Bellows Extension Factor
The extra exposure required when a lens is racked far forward for close focus, spreading its light over a greater distance.
Formula / rule
Factor = ((v) / f)²
Measured in
stops (EV)
In depth
The inverse square law applies inside the camera exactly as it does outside it: doubling the lens-to-film distance quarters the illumination, costing two stops. Cameras with through-the-lens metering compensate silently, but view-camera and studio-flash users must calculate it, since a handheld flash meter knows nothing about how far the bellows are extended.
Examples in practice
A 150 mm lens extended to 300 mm for 1:1 macro needs two extra stops: f/22 on the meter becomes f/11 on the lens.