Konica C 35 AF equipped with Honeywell's Visitronic system. This system was based on the classic rangefinder and triangulation, only two images were brought together not by the photographer himself, but by electromechanical automation, comparing signals from two CCD matrices.

Canon has taken a slightly different path, deciding to do without complex electromechanics. Canon AF35M (1977) introduced active autofocus, which was an optoelectronic version of the classic rangefinder: an LED emitted an infrared pulse, and the distance was determined by the angle of its reflection from an object measured with a CCD sensor. The next model, Canon AF35ML (1981), already used passive autofocusing, based on "solid-state triangulation": no moving parts, and the "mixing" of images was carried out electronically - by the difference between the signals on the two CCDs.

In the first rangefinder cameras, the photographer combined the images, read the distance and set the resulting value on the focusing scale of the lens. The 3A Autographic Kodak Special combines these procedures into one.

Phase shift

The first autofocus SLR camera was the Minolta Maxxum 7000 (1985). This model used the Through The Lens (TTL) phase-detection autofocus (AF) system, which is still widely used today. Its principle of operation is based on the fact that rays passing through two halves of the lens are reflected by a mirror and focused at two different points on the AF sensor - two CCD rulers. The distance between these points for perfect focusing is precisely known, and if the measured distance between the peaks does not coincide with this value, the control system begins to move the lens in the right direction until the peaks are in the right places. V real life, of course, everything is much more complicated - the image is not a point, it may not be located on the optical axis, etc. These problems are solved by introducing different masks and additional condenser lenses, but the principle is the same.

Automatic Rangefinders and True AF The Konica C35 AF was equipped with an electromechanical rangefinder with two CCD sensors. The signals from the sensors were compared, their coincidence meant precise focusing.

Phase detection autofocus is very fast (the system immediately knows in which direction to move the lens, and thanks to this it can even track the movement of an object in the frame), does not require a lot of computing power and has no moving parts. The main disadvantage of this system is its uncertain operation in low light, as well as the fact that it only works when the mirror is lowered: at the time of shooting, the mirror rises, and all the light through the lens falls on the film or matrix, and not on the AF sensor. This means that this system is not suitable for those cases when the frame is sighted on the LCD screen (LiveView), that is, for most compact digital cameras and smartphones.

And the first real AF appeared in the Minolta Maxxum 7000. It was a full-fledged phase-detection autofocus system through the lens (TTL) - the ancestor of all modern phase-detection AF systems.

In the image and likeness

For digital cameras, which have replaced film cameras since the early 2000s, a new principle of autofocus had to be invented. Well, not entirely new. How does a person aim the lens manually? Turns the focusing ring until the observed picture becomes sharp, that is, the maximum contrast. Contrast autofocus works in the same way: it moves the lens, achieving maximum contrast of the image on the light-sensitive matrix.

Such a system works with the main array and does not require complex optical circuits and additional sensors. But, unlike phase detection autofocus, it cannot determine in advance in which direction the lens should be moved, and begins to do it in a random direction - exactly as a person would do. Therefore, the focusing speed is sometimes poor - especially in conditions insufficient lighting or when shooting low-contrast subjects, when the system simply cannot "see" sharp details (just like a person). Nevertheless long time For compact digital cameras and especially smartphones, there were simply no alternatives to contrast autofocus.

The Canon EOS 70D is the first camera to feature a Dual Pixel CMOS AF system. Unlike the hybrid AF system, which uses dedicated dedicated photodiodes on a common CMOS sensor, Dual Pixel AF uses all of the sensor's photodiodes for both focusing and photography.

Hybrid approach

In 2010, Fujifilm released the FinePix F300EXR with a new hybrid autofocus system. On the camera matrix, in addition to the usual photosensitive photodiodes (pixels), two types of specialized ones were evenly scattered - "right" and "left", that is, they perceive light only from the right or left part of the lens (the other part is covered with an opaque mask). The AF system compared the image on the sub-arrays formed by the "left" and "right" pixels. Exact coincidence of the two images indicates accurate focus, and offset indicates how much and in which direction the lens should be shifted. Sounds like phase-detection AF, doesn't it? Almost, but not entirely: the resolution of the sub-matrices is significantly less than that of the entire matrix, and at very small deviations from accurate focusing, the system is unable to see the difference, so at the final stage, contrast focusing is used.

Nothing extra

Hybrid autofocus favorably combines the advantages of phase and contrast AF systems, but it also has disadvantages. To improve AF performance, you need to increase the number of pixels that “work” only by 50%, and this leads to a decrease in the overall light sensitivity of the matrix. But the matrix designers have come up with a clever way to get around this limitation.

In 2013, the Canon EOS 70D pioneered the Dual Pixel CMOS AF system. And in 2016, the first smartphone with a camera equipped with a Dual Pixel system appeared on the market - the flagship Samsung Galaxy S7.

There is a way to make it “sharp” without autofocusing at all. In the era of film cameras, cheap models were usually equipped with a simple focus-free lens at a hyperfocal distance. Such a lens allows you to more or less sharply depict all objects located at a distance from half of the hyperfocal (usually 0.5-1 m) to infinity. Cheap digital cameras and the first smartphones with cameras were equipped with similar lenses. However, this principle only applies to cheap wide angle lenses with a large minimum aperture. Another case is the use of a plenoptic camera, or "light field camera". It records not only the distribution of illumination in the focal plane, but also the direction of the incoming rays (light field). Such an image can later be "refocused" in any desired way (in any plane). The idea for such cameras was put forward in 1908, and a few years ago the Lytro company decided to produce digital versions, although they have not yet received much distribution.

Each pixel of the Dual Pixel sensor consists of two separate photodiodes - "right" and "left". Thus, during autofocusing, the entire sensor is divided into two sub-sensors, "right" and "left", with the same resolution as the main sensor. Comparison of signals from the two halves provides higher accuracy than hybrid ones, and the speed is much higher than that of contrast AF systems (for example, in the Samsung Galaxy S7, the focusing time is less than 0.2 s). Since Dual Pixel is phase-detection AF, it allows you to track the movement of a subject in the frame. And at the time of shooting, both sub-matrices work as a single whole, there is no drop in light sensitivity, which is important for smartphones with their small matrices. Therefore, such a system today represents the pinnacle of the evolution of AF systems. Of course, until engineers come up with something new again.

Sonars, radars and lidars

A separate branch on the evolutionary autofocusing tree is occupied by external (relative to the camera's optical system) rangefinders with direct distance measurement. One of the first cameras with an autofocus system was the Polaroid SX-70 Sonar OneStep (1978), equipped, as the name implies, with an ultrasonic sonar-based rangefinder. Archaic? Not at all, sonar rangefinders for cameras still exist. They are produced, for example, by the RedRockMicro company - though not for automatic, but for remote manual focusing of professional cameras. A newer principle for determining the distance, laser ranging, is now actively used not only in construction and military equipment, but also in some smartphones (LG G3) - in addition to conventional system contrast autofocus. Sony's patents mention radar autofocus, but there are no production samples of this type on the market.

The editors would like to thank Markus Kohlpayntner for help in preparing this article.

Let's start with what autofocus is. This is a system that provides automatic focusing of a camera lens, camcorder on an object (or several objects) of shooting. Autofocus is most often referred to as AF.

There are two modes of autofocus operation: passive and active... The point is that the system needs to determine the distance from the focal plane to the subject of shooting, and active autofocus achieves this due to elements interacting with the subject (ultrasonic or infrared locators), and the passive one does not interact with the object itself and does not emit anything - it only analyzes the light beams entering the camera.

Autofocus performs all its work in a matter of moments and practically without the direct participation of the photographer himself. This device is provided in all modern cameras and differs in its type. As a rule, the following types are distinguished:

• Phase detection autofocus
• Contrast autofocus
• Hybrid autofocus

Let's consider each of them in more detail. Work phase detection autofocus is based on the use of special sensors that collect light rays from scattered fragments that come to them from different points of the frame thanks to mirror systems (in some devices they are replaced by lenses). After that, all the light is split into two streams and sent to the light sensor. The final aiming occurs at a certain moment when the separated beams are at a distance specified by the sensor device. After calculating the required distance, the device itself determines how to change the position of the lenses in order to obtain an image best quality... The indisputable advantages of phase-type autofocus can be safely attributed to the accuracy and speed of focusing, which is especially important if you are shooting a moving scene. A large number of sensors literally follow the image for maximum quality. Phase-detection AF is used in SLR systems.

The next type of focusing is contrast autofocus ... His work is based on special photosensitive elements that conduct research on the contrast of the scene being filmed. Precise focusing occurs at the moment when the given image acquires the sharpness and contrast that is most different from the background. To achieve the best result, the microprocessor of such devices can move the lenses from their original position. The advantages of this type of autofocus include simplicity, rather small dimensions and the absence of the need for any additional sensors. Due to the peculiarities of this system, it is used in "soap dishes", cameras of modern smartphones, etc.

Another view that deserves the attention of a photographer is hybrid autofocus... The original thought was to combine passive and active AF. Modern developments in hybrid autofocus are based on a combination of phase and contrast technology. This type of autofocus is being implemented today in mirrorless systems, where such AF shows more convincing results than contrast AF, which was used before.

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