How does AF work?


TPF Noob!
Jul 14, 2003
Reaction score
Reading UK
As technically orientated as I am, I’ve often wondered how auto focus on a camera actually works. Three theories are that

1. a beam, like radar, is bounced off the subject with the return time depicting the distance and therefore focus position.

2. a rather small computer of sorts looks at the image and says “hmm not in focus, let me twiddle this knob and see…”

3. An invisible measuring stick is protruded out of your camera by leprechauns (small ones) who then twiddle the knobs.

Please help me…
I would think that number 1 is the most nearest to being accurate, but maybe you can sneak up on those leprechauns when they are not looking.
Modern autofocus SLRs use a system of autofocus called "phase detection." I'd try to explain it myself, but I think would help you more than I could here. Please read it before continuing on in this post. Digital SLRs use the same system, since most of them are actually based on film bodies.

"Prosumer" digital cameras (and most below them) use a system called "contrast detection." Basically, the camera examines areas of the CCD for contrast, and adjusts the lens to bring the picture into focus, as far as it can tell. The problem with this is that it's not very exact, and the camera has no way of telling how out of focus the image is, so it may have to hunt more often (especially in low light). With phase detection autofocus, the camera knows exactly how out of phase the two parts of the system are, and can adjust the focus quickly and accurately.

Both of the two previous systems are referred to as "passive" since the camera is only analyzing internal information, and not actively emitting any sort of focusing aid. Then we get onto the "active" systems... You're most likely to see this sort of system on point and shoot autofocus film cameras. Generally focusing is accomplished through sending out an infrared beam to measure the distance to the subject. This works for most people who stick their subject in the middle of the frame and snap away. However, it's terrible for anyone doing serious photography. It's not very exact, because the camera can quite easily focus on something you didn't intend it to (ie an object in the background). It wouldn't go down well with SLR users who like precise control over the focusing point! It's a really cheap system to implement, and doesn't require any input from the user, so you see it in a lot of these film cameras, as I was saying.

Most reactions