Macro Bellows Exposure Factor Calculator

Method, formula and assumptions

For a symmetric thin lens, magnification = added extension/focal length. Exposure factor = (1 + magnification)^2; effective f-number = marked f-number x (1 + magnification).

Input definitions and supported units

• Lens focal length (mm): Measured or assumed numerical value, from 0.1 to 2000; must be positive.
• Additional extension beyond infinity (mm): Measured or assumed numerical value, from 0 to 2000; zero is supported where the stated method permits it.
• Marked f-number: Measured or assumed numerical value, from 0.1 to 128; must be positive.
• Infinity-metered shutter time (seconds): Measured or assumed numerical value, from 0.000001 to 10000; must be positive.

Worked example with answer

50 mm added extension on a 50 mm lens gives 1x magnification and a 4x exposure factor, or two stops. Marked f/8 becomes effective f/16; 0.01 seconds becomes 0.04 seconds.

How to use the results

Use extension beyond the infinity-focus configuration, not the full lens-to-sensor distance. Through-the-lens metering may already compensate, so do not add this twice.

Results retain full precision internally and display up to four decimal places. Very small nonzero results use scientific notation rather than appearing as zero. Check the labels before comparing values across units.

Limitations and references

Internal focusing, pupil magnification and asymmetric lenses change the model. It is not a lens specification or a precise macro exposure calibration.

The method is a self-contained arithmetic, geometric or explicitly stated mathematical model. There are no imported tariffs, empirical presets or equipment specifications; measured values and allowances must come from your own process.

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