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How does auto focus work on modern cameras? How accurate is it?

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3 Answers 3

up vote 17 down vote accepted

An AF system essentially consists of a sensor system that is linked (via the camera's processor) to the AF motor, which will either be in the lens or the camera body depending on the model.

There are 2 kinds of autofocus. Active AF uses methods such as ultrasonics or infrared to measure the distance between the camera and the subject. A pulse is emitted from the camera, bounces off the subject, and returns. The time this takes is calculated in-camera and used to determine the distance. This kind of AF is independent of the lens/mirror system of the camera.

Passive AF analyses the image in the viewfinder instead. 2 methods are used in Passive AF. The first is Phase Detection. Here the image is split into two in the camera and the different phases of the two images are analysed. It achieves range-finding by essentially comparing how the two images diverge on the sensor. This is the system most modern DSLRs use, as it is the most accurate.

The second passive AF system is Contrast Detection. This is most commonly used in video cameras, and in DSLRs when in live view mode (essentially the same as a video camera). It works by analysing the contrast between pixels; the better the image is in focus, the greater the difference in intensity between pixels. So the camera checks intensity, focuses a little, checks again, etc., until it achieves a focus that gives an acceptable (preprogrammed) intensity difference. There is no actual range-finding going on. Contrast Detection is generally slower and less accurate than Phase Detection.

As for accuracy, generally, if used correctly, AF systems are very accurate (Passive Phase Detection being the most accurate). However, they often have problems in low-light (hence the AF lamp that comes on when you try and focus in the dark). The user also has to make sure that they are focusing on the correct point (e.g. focusing on the subject not the background).

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how can the "Active" system be independent of the lens system? At least you need the lens' focus distance to be read by the camera and compared with the measured distance. Thus, a passive lens is not appropriate for such system. Am I right? –  ysap Jan 13 '11 at 16:25
Because Active AF sends an ultrasonic signal from the body and uses the time it takes to return to the camera to calculate the distance. It then passes that distance to the lens in order to focus it. By 'independent from the lens system' I mean it doesn't use the image coming through the lens to focus. –  ElendilTheTall Feb 11 '11 at 16:24
Do any modern cameras use the Active approach? –  mattdm Jun 11 '13 at 12:14
Also, I question the idea that contrast detect is less accurate. –  mattdm Jun 11 '13 at 15:32
Agree with @mattdm: Phase detection is generally faster, but less accurate than contrast detect. At least according to the lensrentals Canon tests: Contrast detection was just as accurate as careful manual focusing. Phase detect was accurate when it hit, but way off in 10-30% of the cases. A few combinations of the latest lenses (300mm f/2.8) and latest bodies (5D Mk III, but not Mk II) were just as accurate using phase detect. In either case, contrast AF was the gold standard in accuracy. –  j-g-faustus Jun 11 '13 at 17:03

Here's the best summary I've found on the subject. It is written by Roger Cicala, the CEO of His blog is one of my favorites.

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Nice reading. I've visited this site for couple of times recently and it seems it has some interesting informations. I'll keep an eye on it now on :) –  Petr Újezdský Jan 29 '13 at 20:16

Here is a good write-up on all things auto-focus. For a very complete review have a read.

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Good to see you moving from questioning to answering. :) But in general, I think pointers to wikipedia articles are better as comments rather than answers. (If you summarize a tl;dr wikipedia article in a helpful way, that's a different story.) –  mattdm Jan 13 '11 at 15:13
@mattdm: Thanks. Answering is another way or learning for me. don't get me wrong, asking has been good for me as well. The problem with comments is I don't see a way to add links. With answers there is built in functionality for links. –  kacalapy Jan 13 '11 at 15:51
You can just type a URL in a comment and it gets linked. Not as pretty but it works. –  mattdm Jan 13 '11 at 15:54

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