The set of simple formulae to use is:

1) Magnification = Extension / Focal length

2) Aperture factor = 1 / (Magnification + 1)

3) Shutterspeed factor = (Magnification + 1)^2

4) Correction in stops = log(Shutterspeed factor) / log(2)

Extension includes the built-in extension of the lens barrel, i.e. the one you add when turning the focussing ring from inifinity setting. With the lens set to infinity, it is the length of the tube added.

Maximum built-in extension is not small enough to ignore. So don't ;-)

Things get a bit more complicated (and more precise) when considering that most (if not all) lenses are not symmetrical. A tele lens, for instance, will lose light faster with than the above formulae would suggest.

To take asymmetry into account, we need to know the sizes of the entrance and exit pupils of the lens. The ratio of these sizes must be used as a factor in the above formula 1). This ratio (let's call it the asymmetry factor, AF) is

5)AF = diameter exit pupil / diameter entrance pupil.

1) should be transformed to 1a)

1a) Magnification = Extension / (Focal length * AF)

But *only* to calculate exposure compensation. Not when determining image size!

The sizes of entry and exit pupils can be found in the Zeiss Lens Data Sheets, available on-line on Zeiss's website (

. Click your way to photography, select Hasselblad and the appropriate lens line, pick the lens you're interested in, and click to see the technical information (PDF-format)).

These simple formulae don't work properly with lenses that have internal focussing.

Oh, and administrator: could you please do something about all these empty posts? Like switching Peter C. Patton's email notification of new posts off? A "very active member" indeed, but he doesn't have much to say, has he?