How this calculation works
Moving a unit-focusing lens farther from its image plane spreads the same light over a larger image. Close-up exposure can therefore require compensation even when the marked aperture is unchanged. This page evaluates the symmetric thin-lens bellows factor from extension beyond infinity focus. It reports a time multiplier, additional stops and an effective f number under an explicit pupil-magnification assumption.
Magnification = added extension/f; exposure factor = (1+m)²; stops = log2(factor)A worked example
A 50 mm lens with 50 mm added extension has ideal magnification one. Exposure factor is (1+1)² = 4, so a 0.01 s reference becomes 0.04 s. The marked f/8 becomes effective f/16 under the symmetric-lens assumption. With only 25 mm added extension, magnification is 0.5 and the factor falls to 2.25.
Good to know
Internal-focus and asymmetric lenses can have different pupil behavior and effective focal length at close distance. This calculation does not calibrate those designs. Flash exposure geometry, film reciprocity failure, illumination changes and automated camera compensation are not included. Use the factor as an explicit ideal comparison rather than an instruction to override an actual metered exposure.
Reference: Nikon MicroscopyU — image brightness and magnification (opens in a new tab) ↗
What this calculation answers
Moving a unit-focusing lens farther from its image plane spreads the same light over a larger image. Close-up exposure can therefore require compensation even when the marked aperture is unchanged.
This page evaluates the symmetric thin-lens bellows factor from extension beyond infinity focus. It reports a time multiplier, additional stops and an effective f number under an explicit pupil-magnification assumption.
The Thin lens real and virtual image magnification calculator connects image distance with linear magnification; close-focus image extension also changes the illumination represented by this symmetric bellows factor.
Use the right input conventions
Extension means extra image distance beyond the focal length, not the complete lens-to-sensor distance. At zero added extension the factor is one. Enter focal length and extension in millimetres.
The model assumes pupil magnification one, as for an ideal symmetric lens. The marked f number is used only to derive the corresponding effective f number; it does not change the extension ratio itself.
Interpret the result
A factor of four means four times the exposure duration at unchanged illumination, equivalent to two additional stops. It does not mean four stops. The logarithm converts the linear exposure multiplier into photographic stop units.
Through-the-lens metering may already compensate for reduced image-plane illumination. Applying this factor again would double-correct such a reading; the entered base time must be an uncompensated reference for the comparison.
Work through the example
A 50 mm lens with 50 mm added extension has ideal magnification one. Exposure factor is (1+1)² = 4, so a 0.01 s reference becomes 0.04 s.
The marked f/8 becomes effective f/16 under the symmetric-lens assumption. With only 25 mm added extension, magnification is 0.5 and the factor falls to 2.25.
Where the model stops
Internal-focus and asymmetric lenses can have different pupil behavior and effective focal length at close distance. This calculation does not calibrate those designs.
Flash exposure geometry, film reciprocity failure, illumination changes and automated camera compensation are not included. Use the factor as an explicit ideal comparison rather than an instruction to override an actual metered exposure.
A few questions, answered
Should total bellows length be entered as extension?
No. Enter only the added distance beyond the infinity-focus image distance.
What changes in the comparison?
Halve added extension and compare the nonlinear exposure factor.