Focal length / depth of field question

[Emmanuel BIGLER]

Another answer : when the magnification ratio is bigger than 0.1 = 1/10 = 1/10, i.e. when the subjet is located closer than 11 times the focal length in front of the lens, DoF no longer directly depend on the focal length but only on
- chosen value for the circle of confusion, CoC, "c"
- magnification ratio "M" = (image size) / (object size)
- and 'engraved' f-number "N" (which actually differs from the effective f-number. N_eff = N(1+M) )

In short, the approx formula for total DoF valid when the magnification ratio M is bigger than 0.1, explains eveything and reads as :
total-dof = ((M+1)/M^2) . 2 N c = (1/M^2) . 2 N_eff c
For example, when M=1 in 2f-2f position, total-dof = 4 N c whichever focal length you are using.
Provided of course that no secret digital edge-enhancement is applied between the sensor and the photographer's digital file

[pdmoylan]

DOF is solely a function of magnification and aperture. It has nothing to do with FL. What is different with different FLs is AOV, and degree of background information.

[Ken Lee]

Given the same camera and subject, while depth of field may be the same whether we shoot a 450mm lens at 15 feet or a 25mm lens at 10 inches, the appearance of the subject will be quite different with respect to foreshortening and relation to background.