Bellows Extension & Exposure Calculator
Enter a focused bellows measurement, a ground-glass image measurement, or a target magnification to find the exposure correction and extension needed for close-up work.
Exposure compensation (stops)1.17
- Magnification
- 0.5
- Reproduction ratio
- 1:2
- Required bellows extension (mm)
- 225
- Extension beyond infinity (mm)
- 75
- Exposure factor
- 2.25
- Corrected shutter time (s)
- 0.28
- Corrected aperture
- f/10.67
- Effective aperture
- f/24
Assumes consistent measurement points and a symmetric thin lens for effective aperture.
Do not add this correction again if through-the-lens metering already measured light at the film or sensor plane.
How to use this calculator
- Choose whether you are calculating from measured bellows extension, image size, or a target magnification.
- Enter the infinity-focus extension for the lens, using a measured value when possible.
- Fill in the measurement fields for the selected method; the other method fields are ignored.
- Enter the metered shutter time and marked aperture to see the corrected time, corrected f-number, and effective aperture.
How the bellows factor is calculated
Close focusing moves the lens farther from the film or sensor. The image is spread over a larger area, so the exposure has to increase unless the camera is metering through the taking lens at the film or sensor plane. This calculator accepts three common field workflows: direct bellows extension, image size compared with the real subject, or a target magnification for planning a setup.
Let B be the lens-to-film distance at infinity focus, E the focused extension, I the image size on the ground glass, O the matching real subject size, m the magnification, Q the exposure factor, t the metered shutter time, and N the marked f-number. The selected method sets magnification:
- Measured extension:
m = E / B - 1. - Image and subject size:
m = I / O. - Target magnification:
mis the value you enter.
Required extension is Erequired = B * (1 + m), and extension beyond infinity is DeltaE = B * m. The exposure factor is Q = (E / B)^2, which is the same as (1 + m)^2. Stops are C = log2(Q). If you keep the aperture unchanged, use tc = t * Q. If you keep the shutter time unchanged, open the lens to Nc = N / sqrt(Q). The symmetric-lens effective aperture is Neff = N * (1 + m).
What changes the correction most
Magnification is the main driver because the exposure factor uses the square of 1 + m. At 1:2, the factor is 2.25, or about 1.17 stops. At 1:1, the factor is 4, or exactly 2 stops. A small error in the infinity-focus reference also matters, especially with telephoto, retrofocus, and asymmetric lenses where marked focal length may not match the measured optical distance.
What this calculator leaves out
The effective-aperture result assumes a symmetric thin lens. Some lenses need pupil-magnification correction, and many large-format setups make exact extension hard to measure when tilt, swing, rise, or fall are applied. The calculator also does not add film reciprocity-failure compensation, shutter-speed rounding, lens detent availability, TTL metering behavior, or manufacturer-specific guidance for very long exposures.
Worked example
Suppose a 150 mm lens measures 150 mm from lens reference plane to film at infinity and 225 mm after focusing. The magnification is 225 / 150 - 1 = 0.5, so the reproduction ratio is 1:2. Required extension is 150 * (1 + 0.5) = 225 mm, and the extra extension beyond infinity is 150 * 0.5 = 75 mm.
The exposure factor is (1 + 0.5)^2 = 2.25, which is log2(2.25) = 1.17 stops. A metered 1/8 second exposure becomes 0.125 * 2.25 = 0.28125 seconds if the aperture stays at f/16. If the shutter time stays fixed, the aperture-only correction is 16 / sqrt(2.25) = f/10.67. The symmetric-lens effective aperture is 16 * 1.5 = f/24.
Common questions
Why does bellows extension require extra exposure?
Close focusing increases the lens-to-film distance and spreads the same light over a larger image circle. The loss follows the square of the extension ratio, so doubling extension from the infinity reference needs four times the exposure.
How do I measure bellows extension on a view camera?
Measure from the same lens reference point to the film plane at infinity focus, then measure the same distance again after focusing close. The iris plane is a useful approximation for many large-format lenses, but consistency matters more than the exact external landmark.
Can I calculate bellows factor from image size instead?
Yes. Measure a feature on the subject and the same feature in the focused image on the ground glass. Image size divided by real subject size gives magnification, and the exposure factor is then one plus magnification squared.
How much extension is needed for 1:1 magnification?
For a symmetric thin-lens estimate, 1:1 magnification needs twice the infinity-focus extension. A 150 mm infinity reference therefore needs about 300 mm of bellows draw, with an exposure factor of 4, or 2 stops.
Does TTL metering already correct for bellows extension?
Usually yes, if the meter reads light through the lens at the film or sensor plane during the actual setup. Do not apply the correction twice; use this calculator mainly with handheld meters, external flash meters, or reverse planning.
Why does effective aperture matter?
Effective aperture describes the working f-number after magnification is included. It matters for diffraction, flash exposure, and depth-of-field estimates even when you decide to compensate by changing shutter time instead of opening the aperture.