AG Schnozz wrote:..
> ....... I figure that there are two schools
> of thought about DoF, one of which is based in 150 years of
> photographic history and the other based on pixel-peeping. No matter
> which opinion I spew forth will be trounced on by the purveyers of
> the opposite thought.
>
It seems to me that there should be a middle ground, one that is based
in the tested truths of traditional DOF calculations - and adjusted to
take into account real changes in photographic technology and usages.
As I understand it, the DOF calculations that go into the markings on
our lenses and the tables in their instructions are based on the average
visual perceptions of people with 20/20 vision viewing 8x enlargements
of 35 mm film images at 10 inches.
Those seem like pretty reasonable assumptions for the early period of 35
mm film technology:
- Amateur images were rarely enlarged more than 8x, with 4x being by far
more common.
- Although the great lenses we recall were pretty darn good, the vast
sea of lenses being used just weren't all that great.
- Films and lenses were slow. Motion blur was a very common problem.
- Film was grainy, so serious cropping was uncommon.
- Film was grainy, limiting detail resolved.
- Many cameras relied on zone focusing.
- Accurate focus with the pre-pentaprism SLRs in all but ideal
circumstances was quite difficult.
- Many enlarger lenses were not top notch and most slide projector
lenses were pretty soft.
Sure, there were plenty of photographers who transcended these
limitations, but they learned through more sophisticated tables and/or
empirical experience that they needed to use more stringent DOF criteria
for their work. I'm sure many here have heard advice to go one or two
stops further on the DOF markings on a lens for critical work. I know
I've heard it here, as well as elsewhere, and used it to my advantage.
Much has changed:
- The average lenses used today are, I believe, better than their forbears.
- Contemporary film is both less grainy and capable of greater resolution.
- Digital sensor systems are pretty universally less noisy than
comparable film.
- These qualities invite greater enlargement/cropping than before.
- Film, and especially digital, is faster, making smaller apertures and
higher shutter speeds more common.
- The technology to make 12x18 prints with high print resolution at home
is common and inexpensive.
I propose that, while the basic methodology of DOF calculation is still
sound, the assumptions used in applying it in the past aren't valid
anymore. Improvements in many aspects of photographic technology have
made significant improvements in many factors that masked DOF
limitations in the past. The thoughtful photographer will adjust DOF
assumptions from the old rules to new ones that maximize their image
results.
So no, I don't think standards based on pixel peeping should be standard
for general use. But yes, I think the old tables/standards based on a
0.033mm CoC need revision.
Moose
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