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01-05-2009, 09:07 PM   #1
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Join Date: Mar 2008
Location: Brisbane, Australia
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Posts: 142
Some thoughts on speeding up AF

This may have already been mentioned here before but I haven't seen anything like this posted recently so here I go.

From what I've read here and on other forums the apparent slowness of the Pentax AF system is due to the camera performing a double-check to confirm focus lock. AFAIK Canon and Nikon cameras don't perform this double check (also note that both systems have numerous complaints about AF accuracy in their forums even on flagship models but hey, at-least it's fast). In my experience most cases of the AF refusing to lock on to the subject are due to the camera being unable to perform the double-check (usually with moving subjects).

It's always been assumed that we have no choice but to deal with the double-check but I've been playing with my K20D and it seems we can actually work around it (I've assumed that this can be done for all current models too). If you AF with the AF button (K20,K10D) or Ok button (K1000,100,200D) as opposed to a half-press of the shutter you can trip the shutter before focus is locked. You can therefore hold down the AF or Ok button until the subject appears focused in the viewfinder and release it before the double check occurs.

This speeds up focus especially in low-light but at the expense of accuracy. However at narrow apertures this is most likely more than accurate enough. At wider apertures where critical focus is required you can still use the more accurate double-check method.

Let me know what you think, I've found this can produce decent results at 1/50th, ISO1600 f/2.8 with the FA50 f1.4 on my K20D.


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