[Leica] Same DOF, but growing COC in my head

[Hans-Peter.Lammerich@t-online.de]

Right, same lens, same focus distance, same aperture, same COC. But the
COC is on film which needs enlarging for printing or projection. LF
needs less enlarging than MF than 35mm than Minox 8x11mm. To use the
"right" COC for DOF calculations you need to know the final image size
on paper/screen and the viewing distance. If you argue that larger
prints are generally viewed from a greater distance, print size becomes
irrelevant.

So the COCs on which DOF scales are based are 1/20mm for medium format,
1/30mm for 35mm and 1/60mm for Minox 8x11. These are not strictly in
proportion to negative size. One assumption behind that convention
probably is that film resultion and grain are limiting factors and you
may generally make smaller prints from smaller negatives.

But even if the above COCs would be corrected strictly in proportion to
negative size, I am not bsolutely shure whether there is still a DOF
advantage for the smaller formats. By advantage I mean wider DOF.

The DOF calculator at http://www.photostuff.co.uk/dof2.htm apparently is
only meant for 35mm, as the pre-set 50mm standard lens might suggest.
This http://www.minoxlab.com/Don_Krehbiel/mpl/dkdof.htm is a better URL
for the DOF discussion.

Hans-Peter