The Darkroom Cookbook, 5th Edition

Appendix 5Time Adjustment for Enlarging and Reducing Prints (Inverse Square Law)

Daughtee Rogers taught me this simple calculation in the early 1970s for adjusting print time. Once learned, it will save you time and paper. The only thing you need is a cell phone with a calculator app. Before your eyes cross, remember that if I can figure this out, you can, too.

T(ime)

L(ength)

$$\frac{{\left(T1\mathrm{ }x\mathrm{ }L2\right)}^2}{({L1)}^2}\mathrm{ }=\mathrm{ }\mathrm{T}2$$

T1 = Original exposure time

L1 = Original length

L2 = New length

T2 = Adjusted exposure time

1. Measure the distance between two points on the print (L1).

2. Enlarge or reduce the image and measure the same two points (L2).

3. Use the formula.

4. If you are using inches as your unit of measure, convert fractions to decimals as follows:

   1. 1/2 inch = 0.5

   2. 1/4 inch ...