Re: How Does Canon Auto Focus Work?

Mark² wrote:
Graham Fountain wrote:
Burt wrote:
I was just thinking, and I realised I actually have no idea how auto
focus works on my Canon DSLR.

No infrared. Anyone know how it works?


What do you mean about Canon and Auto-Focus? Canon DSLR's dont have
autofocus. When people talk about AF on a canon it means "Almost
Focussed" or "Approximate Focus".
Actually when the poor design considerations of canon's AF system are
taken into account, it's quite remarkable they do as well as they do.
Probably most startling is that they use no feedback to ensure that
the lens actually did move to the correct focus position.

Another
oddity is that they don't even try to focus the image, but just try
to get the lens within a DOF field range based on an 8x10 print.

Got a credible source for this idiotic statement?
I guess it was a bit idiotic - I should have double checked before I posted instead of relying on my memory - it isn't 8x10 at all, it is actually 6x9.
http://www.fredmiranda.com/forum/topic2/241524
to quote the relevant bit:
"When the camera determines how far and in what direction the lens must move to cancel the phase difference, it does so within a tolerance of "within the depth of focus" of lenses slower than f2.8 (down to f5.6) or "within 1/3 of the depth of focus" of lenses f2.8 and faster. The depth of focus is the range at the sensor plane within which the image of a point will be reproduced as a blur smaller than the manufacturer's designated "circle of confusion" (CoC). Canon's designated circle of confusion is 0.035mm for the 24x36mm format and 0.02mm for the APS-C format. The CoC is based on maintaining the appearance of sharpness in a 6x9 inch print at about an 10 inch viewing distance (as revealed by the Euro-Canon web site). There is no guarantee that images enlarged any greater than this will appear sharp."

Also on the same site:
"In one "look," it calculates the distance and direction the lens must be moved to cancel the phase differences. It then commands the lens to move the appropriate distance and direction and stops. It does not "hunt" for a best focus, nor does it take a second look after the lens has moved (it is an "open loop" system)."
This web site posts links to the patent, and references to various Canon documents.
Is that credible enough for you?
I stand by my original statement of "Approximate Focus" or "Almost Focussed", and that given these stupid design considerations, it is amazing they do as well as they do. I find it really amazing that the body doesn't take a 2nd look to make sure the lens went to where it was supposed to. This is just begging for problems once lenses get a little age on them.




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