But this can get complicated if you are not enlarging negs full frame. If I
am enlarging just a portion of a neg, don't the measurements become
complicated? No?

RON
"John Stockdale" wrote in message
om...
"rgans" wrote in
message news:
...............
For example, if I
determine that the exposure I want at 58cm is 12 sec at f/8, but then I
change the distance to 68 cm, how would I calculate the new exposure?
(I
know that light intensity falls off as the square of the distance)
...............


Actually, the formula for calculating enlarger exposure when the
magnification is changed is actually a bit different to simply using
the inverse square law.

new_time = old_time x (new_Mag +1)^2 / (old_Mag +1)^2

where new_Mag and old_Mag are the magnifications from negative to
print. So if you wanted to enlarge a 35mm negative to 8inch x 12inch
full frame, (say near enough to 8x magnification) and you had a time
for 4inch x 6inch full frame, (say 4x magnification) the time
adjustment would be:

new_time = old_time x 9^2 / 5^2 = old_time x 3.24

The inverse square law would give x 4. For smaller changes, say 7x to
8x, the error is less. Also, the formula ignores considerations like
a bigger print needing more contrast and maybe more density in order
to look its best.

Note that if the test print were from a cropped part of the negative,
then the magnifications would be different.

For more detail, search in rec.photo.darkroom for

Michael Gudzinowicz enlargement/reduction exposure formula