Do you mean the image plane to the ccd? (ie, a + b above) Are you sure?


In that case, mfd = a + b above, so according to the lensmakers equation, I get mfd = f[(m+1)^2]/m, where f is the focal length and m is the max magnification. This formula seems to overestimate the mdf reported in lens specs.


If mfd is the distance between the "ideal optic" and the focal plane (a above) I think the formula would be mfd = f(m+1)/m, which agrees much more closely with specs in Bryan's reviews for lenses like the 500 f/4.


Possibly I've made a mistake in my algebra, but I don't think so


Anyway, it doesn't really matter what mfd is. Bob- if you want the distance from the front element to the subject, use the second formula. If you want distance from the subject to the ccd, use the first.


Just know that real lenses differ from ideal, so neither formula will be exact.