At 09:43 1/13/02, Moose wrote:
I didn't say better or worse DOF, just different. I skimmed through all
that DOF thread and saved some for later study and elucidation. I do think
that, for the same final image size, the MF confused circles ( ;-) ) are
magnified less than the circling confusion of 35mm, leaving the final
confusing circles about the same size (on the final viewed image, not the
film) for equivalent coverage lenses on both formats. Maybe a DOF maven
will confirm or deny - treatise not necessary, but possibly unavoidable .
For the same field of view DoF will change if you change the film format size:
a. Focal length must increase linearly as size of film increases to
maintain same field of view (linearly means f^1).
b. As focal length increases, DoF decreases at the square of the focal
length (f^2) if maximum allowable circle of confusion diameter remains
unchanged.
c. Maximum allowable circle of confusion diameter increases linearly as
film size increases (c^1). This increases the DoF linearly *if* focal
length remains unchanged.
Final result of using a larger film format (size) with same field of view:
a. Take one step forward with increase in DoF from linear increase in max
CoC diameter allowed.
b. Focal length must increase linearly to maintain same field of
view. Take two steps backward with decrease in DoF at a rate of the square
of the focal length change.
To maintain the same DoF between my OM-1n with a 50mm lens and my M645 with
an 80mm lens, I must stop down the 80mm lens on the M645 by about a half
stop more than the 50mm lens on the OM-1n. About the same on a 6x6. IIRC,
a 6x7 format is about a full stop more and a 6x9 is about 1-1/3 stop more.
-- John
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