At 17:43 12/30/01, John Robinson wrote:
[snipped out portion about insomnia]
What I tried to say was that if you set up your OM to photograph say a
head and shoulders portrait with a leafy background and as an expirement
you first
mounted your 24mm, set apature at f4 and placed camera 2 and 1/2 feet from
subject, exposed frame 1, now mount the 50mm at f4 at 5ft from subject,
frame 2, now mount your 100mm at f4 at 10ft from subject. frame 3,
woulden't all 3 frames have about the same DOF and the background have
about the same degree of fuzzyness?
DOF? Absolutely. You have kept magnification constant on the same film
size (format). The DOF remains the same. However, the circle of confusion
grows faster as you move farther outside the DOF. The circle of confusion
at "infinity" will be much larger from the 100mm lens than it will be from
the 24mm lens. This was a revelation some time back when I started playing
with the DOF equations. Something about a common (and incorrect)
recommendation to "use a longer lens to reduce DOF" didn't make sense if I
then moved to keep the subject the same size. What does change is
perspective and the size of circles of confusion well outside the DOF! Not
certain I would want a tight head/shoulders done with a 24mm (unless I was
a circus clown and wanted the big red nose to look even larger)!
Or am I all wet?
Nope! It's all about magnification on the piece of film. I should make a
clarification about "magnification" as used in the posting just prior to
this one. I wrote:
For the same angle of view, aperture and focus distance (magnification),
an increase in film size shrinks the DOF!
This is not the pure definition of magnification, which is object size on
film. I used it loosely (and probably improperly) to describe filling the
viewfinder or film frame which actually makes it occupy a bigger piece of
film. In changing from a 50mm lens on a 35mm camera to an 80mm lens on a
645 camera, the image size on film is bigger and technically it is a larger
magnification (on film).
Whoever advised using a larger film format didn't understand the real
factors the determine DOF. Reason? If an image is cropped, it requires
greater enlargement to make the same size print. Greater enlargement for
the same size print reduces the maximum diameter for the circle of
confusion on film. When it's all over with you end up with the same
maximum circle of confusion and the same magnification as if you had
started with 35mm film to begin with. It's a zero sum game except more
effort is expended (and probably $$) using a larger film format and making
a custom cropped print from it.
Thought Experiment #1:
Change from a 50mm lens on a 35mm camera to a 50mm lens on a 645
camera. Keep distance and aperture the same. Crop the 645 image to match
the 35mm film image. You end up with the same magnification on film and
therefore the same size piece of film, 24mm x 36mm, as if you had used 35mm
film! For the same size print this reduces the maximum circle of confusion
in the 645 image to that for a 35mm film image.
Thought Experiment #2:
Change from a 50mm lens on any camera to a 100mm lens and double the
distance, but keep aperture constant (part of your thought
experiment). DOF remains the same, but objects well outside the DOF look
more out of focus; the farther from the near/far DOF boundaries the more
out of focus they appear. The circle of confusion grows faster.
Thought Experiment #3 (two parts):
[Note: DOF equations typically give the one for hyperfocal distance first,
and then use that value in two more equations for the near and far DOF
limits. If the hperfocal equation is substituted into the ones for near
and far limits the relevant portion of it for this thought experiment ends
up with "f^2/CoC" (focal length squared divided by the maximum circle of
confusion diameter).]
(a) Use a 50mm lens on a 35mm camera and make image #1. Use an 80mm lens
on a 645 camera keeping distance and aperture the same and make image
#2. No cropping is required as you end up with approximately the same
angle of view (but it's on a larger piece of film). The 645 image #2 has a
slightly narrower DOF than the 50mm lens with 35mm camera image
#1. Why? Focal length and the allowable circle of confusion both increase
proportionately in keeping angle of view constant. However, in the DOF
equations the focal length is squared and the circle of confusion
isn't. The DOF shrinks comparing image #1 to image #2.
(b) Now make image #3 using an 80mm lens on the 35mm camera keeping
distance and aperture the same as for #1 and #2. Its DOF will be the
narrowest of the three. Why? The maximum circle of confusion for image #3
is smaller than for image #2 (it's the same as for image #1, therefore its
DOF shrinks even more.
[snipped out self-flagellation about math]
And John L., love your site, it's a standard place of reference I go to
often. John Robison
Thanks.
-- John
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