Peter,
The standard reference used for DoF scales has been an 8x10 inch
target print since almost forever. If you're printing larger than
that, the usual assumption is that the increased viewing distance
compensates for the larger size to give the perception of sharpness
consistent with normal viewing distance for an 8x10 inch print. So the
CoC, dependent upon format, is indeed the arbitrary constant which
will cause the DoF to vary dependent upon format.
The whole notion of DoF is somewhat arbitrary ... what looks sharp me
might not look sharp to you. Some people prefer a smaller CoC even for
an 8x10 inch print, meaning a reduction in DoF.
Godfrey
On Nov 13, 2008, at 6:49 PM, Peter Loveday wrote:
For me print size is definitely a factor.
When considering if something is adequately in focus, or within
acceptable DOF, it is really the final result that interests me. Of
course not only print size, but viewing distance affects the overall
'enlargement' of the image... Certainly the average 10x15 print may
be viewed a lot closer than a 2m x 3m wall print, and this can
change what CoC is acceptable, and what falls within acceptable focus.
Still, in the majority of cases, where I'm considering a 10x15 vs an
A4 vs a web image, the viewing distance is similar, and acceptable
CoC (and therefore DOF) varies with enlargement size.
I think, for me, unless I'm contact printing from film (or
sensor :), the original format is less relevant than the overall
enlargement of the 'system' - that is sensor, print, viewing
distance considered as a whole.
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