Reciprocal Rule
A handholding guideline stating that the slowest safe shutter speed is roughly one over the focal length in use.
Formula / rule
t ≤ 1 / (focal length × crop factor)
Measured in
seconds
In depth
The rule dates from 35 mm film and 6×4 inch prints, so it is optimistic for high-resolution sensors examined at 100 per cent, where many photographers double or quadruple it. Crop sensors need the focal length multiplied by the crop factor first, and image stabilisation buys back anywhere from two to six stops.
Examples in practice
A 200 mm lens on a full-frame body suggests 1/200 s handheld; on an APS-C body the same lens suggests 1/300 s.