On 8/14/2010 4:31 PM, Chuck Norcutt wrote:
> No, the distances were not arbitrary. If you go back and read my
> original post on this subject and (I think) my last comment, the
> distances were chosen such that an error of 1 meter in focusing would
> alter the total DoF by a considerable amount. From 5 to 6 meters on the
> zoom at 24/3.2 the DoF gets extended by 5 meters on the long side. It
> was intentionally sized to say you can't claim extra DoF for the lens
> since, at that distance, you can't focus accurately enough to say they
> were actually focused at the same distance.
Wait a minute here. Yes indeed, the original question involves DOF. However,
this test is simply about whether AG can
accurately focus within a specified difference of subject distance. The choice
of a shorter difference of subject
distances does not invalidate this particular test.
IF - the test proves that AG can focus over a narrower distance than you
specified, it also proves he can do so over the
longer distance.
Your specified distances may still be correct for the DOF test, but don't get
the two tests confused.
As you have separately posted, you don't agree with me that the test succeeded,
but that doesn't change the fact that if
it did, it proves the focus differentiation/accuracy point quite nicely.
> If you can't claim exactly the same focusing distance (within a given error
> range) you can't claim
> unusual DoF behavior on the part of the lens.
I agree.
Moose
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