HTML Code:
This is one of my old rpd posts. It doesn't include reciprocity failure, but for
LF's somewhat limited print magnifications, that isn't a problem.
Suppose that your 15 sec, 8x10, was an 8X enlargement (old_M = 8) from a 35
mm negative, and the new magnification (new_M) would be 11X, for an 11x14
(printed so the 24mm dimension of the negative just fits the paper; the
long dimension is cropped). The exposure factors are equal to the
magnification plus one, squared, so:
new_time old_time
------------ = ------------
(new_M +1)^2 (old_M+1)^2
or...
new_time = old_time x (new_M +1)^2 / (old_M+1)^2
so:
new_time = 15 sec x (11+1)^2 / (8+1)^2 = 15 x 1.777 = 26.7 sec.
To simplfy matters, you can place a masking tape scale on the baseboard so
overall magnification can be read directly, even for cropped prints (use
overall magnification, not the cropped print dimension).
Using a spreadsheet, you can indicate new and old magnifications in a
lookup table, with the intersection formula equal to the ratio of
magnification plus one squared, as above. For magnifications from 4X to
12X, the table would be:
Old New Magnification
Magn.
4 5 6 7 8 9 10 11 12
4 1.00 1.44 1.96 2.56 3.24 4.00 4.84 5.76 6.76
5 0.69 1.00 1.36 1.78 2.25 2.78 3.36 4.00 4.69
6 0.51 0.73 1.00 1.31 1.65 2.04 2.47 2.94 3.45
7 0.39 0.56 0.77 1.00 1.27 1.56 1.89 2.25 2.64
8 0.31 0.44 0.60 0.79 1.00 1.23 1.49 1.78 2.09
9 0.25 0.36 0.49 0.64 0.81 1.00 1.21 1.44 1.69
10 0.21 0.30 0.40 0.53 0.67 0.83 1.00 1.19 1.40
11 0.17 0.25 0.34 0.44 0.56 0.69 0.84 1.00 1.17
12 0.15 0.21 0.29 0.38 0.48 0.59 0.72 0.85 1.00
Bookmarks