I recently wrote a computer routine to calcute the precise exposure setting for my Nikon Multiphot photomacrographic camera. When you use 4x5" sheet film (at 10$ each), heavens forbid you should have to guesstimate the ensuing exposure and bracket that value in additional. Since there is no TTL or any other means of getting the exposure reading, except for using a hand-held meter, you are thrown back into the dark ages of pre-TTL photography.
Given data on the lens and its aperture setting, the amount of bellows draw, and the required final shutter speed, I now can adjust my lights (cold-light fibre-optical halogen ) to obtain exposure accurate well within 1/3 stop. Thus, I'm ensured not to waste any of those expensive 4x5" sheet films.
While working with the program I found that the formula given by Szen is not correct. Or rather, the equation *is* correct, but not the interpretation of the parameter "p", which should be the inverse (p=1/P). The pupil magnification "p" is 1 for the symmetric lens case, <1 for a telephoto design, and >1 for a wide-angle lens. This is exactly opposite of stated in Szen's post. I suspect he has gotten this from an earlier version of the tutorial by David Jacobsen on photo.net, which previously carried the same flawed interpretation, but now is amended.
The consequence of using the correct parameter is that telephoto lenses are less well suited for close-ups, since the M/p term will be large to give much longer exposures than with the standard case of p=1. In fact, with my 270 mm f/6.3 ED T-Nikkor (with p=2.86), nearly 4 stops of light are lost at 1:1 (2 stops more than the unadjusted p=1 case, and 2 2/3 stops off the wrongly computed formula using the false "p" = 1/P = 1/2.86 = 0.47). Wide-angle lenses would be more appropriate since much less light is lost (because of p >1); however, you can get too close when going towards 1:1 so that the focused plane is within the lens assembly (!).
A final note for the AF Micro 105 is that it cannot have a fixed pupil magnification because its focal length shortens while the lens is focused closer.