Actually some DSLR's do a double check, and others don't due to whoever
was "hating" double checks. Without the double check, an acceptable
range of what's considered "in focus" must be established. For example,
the Canon 400D is the type of camera to have eliminated this double
check for the appearance of AF "speed". Wide aperture lenses are
notorious for being not that well focused with that camera, as posted by
many who don't understand the limitations of calculating the "coast".
Unless by accident, those that do a quick double check are more dead on
than those that don't. (Though it seems that DSLR's that do the double
check permit you to take the photo on the first pass anyway.) Its just
one of the misconceptions and accuracy tradeoffs that are not considered
when people say one camera has "faster" AF than another.
Larry
Winsor Crosby wrote:
> My last sentence was inept.
>> It does not check final focus. People hated that with the lens
>> hunting back and forth with early AF and with the limited control of
>> digital point and shoots. Modern AF calculates how far out of focus
>> it is and directs the focusable part of the lens to move to a
>> calculated position where the lens will be in focus and it takes into
>> account how much things will coast. Maybe the next stage of design in
>> this stuff is not to require the lens to be operating at
>> specification, but for the focus control computer to calculate the
>> focus for each lens as it is being used.
>>
>
>
>
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