Comments interspersed:
<snip>
Second, again holding format constant, a shorter focal length will yield
smaller
circles of confusion than a longer focal length. Thus, a shorter focal length
will have greater DOF at a given f-stop than a longer focal length.
*Yes, but don't confuse f-stop with aperture dimension. F-stop is not an
absolute.
*It's a ratio only. It's not the FL that is the general concern. Aperture
*is the controlling factor.
Longer focal lengths are associated with stacking up an image or compressing
perspective.
Shorter focal lengths are associated with elongating or decompressing
perspective.
This is one reason that short teles and mid-range teles are preferred for
portraits;
they tend to yield just a slight amount of flattening of the subject's
features, rather than elongating the nose, for example.
*No.
*135mm on my 4x5 does no more compression than 40mm on 135.
*It only affects working distance.
A smaller format will give the impression of larger circles of confusion as its
results are enlarged to some typical print size. Larger formats will offer the
impression of smaller circles of confusion, because less enlarging is necessary
to get the same sized print. This means that smaller formats (holding
everything
else equal) will appear to have narrower depth of field, and larger formats
will
appear to have greater depth of field.
*CofC is determined by the size of the aperture. It's not the f-stop. (That's
*merely a functional ratio.) It's affected somewhat by lens design.
Now here's where I get a little foggy.
Let's take a 50mm lens, and put it on an APS sized format camera. Let's take a
75mm lens and put it on a 35mm sized format camera. Let's assume they both
share
equal optical and mechanical characteristics except for focal lengths. And lets
assume we're making two prints, one from each lens, both enlarged to 8x10. The
50mm lens on the APS format camera (or DSLR) is going to have its depth of
field
made to appear narrower due to the "crop" factor (or more accurately, as its
image
is enlarged to some print size). But the 75mm lens on a 35mm camera will show
narrower
depth of field due to the increased focal length. Will the results in this
respect
be identical?
*No.
*Enlargement distorts focus. It's what is on the neg that counts. Apparent
loss of
*of DOF by enlarging is not a loss, but merely a distortion because of the
aperture effect
*of the holes in the emulsion.
*The results will not be identical. If both are shot @ the same f-stop number,
the 75mm
*will always have less DOF. Period. (given the assumptions made in the
paragraph.)
*f8 on 50mm is not f8 on 75mm. It's DOF equivalent is more like f9.5.
What other differences will be detectable, using those same two lenses, same
subject distance?
*Keep in mind
Sincerely,
Collin
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