Darkroom Test Strip Calculator (F-Stop Timing, Burn & Dodge Times)

How it works

segment time = base × 2^(offset in stops) · progressive increment = tₙ − tₙ₋₁ · burn seconds = base × (2^stops − 1) · dodge seconds = base × (1 − 2^−stops)

Photographic paper responds to light logarithmically, so equal steps of seconds (5, 10, 15, 20…) crowd the dark end of a test strip and leave big jumps at the light end. Stepping in f-stops — each segment a fixed ratio more than the last — gives evenly spaced tones and lets you think about burns and dodges in stops, the same units as the camera. A ⅓-stop strip of seven segments covers two stops, enough to bracket most negatives; the winner is refined with a finer strip. Burns expressed in stops transfer to other print sizes without re-testing.

Worked example

A 12 s base with seven ⅓-stop segments runs from 6 s to 24 s: 6.0, 7.6, 9.5, 12.0, 15.1, 19.0, 24.0 s — as a progressive strip that is 6.0 s then increments of 1.6, 2.0, 2.5, 3.1, 3.9 and 4.9 s. A one-stop sky burn on the 12 s base is a further 12 s; a half-stop dodge means shading the shadows for about 3.5 s of the base exposure.

Frequently asked questions