Cropping / Enlargement Factor
Pick paper size and negative format; the calculator works out the magnification.
Where to find it
Tools tab Darkroom Enlarger Enlargement section
Summary
Two chip rows let you pick negative format (35mm, 6x6, 6x7, 6x9, 4x5, 8x10) and paper size (5x7, 8x10, 11x14, 16x20, 20x24). The calculator divides paper short edge by negative short edge to get the enlargement ratio, then squares it for the inverse-square exposure adjustment.
Detail
How it works
What it does
Tap a chip for your negative format and another chip for your paper size. The calculator shows the enlargement ratio (for example, 4.0x for a 35mm negative on 8x10 paper) and the extra stops of exposure that magnification adds.
Why magnification changes exposure
When you enlarge a negative, the same amount of light from the bulb spreads over a much larger paper area. By the inverse-square law, doubling the print dimensions quarters the light per square inch, so you need four times the exposure (two stops more).
Negative formats supported
35mm (24mm short edge), 6x6 (56mm), 6x7 (56mm), 6x9 (56mm), 4x5 (97mm), and 8x10 (203mm). The short edge is what drives the math because that is the dimension that gets blown up to fit the paper's short edge.
Paper sizes supported
5x7 inch (127mm), 8x10 inch (203mm), 11x14 inch (279mm), 16x20 inch (406mm), and 20x24 inch (508mm). Pick whichever matches your enlarger easel.
Reading the result
The Enlargement ratio row tells you the linear magnification (1.5x, 4x, 13x). The Exposure adjustment row tells you the extra stops to add to your base test-strip time. Both update live as you change either chip.