P�l Audun Jensen, quoting me, wrote:
(snip)
"But print magnification has EVERYTHING to do with it. Until now I have
kept my silence WRT this subject but no more. Too frequently anecdote
and intuition have been been offered as informed knowledge. The facts
follow.
The DOF concept is based on the circles of confusion (COC) of the out of
focus part of a photograph being either smaller than the unaided eye can
appreciate, in which case the subject matter will seem acceptably
focussed, or large nough for the same unaided eye to see the
unsharpness of it. Quite simply, when the enlargement factor of a print
is increased then the COC of nearly focussed areas will become larger
and will cross the threshold between small enough to pass as sharp and
large enough to be deemed unsharp."
And P�l replied:
"This is a mix-up of terms. The original poster is right; DOF has
nothing whatsoever to do with enlargement factor in the finished print.
Sharpness, not DOF depends of magnification when the area is constant.
Otherwise, both DOF and sharpness are constant the moment you press the
shutter. The scale on the lens says what sharpness is adequate (which is
a highly subjective matter - DOF is not). If you increase reproduction
ratio verything will experience reduced sharpness at a certain area
equally but DOF is constant.
DOF depends of four factors only:
1) F- stop.
2) Focal lenght.
3) size of the subject.
4) camera- subject distance.
The reason why medium format display less DOF than 35mm is that for a
certain F:stop, certain size of the subject and certain camera to
subject
distance focal length will be longer on MF hence displaying less DOF."
------------------------------------------------------------------------
REPLY:
The DOF scale on a lens as not a sharpness scale, if it was it would be
call a "sharpness scale". It is exactly what it is called, a "Depth of
Field Scale". And DOF most certainly IS dependent upon print size, it
just happens that the scales printed on most lenses are calculated so
that an 8x10 inch enlargement results in circles of confusion of a size
that is as small as the human eye accepts to be a point. Another source
(Ilford Manual of Photography) cites a circle of least confusion (COLC)
of 1/100th inch (0.254mm) for a print viewed at a distance of 10 inches.
This appears to agree with other sources because scaling the numbers
down from 8x10 inch (203x254mm) to 24x36mm (in practice 24x30mm because
this is all that can be enlarged to 8x10 inches) gives an ~on film~ COLC
of 0.03mm for a 24x36mm negative/slide, and this figure has recurred
regularly in this thread.
It is important to note is that DOF is based upon a set of assumptions.
It is assumed that a print will be a certain size, and it is assumed
that a certain sized COC is acceptable. YMMV.
Today I looked at my DOF program. Unlike Bill Peifer's program this one
~does~ have a checkbox to select format: 35mm; 6x6cm; or 4x5 inch, but
when I looked at the comparative COLCs that had been input into the
calculator I smelled a rat. They were:
35mm format d = 0.03
6x6cm format d = 0.06
4x5in format d = 0.15
These numbers DON'T scale up and down accurately. The equations used by
the calculator are attributed to "Applied Depth of Field", by Alfred A.
Blaker, published 1985 by Focal Press, but I don't know if the numbers
are Blaker's or Thomas Hallstein's of Outsight Photography in Santa
Rosa, CA who designed the calculator and posted it on the web. But what
is apparent is that either the numbers were too coarsely rounded up/down
or the enlargement factors between the various negative formats and 8x10
inch were incorrect. The numbers ~should~ be:
35mm format d = 0.03mm
6x6cm format d = 0.055mm
4x5in format d = 0.122mm
based on enlargement factors of 8.46X for 24x36mm to 8x10 inch, 4.62X
for 55x55mm (the actual size of a 6x6cm neg, the film is 6cm wide), and
2.09X for 4x5 inch (97x122mm useable). These small differences are
significant when multiplied out through the equations.
Which brings me to the next issue. The 3 sources I examined: the
program previosly mentioned; the Ilford Manual of Photography; and the
Gevaert Manual of Photography, all gave different equations and did not
produce identical results. It is very likely that the DOF scales on a
German 35mm camera and a Japanese medium format camera were calculated
with different equations and using different COLC standards. How can I
compare them without getting bogged down in endless arithmetic?
So Bill Peifer is spot on to say that comparing the DOF of different
formats is like comparing apples to oranges, not only because of the
ultimate subjectivity of acceptable sharpness but, as I have found,
because there seems to be an astounding lack of consistency in the
standards applied to different formats. For unexplained reasons,
the "level playing field" has been tilted and any valid comparison
thwarted.
Some day I'll run a practical test of my beliefs re DOF and post the
results on the web. But first I have a one year old son whom I stay
home and care for, and he's teething and grumpy and is being very needy
so
I'll just add it to my "to do" list......
Regards,
Anthony Farr
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