Quote Originally Posted by ic-racer View Post
The equation I use is:
new_time = old_time x (new_M +1)^2 / (old_M+1)^2
where M = new magnification (print/neg) and m = old
magnification (print/neg)

The exposure time factor would be:
Factor = (M + 1)^2 / (m + 1)
My quick study of this matter a few years ago had
me turn my attention to the purchase of an OM-10.
A more thorough study this pass confirms the
equation you use to be THE one to use. Print
size divided by negative size is specified
to be the method for deriving M&m.

Although a + 1 in each term appears to compensate
for aperture changes and the equation is described
as being accurate, I've not encountered any claims
of it being exact. So I wonder if the equation is
the last word?

Without the + 1s results will equal those of the
equation Nt = Ot x Ne^2/Oe^2; where the terms
are Old, New, time, and edge.

For more information on this subject and derivation
of other exposure related variables search Google
for, exposure enlargement calculations . Dan