FYI for you digital camera lovers:
Thought experiment #3 part (a) is the reason it may seem difficult to exert
DOF control to get a shallow DOF with your digital camara! The CCD is
small in size compared to a 35mm film frame. It's the reverse of what
happens when going from a smaller to a larger film format. The DOF expands
when going from your 35mm format OM to the equivalent focal length (for
same angle of view) on your digital camera.
At 14:10 12/30/01, I wrote:
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.
-- John
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