I understand what you are saying and completely agree. But you stop too
soon. (Or maybe I go one too far.)
I should comment that what you are calling the "circle of confusion" I
would call a "defocused point" - i.e. any point that is not on the focal
plan. I use the term "circle of confusion" the describe the size at
which a defocused point loses apparent sharpness.
A little thought experiment: Take a photo on 35mm with a 50mm lens at
f8. The physical diameter of the aperture should be 6.25mm (50mm /8).
Assume there is a point outside of the focal plan that is defocused but
is within the DOF area and so appears to be sharp. Call the point P and
its diameter D. Now I take the same shot from the same distance with a
6x7 using a 100mm lens at f8, of course needing the longer focal length
to get the equivalent frame at the same distance. The physical diameter
of the aperture is 12.5 (100mm / 8). Since the physical size of the
aperture is now larger, the defocused point P is correspondingly larger
and is 2xD. Being larger it would have less apparent sharpness. That
would seem to confirm the idea that as the format gets bigger there is
less DOF.
But - now enlarge both those images to be 700mm wide. The 35mm exposure
is enlarged 20x. The 6x7 exposure is enlarged by 10x. In both
enlargements the diameter of point P is now 20xD. If the defocused point
is now the same size in both prints, why would its apparent sharpness
differ? (I'm obviously taking liberties by ignoring differences in
aspect ratio and assuming that 6x7 is twice the size of 35mm, which it
is not exactly.)
Guidelines for circle of confusion size for use in DOF calculations
scale with the size of the format. For example, Wikipedia's circle of
confusion page says that the circle of confusion for 35mm is 0.029mm and
for 6x7 is 0.059mm.
That's what I don't get about the circle of confusion - it seems like
all the DOF gained in small formats would be offset by the additional
enlargement of the image to get it to viewing size. But that is not how
things actually work.
Mark
On 3/7/2015 2:03 PM, John Francis wrote:
On Sat, Mar 07, 2015 at 11:55:07AM -0500, Mark C wrote:
The one factor I am ignoring is the "circle of confusion" and effect that
enlarging the image itself has on the DOF of the final print.... and that is
not something I have never been able to grok and so won't comment on. Maybe
someone else can.
Mark
I consider that to be the most significant factor in the DOF of the final image.
Ignore all mystical numbers like format, f stop, focal length, ... for now.
The basic problem is you have three defining factors to consider. They are:
1) The size of the subject you are tring to capture
2) The size of the final image you are trying to create
3) How far away from the subject you can place the sensor
You can juggle all the other numbers (how much of your sensor does the image
cover says how much cropping and magnification you will have to do, etc.),
but after a whole lot of maths you end up with one simple result:
The size of the "circle of confusion" is basically defined by the
actual diameter of the aperture (focal length divided by f stop);
all the other numbers cancel out.
That's why depth of field increases when you go to a smaller sensor;
it's because you generally want to keep the exposure variables (shutter
speed and f stop) around the same, and using a smaller camera system
means that you can use a physically smaller lens aperture. This reduces
the size of the circle of confusion (and thus allows for parts of the
subject that are further away from the plane of true focus to still
yield images which are perceived as being acceptably within focus;
this is another way of saying that the depth of field is increased).
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