CDAF question

Started Apr 30, 2012 | Discussions thread
kenw
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Re: CDAF question
In reply to Detail Man, May 1, 2012

Detail Man wrote:

Interesting, Ken. It makes sense that (at least) two samples from the involved photo-sites would need to be compared - after each have been analyzed for the averaged site-to-site magnitude of differences in luminosity. It seems like it might make sense to gradually increase the precision with with such an assessment of individual sample "contrast" is made - in order to minimize the total iteration-time (as it seems clear that the focus-distances in the initial phases of the iteration would be very unlikely to correspond to the more accurate result to eventually be converged upon) ?

I suspect this is how it would work, coarse measurements for initial movements and finer measurements as you approach focus. One thing about CDAF is you actually have to move past focus before you can detect it. You don't know what the sharpest focus is until you start seeing the image get softer again. That would probably be performed with very fine movements. A PDAF system on the other hand doesn't have to do such things at all.

  • The focus measurement degrades quickly as you move out of focus. Therefore CDAF is much more likely to need to "hunt" for focus than a PDAF system.

Physical effects of the Focal Length at infinity of the lens-system being adjusted are no different than would be the case for any other method of auto-focusing - the measurement itself is tweaky . A good reason not to wait long after AF indication - though I myself do often have a tendency to do that [in the belief, or superstition, that the OIS (Mode 2) may need a bit of time to "servo-in"].

I think I follow what you are saying, but not sure how it relates to the part of my post you quoted. All I meant was that for a random starting point of mis-focus the PDAF system is much more likely to get an initial measurement that is valid and useful. For a CDAF system it can be more likely that it finds it is too far out of focus to get a measurement and must "guess" which way to move focus to check again. That's what I meant about hunting. And my statement is probably too broad really - I suspect CDAF could do clever things to help itself in these cases (like step down the aperture).
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Ken W
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