Unraveling Auto Focus: Cameras and Math

[Dilbert]

hello,

i am not sure that this is the correct forum to post in, but i am confident that the moderators can swiftly correct my error if that is the case.

I was wondering a little about cameras and their auto foucs. How does it work and what is the math behind it?

 

[matt grime]

the camera divides the picture it, erm, sees, up into different regions, the shape of this initial division varies from camera to camera, and some have several different divisions that it will use depending on if you're taking a landscape shot or a portrait. a portrait shot will put more emphasis no hte central area. now in each area it further subdivides into tiny little areas and then tries to work out what are edges. if it's in focus then the edges are sharp which basically means that the colours in two closely position areas will be very different each side of the edge. if it were out of focuse the drop off, or change in colours etc would be less sharp. it then adds these differences up (i don't know wxactly how it assigns numbers, it will differ in different cameras) and then weights it for the larger area it's in - more weighting for the centre) and then if the number it comes up with is small enough it decides if it's in focus enough. that is roughly how it works, i think.

 

[Dilbert]

so, it is basically edge detection?
Then i have solved the math already, i just thought it was more complex.

Thanks Matt.

 

[mewmew]

Actually, there are two types of auto focus, one is active, the other is passive. Active sends out an infrared beam of light and it bounces back to the camera and then gets measured to determine how far away things are. Passive is like described above, it is a pretty simple idea, but making it work is not as easy as it sounds.

 

[chroot]

Yes, auto-focus just applies a digital edge-detection filter to the incoming images, and adjusts the focus until the edges are as sharp as possible. Alternatively, the processor may just look at a histogram of the incoming images and adjust the focus to maximize the spikes on the histogram.

- Warren

 

[Dilbert]

thanks Warren.

Obviously having a histogram would contribute a lot.

 

[Dilbert]

might be a stupid question but, there are several edge detection techniques. Sobel Edge Detection for instance.

What kind of edge detection does the cameras use ? i guess that it can vary from the different brands but is there any mainstream edge detection filter? In that case, which?