Bellows Extension Exposure Calculator
In close-up and macro work the lens is moved far from the sensor to focus on nearby subjects, which spreads the light over a longer path and darkens the image. The bellows factor measures how much extra exposure is needed, and it follows the inverse-square law: it equals the lens extension squared over the focal length squared. This calculator returns the bellows factor, the extra stops to add, the magnification, and the corrected shutter time from a base exposure. It matters most with handheld meters and manual flash, where the camera does not auto-correct.
Bellows factor formula
Bellows factor = (extension / focal length)^2
Extra stops = log2(bellows factor)
Magnification m = (extension / focal length) - 1
Corrected shutter = base time * bellows factor
The factor follows the inverse-square law because the image distance increases. Through-the-lens metering already applies this correction automatically.
Macro exposure context
- At infinity focus the extension equals the focal length, so the factor is 1.
- At 1 to 1 (life-size) the extension is twice the focal length: factor 4, two extra stops.
- The correction matters for handheld meters and manual flash, not TTL metering.
- Equivalent form: bellows factor = (1 plus magnification) squared.
- Add the extra stops by slowing the shutter, opening up, or raising flash power.
Bellows exposure: frequently asked questions
What is the bellows factor in macro photography?
When a lens is racked out for close focus, the image is dimmer because the light spreads over a longer distance to the sensor. The bellows factor is the ratio by which exposure must increase, equal to the lens extension squared divided by the focal length squared: (extension / focal length) squared.
How many extra stops does close focus need?
Extra stops equal the base-2 logarithm of the bellows factor. A bellows factor of 2 needs 1 extra stop, a factor of 4 needs 2 stops. At 1 to 1 (life-size) magnification the extension is twice the focal length, giving a factor of 4 and 2 extra stops of exposure.
What counts as the lens extension?
Extension is the distance from the lens's rear nodal point to the sensor, which at infinity focus equals the focal length. As you focus closer, that distance grows. For a simple lens you can read it from the focusing scale or measure it; this calculator takes the total extension as an input.
Does TTL metering already correct for this?
Yes. A camera that meters through the lens (TTL) sees the dimmer image and compensates automatically, so the bellows factor is built in. The correction matters when you use a handheld meter, studio flash with manual settings, or an external meter that does not see through the lens.
How does magnification relate to the bellows factor?
The bellows factor also equals (1 plus magnification) squared, where magnification is the image size divided by the subject size. At 1 to 1 magnification the factor is (1 plus 1) squared, which is 4. This is an equivalent way to compute the same correction.
Official sources
Reviewed by the CalculatorHub team, edited by James Graham, 17 June 2026. See our methodology.