At 17:04 03.03.2003, Paul Farrar wrote:
It's a problem with all formats, not just digital. It may, or may not
be an actual problem for a particular case. Large format photographers
have a problem with too shallow a depth of field in some cases, but they
get a break on diffraction limitation, so they can stop down more.
I'd like to confuse this issue further. I think it's misleading to say that
one format has inherently more or less DOF, and it's certainly wrong to say
that LF photographers get a break on diffraction limitations. There are
many DOF calculators online to play with, and all of them are based on the
same assumption, that you're going to enlarge a print to a certain size.
When you do that (and view the print from a normal distance) sharpness
beyond a certain level is strictly unnecessary because your eyes can't see
the details anyway. This is the basis for the Circle of confusion, or the
biggest circle on the negative that the eye will think is a point when
viewed in a print (again, from a normal viewing distance). Obviously a
small film (or CCD, CMOS) will need higher magnifications than a large
format negative to reach the same size, and thus the circle of confusion
must also be smaller for the smaller format.
If you play with one of the DoF calculators and compare DoF for different
formats you'll discover that for any angle of view DoF only depends on
absolute aperture. In other words a normal lens with a 2cm opening will
have the same DoF for any format. This means that a 50/2.5 on 35mm will
behave much like a 300/9 on 8x10". The trouble starts when you try to go in
the other direction, to smaller film or sensors. How can you get a 2cm
opening from a 10mm lens? (BTW, simple division to fails in this extreme
case, f/0.5 is the optical limit for lenses with air on both sides, and
that would require an infinitly large opening)
For the 4/3 system this means that while the lens is 300mm f/2.8 it will
behave like a 600mm in terms of angle of view, but then only as an 600mm
f/5.6 in terms of DoF. The car fanatics say there's no substitute for cubic
inches, and if you're after shallow DoF then there's no substitute for
inches of aperture either. If Olympus should one day make a 25mm f/1.0 for
4/3, the DoF would still be the same as for a 50/2.0 on 35mm film.
Oh, and the LF photographers have to struggle with diffraction as well.
They're into diffraction limited territory all the time, but compensate
with larger film area = lower magnifications = larger CoC.
Anyone not confused yet? Want to talk about F280 fill-flash instead? ;-)
Thomas Bryhn
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