Test Strip
A piece of paper exposed in bands at increasing times, used to determine the correct exposure for a print.
Measured in
seconds
In depth
The strip should cross the most important tonal areas of the image rather than a random part of the frame, since the aim is to judge one specific tone. Doubling exposure adds one stop, so a strip in equal time steps gives unequal tonal steps — which is why experienced printers work in ratios rather than in fixed seconds.
Examples in practice
A strip exposed at 4, 6, 9, 13 and 20 seconds shows the highlight tone landing correctly at 9.