Walt said:
Ah, but now you've injected the notion of "greater enlargement." That's
a different ballgame. Carried to its extreme, DOF becomes practically
non-existant. I can blow up a 6x9cm Provia 100F tranny enough so that
anything not in the plane of exact prime focus starts to get soft and
fuzzy. DOF can cover up for only so much nearly in focus stuff, and it
is strictly governed by the law of diminishing returns.
You have to live with what you start with, and if you have a little
thing in the beginning, you can't expect to have a big thing in the end.
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Ah, you've figured out the answer but haven't realized it yet. DOF is a
function not only of focal length, aperture and distance but also of
final print magnification, viewing distance and resolving ability of the
human eye. Many folks don't realize this because they're used to
looking up DOF in a table which makes no mention of magnification or of
Circle of Confusion aka resolution or ability of the human eye. The
reason is that most DOF tables have built-in *assumptions* about final
print magnification which is based on using 35mm film, magnifying 8
times to make an 8x10 print and viewing the print from a distance of 10
inches (normal reading distance) with an average human eye.
Fussing about with assumptions about such things as human visual acuity
is a bit esoteric for most of us and don't really make much difference.
So, some simplifying assumptions are necessary and are all rolled up in
what's called Circle of Confusion or desired resolution on the film or
sensor.
Film format or sensor size is something that can't be ignored, however.
As you posted earlier if you attach your medium or large format lens
onto your beloved OM or E-1 the lens will deliver exactly the same
resolution to film or sensor that it always did on the larger camera.
But, in order to make a print with the same angular view as the larger
camera we'd have to use a shorter focal length lens AND magnify the
final image more than we'd have to with the larger format. It's the
difference in final print magnfication that changes the equation. In
order to make a sharp looking 8x10 from a 35mm film frame I need to have
greater resolution on the film. In other words, I have to have a
smaller Circle of Confusion if I'm going to blow it up more and still
have it look sharp.
In the case of the E-1 with it 2X crop factor we have to blow images up
about 16X to make an 8x10 print instead of 8X with the 35mm. The image
on the sensor has got to be sharper (smaller CoC) in order to look sharp
on the final print.
Now, back to the DOF scale on the lens. When using an OM lens on an E-1
the 2X difference in final print resolution equates to a 2 stop
difference in DOF for the same focal length. If the OM lens is set at
f/8 then add 2 stops and read the DOF from the scale for f/16. For
example refer to the calculator at <http://www.dofmaster.com/dofjs.html>
Set the format for 35mm, focal length at 50mm, focus distance at 10 feet
and aperture at f/8. You'll find that the DOF is shown as 7.77 to 14
feet. Now change the format to E-1 (which is actually just cutting the
CoC in half) and set the apeture to f/16. You'll find that the DOF is
exactly the same.
ps:
Brief explanation of Circle of Confusion: Consider that the lens is
focused on a tiny dot like the period at the end of a sentence. The
light passing through the lens forms a cone whose diameter is decreasing
from the rear nodal point of the lens to a minimum at the point of best
focus. Hopefully this is where the film is located. If the light beam
could keep going it would start to diverge again and form a cone with
increasing diamter beyond the film plane.
Now we ask if the dot is in focus if the film plane is, say, 1/10 mm
ahead of or behind what we consider the point of best focus. The answer
is maybe. The dot doesn't suddenly go out of focus but instead starts
to grow fuzzier and fuzzier the futher away it is from the ideal. But
how far can it get from the ideal before it *looks* fuzzy. Depends on
how sharp our eyes are, how far away from the print they are and how
large we've blown up the original image. The "what's acceptable" is all
rolled up in Circle of Confusion. How fuzzy is fuzzy.
Sorry if I've repeated somebody's response. I've been out of town for
nearly a week and am trying to catch up on digests.
Chuck Norcutt
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