Many of my readers have complained about the poor performance of the autofocus in the camera. Let's take a 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 would 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 the 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 get a more contrasting image, and, accordingly, a more accurate focus. Therefore, the camera simply moves the lens 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 - overflight 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 at the moment. 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 camera manufacturers. You can make large jumps and gradually decrease the range, catching the maximum contrast (similar to the method of searching for a dog), 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 mostly 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 the phase detection autofocus. Many people have seen the main mirror of the camera, which rises at the moment of shooting and makes a popping sound, but does everyone know about the additional mirror, which provides 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 a small mirror that works through 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)
