Contrast autofocus in mirrorless cameras. Single point focusing area

Autofocus, including tracking, in comparison with manual focusing, is a preferable setting. In the hands of an experienced photographer, autofocus works much faster, and most importantly, more accurately than the average amateur photographer can do. However, autofocus is not such a simple thing as it might seem at first glance, but it should also be said that this is already a fairly deep jungle, into which they do not immediately enter.

Let's look at how to use autofocus, as well as the subtleties that will allow you to effectively control it and get the best possible result.

To get started, I recommend that you reread the autofocus pages of your camera's manual for a great start. For a normal perception of the material, you need to know and understand where and how the main controls for autofocus and focus points are located.

Autofocus modes

Most cameras, including Canon and Nikon, are equipped with three types of autofocus modes: single, tracking and hybrid autofocus.

Single or spot autofocus

This mode is intended for photographing still scenes such as portrait, landscape, still life, etc. In this mode, when you half-press the shutter button, the lens begins to focus on the subject that is located within the selected focus point. After focusing is complete, the focusing operation is locked, allowing you to re-compose the frame (when the distance to the subject changes, focus is lost) and take the frame.

Understanding this topic, you need to understand that the lens focuses not on the subject, but at a certain distance. A very simple conclusion follows from this. If I focus on an object that is at a distance of five meters, then, accordingly, all other objects at a distance of five meters fall into the field of focus. In other words, after focusing and locking the autofocus, keeping the distance to the subject, I can turn the camera as I like, getting the composition that I need, while not losing focus.

This method is very good in cases where the distance to the object is large enough and measured in meters. At close distances, especially in macro photography, recomposition of the frame, which can lead to a change in distance by several centimeters or millimeters, leads to noticeable loss focus, which is even more critical when narrow depth sharpness.

AF Tracking Mode

This mode is also called continuous, it is simply indispensable when photographing dynamic objects - animals, sports, auto racing, etc. As long as the shutter button is half-pressed, autofocus works continuously, thereby keeping the subject in the field of focus. Naturally, autofocus locks, as in previous version does not occur due to the constant movement of the lens mechanism, which catches focus following the movement of the subject.

An obvious conclusion suggests itself. In this shooting mode, you cannot change the composition. This is because autofocus reacts to a subject in front of a certain focus point, if you change the composition, the subject will be lost and focus will be made on something else.

So, we understood what tracking autofocus is, now there is the last mode - automatic or hybrid. It was created in order to decide for yourself whether you need to block autofocus or not. This mode, unlike other automatic camera functions, seems strange and useless to me, but this is my subjective opinion, perhaps someone will find usefulness and convenience in it.

Release or Focus Priority

For many newbies, the existence of such settings will seem surprising. But they exist and it is simply not possible not to pay attention to them when analyzing autofocus. The first of the priorities (release) means that the moment the shutter button is fully pressed, the sharpness does not matter, in other words, the control over this is completely assigned to the photographer. This mode is enabled by default for tracking autofocus.

Focus priority, means that after pressing the shutter button completely, there should be a focus on specific or specific focus points. If this is not the case, the camera will not allow you to take a picture. This mode is usually enabled with standard settings in single shot mode.

Contrast and phase detection autofocus, which is better?

In digital cameras, manufacturers use two types of autofocus, as you already understood from the name, these are contrast and phase autofocus. It will be very good if we understand these concepts.

Contrast autofocus system

This autofocus method is used in so-called digital cameras and DSLR cameras, but only when the Live View function is enabled. This type of autofocus does not require additional focusing sensors, since it uses exclusively the camera's matrix for focusing. The picture, which comes from the camera matrix, is analyzed by the camera processor for the presence of contrast changes. If more precise focusing is needed, the processor instructs the motor to slightly change the position of the objective lenses in any direction. If, after this manipulation, the image contrast decreases, then the direction of movement of the lenses is reversed. Movement in the right direction continues until the contrast begins to fall again, reaching this limit, the processor tells the motor to return the lens to the step at which the contrast was maximum. After reaching this value, focusing is considered complete.

As you understand, due to such peculiarities of autofocus operation (it is not known in which direction the motor should be turned), a lot of unnecessary movements are made. That leads to the main disadvantages of this focusing method - low speed, which does not allow using it on professional cameras. The second minus, maybe not so critical, is the increased power consumption.

Pluses this method, is the simplicity of design and the ability to focus almost anywhere in the frame.

Phase detection autofocus

As you can imagine, manufacturers of photographic equipment have long since answered the question of which autofocus to choose. Of course, the phase system won. Let's see why this is so.

This type of autofocus is used in digital SLR and film cameras. There is little interference in the optical image transmission system, so in addition to the main mirror, the camera is equipped with an additional mirror, which transmits part of the light to the module phase detection autofocus... Any light beam that passes through the beam splitting prism and microlens is split into two beams, each of which is then directed to the AF sensor. If focusing is accurate, then the beams fall on the sensor at a strict distance from each other.

If the distance between the beams is less than the reference, then the focus is brought closer than necessary (front focus).

If the distance is greater, then the focus is accordingly farther (back focus).

In this case, the amount of shift indicates the distance from the ideal focus. Thus, the processor immediately receives information about in which direction and how much it is necessary to adjust.

Phase-detection autofocus sensors can be cross-shaped and linear. Linear are also divided into vertical and horizontal. The latter are sensitive to vertical lines (tree trunks, fence), while vertical ones are more sensitive to horizontal objects (horizon, road). Accordingly, cross-shaped sensors are versatile, they are sensitive to objects of any shape. You can find out exactly which sensors and where they are located from the instructions for your camera, but the most sensitive sensor is located in the center of the frame.

The main positive quality phase detection autofocus is its speed, which makes it indispensable for shooting action scenes. The main disadvantage is the complexity, cumbersomeness, the need for the finest adjustment of all components, less accuracy than contrast autofocus and a small number of focusing points.

Hybrid system

There is also a third type of autofocus - hybrid. From the name it becomes clear that this is a combined system that includes the qualities of both systems described above. Such autofocus is used in mirrorless and some SLR cameras.

The essence of this system is that the sensors are mounted directly into the matrix. Due to this, focusing occurs according to the following system. First, the basic focus focuses on phase detection autofocus, and then based on the obtained data, contrast starts to work. At the same time, the slowness of contrast autofocus is reduced by more than 75%. Phase detection autofocus does not require such precise alignment, and the entire system takes up much less space.

Having analyzed all the technical aspects of autofocus operation, I would like to conclude with a few words about those factors that also affect its operation:

Lens aperture. Here everything is as always, the higher the aperture, the better the autofocus works.

Focal length. Everything is quite confusing here, but in a nutshell, the larger the focal point, the more visible any autofocus miss. In addition, there is movement, breathing and other factors. I draw a conclusion that the longer the focal length, the worse the autofocus works.

Detail and illumination of the image. The less detail and light in the focus point area, the worse the autofocus performance.

The photographer himself, or rather your ability to use your technique. If a person speaks skillfully, then everything works, if carelessly and incorrectly, then even the most modern autofocus system will miss.

The most important thing in your autofocus work is practice. The correct balanced approach to the work of automation will give you the opportunity to focus quickly and efficiently. What is actually necessary.

Successful photos!

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, and the active autofocus achieves this due to the 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 undeniable 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 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 shot. Accurate focusing occurs at the moment when the given image acquires sharpness and contrast as much as possible 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 no 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 now being implemented in mirrorless systems, where such AF shows more convincing results than contrast AF, which was used before.

Materials

Autofocus is one of the most rewarding achievements of modern photography. Most 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. As with poor eyesight, 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. Here 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 almost all modern cameras, from advanced 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 strange 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 is very clear. What a nice trick.

This is the viewfinder screen of the D3100, which has an 11-point autofocus system. More advanced cameras currently work up to 61 AF points.

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, which do not particularly need to process what the lens sees.

This may seem like a drawback at first glance, but this autofocus 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 the iPhone, google it).

Now that we know that 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, in 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 the objects in the center, you must 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 are happy with the composition, press the shutter button all the way.

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

Select the focus point manually.

It is very rare to have a focal point 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 well 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 instant 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 the information it uses 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 allow you to 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 this mode cannot be activated 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 more the lens has the maximum aperture potential, the easier the autofocusing process will be.

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 1970, Leica made a small revolution in photographic technology by inventing the system auto focus lens to the subject. Over the years, we have become so accustomed to this invention that we take it for granted and are perplexed not to find it in the gadget. To date, two systems have become widespread - contrastingbased on measuring the contrast of the image and phasecomparing the antiphase portions of the point-forming beam. And more recently, literally before our eyes, appeared new system autofocus - hybrid, combining the speed of phase detection autofocus with the precision of contrast detection (as claimed by Samsung's advertising slogan).

Contrast autofocus.

The principle of operation is based on the microprocessor calculating the greatest contrast between the details of the image on the matrix. Next, the program forces the lens to move back and forth until the maximum contrast is found (maximum difference in brightness). Similarly, we focus manually.

The disadvantages of this system are low speed, impossibility of tracking focusing, high accuracy... After all, the lens unit will first have to go through the maximum point, and then go back, and, possibly, repeat the action.

Pros - cheapness, no complicated parts and the need to adjust the optical system, independence from the lens aperture, the ability to use in any system: compact cameras, mirrorless cameras and video cameras.

Phase autofocus.

I think that I will not give here a very complex mechanical and optical phase detection autofocus scheme, sending those interested to the depths of the Internet (here, for example, is a good start). I will only note that the phase detection autofocus system requires special sensors that calculate phase difference luminous flux separated by special mirrors. The first devices had only one such sensor - horizontal, further progress made it cross (actually combining two sensors - horizontal and vertical), then high-precision, then the number of sensors began to increase.

Dual cross sensor

Even entry-level DSLRs today boast 9-11 cross-type sensors, and in professional models their number reaches 60.

The main disadvantage of the phase detection autofocus system is its complexity, the need for precise alignment and adjustment, including software, and hence the price.

Pros - maximum speed, since the magnitude and direction of movement of the lens are immediately known. Thanks to numerous sensors and a powerful processor, it is possible to track the subject and even predict its movement in the frame.

Hybrid autofocus.

Recently, an interesting shooting mode has appeared in many DSLR cameras - LiveView, which allows you to take pictures or conduct video, observing the picture on the monitor in real time. At the same time, the mirror is raised, so only contrast autofocus can be used. A mixed autofocus mode is also possible - when you half-press the shutter button, phase mode is activated, and after focusing the camera switches back to LiveView mode. It is clear that such trade-offs make designers come up with more interesting solutions.

In some modern devices - both SLR (for example, Canon 650D, Canon 70D) and mirrorless (Nikon 1, Samsung NX300), engineers managed to combine the "phase" focusing system with "contrast" - sensors phase detection embedded right into the matrix.

Such a "pseudo" phase system works less accurately and quickly than the real one, and on this, apparently, its minuses end, and pluses begin. Relative "simplicity" of design - no need for complex optical and mechanical schemes... All the work falls on the shoulders of the matrix and the processor, and its power grows, we all know at what speed, so the price of this solution will only decrease ..

One of the unobvious advantages of hybrid autofocus is the lack of front and back focus of the lens, since focusing occurs directly on the matrix.

Moreover, it is very likely that the main forces of engineers will be thrown into the development of a hybrid method of focusing in the next 10-15 years, and maybe less. If the forecast is correct, then in fact it means rejection of the mirror apparatus as a class.

Date of publication:04.09.2015

Sharpness is one of the most important components of high-quality photography. Sharp enough, the shot can convey the scene in great detail and detail.

Focusing is primarily responsible for the sharpness of the photo. Today we'll talk about what it is and how modern cameras work with it.

A bit of theory and history

The lens does not focus on a specific object, but at a specific distance. A lens, like any optical device (for example, a projector, binoculars, microscope, magnifying glass), can be focused only at a certain distance. And only objects that are at this distance will be sharp in the frame. Some lenses even have a special scale that shows the focusing distance in meters. During focusing, the lens unit moves back and forth in the lens, just like we move an ordinary magnifier, looking at small objects: the magnifying glass will show them sharp only when it is at the required distance from them.

When focusing, we adjust the lens to a certain focusing distance.

Nikon D810 / Nikon 85mm f / 1.4D AF Nikkor

An error with this parameter threatens that the main subject of the picture will be out of focus.

An interesting consequence from the previous point: if there are several objects in the frame that are located at different distances, then it will simply not be possible to focus on all of them. But there is a solution: fit all objects in depth of field. We wrote about how to work with it in separate lessons. Note that on devices with a very small sensor (for example, smartphones or compact cameras), the depth of field will be very large. That is why it is easy for such devices to make a frame where both the foreground and background will be sharp. But for the same reason, it is almost impossible to blur the background in the picture with them.

In the past, photographers focused their lens on their own. Today, the manual focus function is retained in almost every camera. And in mirror photography it is always present. The downside to manual focusing is that it takes a long time to get accurate focusing. And if your subject also moves, then manual focusing becomes a real test of the photographer's nerves, coordination and vision. Since the 80s of the last century, autofocus systems began to develop. Then Nikon introduced its first camera with autofocus - Nikon F3AF.

Since then, autofocus-enabled cameras have supplanted more simple modelsdeprived of it. Today, cameras without automatic focus are practically not produced.

We can say that these days autofocus has become an integral part of modern cameras. Auto focus systems are improving every year, becoming faster, more sensitive and more flexible in work.

How does autofocus work?

An automatic focusing system is a complex of sensors and mechanisms. The device needs to evaluate the future frame, understand at what distance you need to focus, and then also move the lens unit in the lens accordingly so that it projects onto the sensor sharp image.

According to the principle of operation, there are two main types of autofocus systems.

Phase focusing

Time-tested type of auto focus. This type of autofocus is essential for SLR cameras. We know that the key element of a DSLR is the mirror itself. Thanks to him, we can see the image obtained directly through the lens of the device. But the functions of the mirror do not end there. And by the way, there is more than one mirror in a cell: there is a whole system of mirrors. It is designed in such a way that part of the reflected light is sent to the viewfinder, and part of it falls on a special module on which the sensors are installed. A modern autofocus module can contain dozens of such sensors. Manufacturers try to position the sensors so that they cover the maximum possible area of \u200b\u200bthe frame, so that the photographer can focus on any fragment of the future photograph.

To the photographer, these small sensors appear as focus points in the viewfinder. I think they are familiar to everyone. The photographer is free to choose the desired point on his own (read "a separate sensor on the focusing module"), or can entrust this choice to the automatics of the device.

To analyze the image, each sensor is equipped with its own miniature matrix 1 pixel wide and several tens of pixels long. However, some sensors are equipped with two such matrices installed in a cross. Cross-type sensors are more sensitive, so they are placed in key locations with conventional sensors around them. For example, a cross-type sensor is almost always located in the center of the frame. Photographers know that the center AF point is the most tenacious and sensitive.

The autofocus system comes into play when you press the shutter button halfway. Also on some devices there is a special button for activating autofocus. The focusing module tells the camera how far to focus the lens in order to get a sharp image at the selected point. For this, a special motor is started, which moves the objective lenses, aiming at focus.

Now the camera remains to focus the lens, and when this happens, you can take a picture.

Pros of phase focusing type:

• Work speed. This type of focusing is the fastest to date. Note that the speed of the entire autofocus system will depend on other factors (for example, the speed of the focusing drive in the lens).
• High sensitivity. Phase focusing sensors can work even in very poor lighting conditions.

Nikon D810 / Nikon AF-S 50mm f / 1.4G Nikkor

Weak evening lighting did not prevent me from quickly focusing where I wanted.

• High precision and fast tracking autofocus. Thanks to sensitive sensors and advanced electronics, modern devices in the tracking focus mode allow not to lose focus even very fast moving objects, tracking them across the entire field of the frame.

Nikon D810 / Nikon 70-200mm f / 4G ED AF-S VR Nikkor

Due to its high operating speed, phase-detection autofocus is ideal for capturing action scenes, including those involving children and animals.

Cons of phase focusing type:

• The ability to work only through the optical viewfinder. After all, it is only when the camera mirror is down that light enters both the viewfinder and the focus sensors.
• The second point follows from the first point: the impossibility of using phase autofocus at the time of video recording.
• Due to the complexity of the entire system, phase-type focusing can suffer from back and front focus. In this case, the camera will systematically focus a little further, or slightly in front of the subject. The bottom line is the same: the object itself, on which the camera focused, will end up being a little out of focus. The problem of back and front focus is solved by setting the equipment in service center... In the case of advanced cameras (starting with Nikon D7200), you can adjust the focus yourself right in the camera's menu.
• Incomplete coverage of the frame area by focusing sensors. You've probably noticed that all the focus points are usually located closer to the center of the frame, while there are none at the edge. It's connected with design features the entire phase focusing system. Here general pattern is simple: the more advanced the camera you hold in your hands, the more focus sensors are installed in it, and the more big square the frame is covered with them. However, it should be said that some professionals often use a single central focus point at all and almost never use others. After all, the central focus point is the most sensitive, and after focusing in the center of the frame, the picture can always be recomposed.

Contrast focusing type

This type of focusing is simpler: it does not require a special separate module and a system of mirrors, because the "focusing sensor" is the camera matrix itself. The camera electronics analyzes the image received by the matrix and evaluates its contrast at the selected point. If the contrast is not maximum, she tries to refocus the lens so that the contrast is increased. So the automation gradually achieves the maximum detail of the picture at the selected point.

In modern SLR cameras, this type of focus is used when working in Live View mode. In mirrorless cameras, it is the main one.

Pros of contrast focusing type:

• Simplicity and reliability of the design. To implement this type of focusing, additional sensors, mirrors and other things are not required. It is thanks to this simple design that mirrorless devices, which use only contrast autofocus, are so compact: they removed the mirror system and the phase focusing module. In addition, since the device is guided not by the readings of a separately located focusing module, but directly by the camera matrix, with contrast focusing, cases of back and front focus are excluded.
• You can focus over the entire field of the frame, not just within the existing focus points. Phase detection autofocus modules often have the sin that all of them (albeit numerous) sensors are located exactly in the center of the frame. This prevents focusing at the edge of the picture - there simply are no focus points. Contrast focusing does not know such problems. Here we can choose any place on the plane of the future image for focusing (even from the very edge), because it is possible to use any area of \u200b\u200bthe camera matrix in focusing.

But here it is worth making a reservation: in mirrorless cameras, the contrast autofocus function is implemented in such a way that the choice of focusing points is somewhat limited, they do not cover absolutely the entire area of \u200b\u200bthe frame.

• The ability to implement additional focusing functions such as face detection.
• The ability to use autofocus at the time of video recording. However, this feature is not yet available in all devices equipped with contrast focusing.
• Theoretically high focusing accuracy. The very principle of contrast focusing allows you to achieve perfect focusing quality. However, there are a number of reservations regarding its accuracy, depending on the implementation of the system in specific devices. We will talk about this in the "cons".

Cons of contrast focusing type:

• Slow work speed. Surely all DSLR owners have noticed that the camera focuses more slowly in Live View. It's all to blame for the contrast autofocus used in this mode. Let with each subsequent generation of cameras the speed of the phase detection autofocus increases, so far, according to this criterion, it is inferior to the phase type.
• Demanding lighting. The speed of contrast focusing will drop even more if you shoot in low light.
• Practical implementation contrast autofocus is not always ideal. We have already said that the practical implementation of contrast autofocus in various cameras does not always reveal its theoretical merits. And here's another feature: some camera models offer the photographer to focus not on a specific point in the image, but on a large frame that moves along the plane of the frame. Many different details can fit inside it, which means that focusing errors are possible: who knows what exactly inside this frame the camera will want to focus on? .. Therefore, inexpensive mirrorless cameras (in which this situation happens) is not very convenient to use with high-aperture optics. First of all, they are designed to work with universal kit lenses that do not have a high aperture ratio. Therefore, having an inexpensive mirrorless camera, do not rush to complement the kit with aperture portrait optics: it is quite possible that most of your shots will not be quite sharp. On the other hand, if the camera allows you to focus on a very small area of \u200b\u200bthe frame, on the contrary, you can achieve accurate focusing when working with high-aperture optics.