Bellows Extension Factor Calculator
In macro photography the lens is racked far from the sensor to focus close. That extra extension spreads the projected image over a larger area, dimming it and lowering the effective aperture. The bellows extension factor is how much extra exposure you must add to compensate. This calculator returns the exposure factor and the equivalent number of stops from your lens focal length and the total lens-to-sensor extension. Use it when metering with a handheld meter or setting manual flash.
Bellows extension formula
Factor = (extension / focal length) ^ 2
Stops = log2(factor)
Magnification = (extension / focal length) - 1
Extension is the total distance from the lens optical centre to the sensor. At infinity focus this equals the focal length and the factor is 1 (no compensation). As the lens racks out, the factor and the stops both grow.
Worked example
A 100 mm lens extended to 150 mm: factor = (150 / 100)^2 = 2.25. Stops = log2(2.25) = 1.17. Magnification = (150 / 100) - 1 = 0.50, or half life size. You need about 1.17 stops more exposure when metering externally.
Bellows extension: frequently asked questions
What is the bellows extension factor?
When you focus close, the lens moves farther from the sensor, spreading the same light over a larger projected image and reducing the effective aperture. The bellows extension factor is the multiplier by which exposure must increase. It equals the square of the total extension divided by the focal length.
How do I calculate bellows exposure compensation?
The factor is (extension / focal length) squared. The compensation in stops is the base-2 logarithm of that factor. For example, a 100 mm lens racked out to 150 mm gives (150/100)^2 = 2.25, which is log2(2.25) = 1.17 stops of extra exposure.
Does this matter for autofocus lenses?
Through-the-lens metering already accounts for the light loss, so cameras metering through the lens compensate automatically. The factor still matters when using a handheld meter, flash in manual mode, or any external exposure setting that does not see the dimmed image.
Sources
- NIST: Office of Weights and Measures (length units).
- The factor (v/f)^2 follows from the inverse-square fall-off of image illuminance with extension in thin-lens optics.
Reviewed by the CalculatorHub team, edited by James Graham, 19 June 2026. See our methodology.