Today we’d like to cover something that is believed to be the most difficult thing in photography both for beginners and professionals. It’s a huge topic which causes a lot of questions that are followed by shoulder-shrugging and eyebrow-raising instead of clear answers. The name of this thing is autofocus and everything that it involves.
Since there’s so much to talk about on this theme, we decided to break it into several parts. This one will feature information about the way autofocus functions, autofocus types, and autofocus points.
Having your subject—whether it’s a person or a building—in perfect focus is crucial to making a good photograph. We are lucky because the times of manual focus have long passed and now literally the only thing that you have to do to make your camera focus is to press the shutter button.
However, the way the camera actually focuses remains to be a mystery for most photographers. Let us explain you how it works.
There are two AF types—active autofocus and passive autofocus—that exist in the world of point-and-shoot and DSLR cameras.
The first one, active autofocus, is a relatively outdated AF type that uses either infrared light or ultrasonic sound waves in order to find out the exact (at least more or less exact) distance between the camera and the subject by emitting them from the camera. These waves or light are then reflected back to the camera and the time during which these waves travel between the camera and the subject and back is used for the distance calculation.
This AF method works really well in low-light conditions, but it won’t work if your subject is located too far away from the camera or behind a glassy surface. This is the reason why this AF type is not widely-used nowadays.
Unlike active autofocus, passive AF doesn’t use any physical energies in order to find out the distance between the subject and the camera. Instead of this, it uses contrast. Let us expound on this.
There are two kinds of passive autofocus, Contrast Detection and Phase Detection, and both of them use contrast on a certain part of the image. If this zone on the image has enough contrast, then the camera will assess it to understand whether it’s blurry or not and then refocus automatically.
In case of Contrast Detection passive AF, the camera uses the sensor in order to detect contrast, whilst in case of Phase Detection, the camera detects contrast using light that enters the lens and falls on the sensor.
Active AF is the most widely-used AF technology nowadays and this is exactly the reason why your camera cannot autofocus properly when there is not enough light. Not enough light means that there’s not enough contrast. Also, this is why AF assist beam—which illuminates everything in front of your camera—helps so much in getting your subject in focus.
Every DSLR has a certain number of focus points and these focus points can be of two types: vertical and cross-type. You can see focus points as small dots/squares through your viewfinder when you are shooting in the automatic mode or switching between different AF area modes.
Focus points are used by your camera in order to automatically select the area that should be in focus as well as to track moving subjects and keep them constantly in focus.
Cheaper entry-level DSLRs usually have fewer focus points and almost no cross-type focus points. Usually, entry-level cameras have 9 to 39 focus points and one to nine of them are cross-type. Professional DSLRs may have 51 focus points or more and all of them may be cross-type.
Well, we think that it’d be a good moment to explain your the difference between the two. As one can understand from the name, vertical focus points detect contrast only vertically because they are one dimensional. Cross-type focus points are two dimensional and they detect contrast both vertically and horizontally, ensuring higher precision in focusing.
We are sure that by now you have a question about focus points. Are they really so important? Yes, they are. The number of focus points in your camera is one of the most important things that you should look at if you are seeking perfection in focusing.
In our next article on autofocus we will cover all most widely-used autofocus modes and explain you which one you should use in different situations. Good luck!
We hope that this article was of help to you and we’ll be glad to answer your questions in comments. Feel free to shoot us a message!
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