How do I convert a bellows factor to stops?

Added stops equal log base 2 of the exposure factor. A factor of 2 is one stop, 4 is two stops, and 8 is three stops.

Where should I measure bellows extension?

The formula uses image distance from the film or sensor plane to the lens's rear nodal point. The lens board or diaphragm position is a field approximation for many conventional large-format lenses.

Does TTL metering already correct for bellows extension?

A through-the-lens meter normally observes the light reaching the camera and may already include extension loss. Do not apply the correction twice; consult the camera system instructions.

Why may the result differ for telephoto or retrofocus lenses?

The lens board may be far from the rear nodal point in asymmetric designs. Internal focusing, pupil magnification, and extension measured from the wrong reference plane can make the simple formula inaccurate.

Is bellows compensation the same as reciprocity correction?

No. Bellows compensation corrects geometric light loss. Film reciprocity failure is an additional material-specific correction that may be required for long exposures.

Are my camera settings uploaded?

No. Calculation, copying, and CSV generation happen locally in the browser.

Bellows Exposure Compensation Calculator

Calculate the extension factor and added stops caused by close focusing, then convert a metered shutter time, aperture, or ISO into a compensated setting. Inputs stay in your browser.

Lens, extension, and metered settings

Length unit Focal length and extension must use the same unit.

Lens focal length (mm) Use actual focal length, not a crop-equivalent value.

Total optical extension (mm) Film/sensor plane to rear nodal point after focusing.

Metered shutter time (optional) Seconds or a fraction: 1/125, 0.5, or 2.

Marked aperture (optional) Enter the f-number only, such as 8 or 16.

Metered ISO (optional) Used only to show the ISO-only compensation option.

No input tracking: calculations, copying, and CSV generation happen locally on this device.

Exposure compensation

Bellows exposure factor 2.25× Add 1.17 stops of exposure

Extension ratio1.500×Total extension ÷ focal length

Estimated magnification0.50×Simple thin-lens estimate

Corrected shutter time1/55.6 s0.018 s exact

Effective aperturef/24At the entered f/16 setting

Aperture-only settingf/10.7Instead of changing shutter or ISO

ISO-only settingISO 225Instead of changing shutter or aperture

Choose one compensation method, not all three. Lengthening shutter time preserves the selected aperture and depth-of-field intent.

Bellows extension reference

The table uses the focal length and optional metered settings above. The highlighted row is the current extension ratio.

Extension ratio	Total extension	Exposure factor	Added stops	Corrected shutter	Effective aperture
Enter valid values to build the reference table.

How to calculate bellows exposure compensation

Compose and focus first. Bellows draw changes with focus, so measure the camera in its final focused position.
Enter the actual focal length. A 150 mm lens remains 150 mm regardless of film or sensor format.
Measure total optical extension. Measure from the film or sensor plane to the rear nodal point. For many conventional large-format lenses, the diaphragm or lens-board plane is a practical field approximation.
Enter the meter reading. Shutter, aperture, and ISO are optional; the factor and stops require only the two lengths.
Apply one compensation. Multiply shutter time by the factor, open aperture by the displayed stop amount, or multiply ISO by the factor. Do not apply all three.
Check the capture. Bracket film or inspect a digital test frame when the subject is critical or the lens design makes physical measurement uncertain.

Bellows factor formulas

Let E be total optical extension, f lens focal length, F exposure factor, and C the added compensation in stops. Both lengths must use the same unit.

F = (E / f)²

C = log₂(F) = 2 × log₂(E / f)

corrected shutter time = metered time × F

A 150 mm lens at 225 mm of total optical extension has an extension ratio of 1.5. Its factor is 1.5² = 2.25, equivalent to log₂(2.25) ≈ 1.17 stops. A metered 1/125 second becomes 2.25/125 = 0.018 second, or about 1/55.6 second.

Magnification relationship

For a simple thin lens, image distance is approximately f × (1 + m). Therefore m ≈ E/f − 1 and the same factor can be written (1 + m)².

Effective aperture

Under the same symmetric-lens assumption, effective f-number is the marked f-number multiplied by E/f. At f/16 and a 1.5× extension ratio, the effective aperture is approximately f/24.

Aperture-only correction

To compensate entirely with aperture, divide the marked f-number by E/f. This changes depth of field and may be impossible when the lens is already near maximum aperture.

Shutter or ISO correction

Multiply either shutter time or ISO by F. ISO 100 becomes ISO 225 for a 2.25× factor. These are alternatives, not cumulative changes.

Reference: the official Kodak Gray Card guide gives effective f-number as f × (M + 1), exposure factor as (M + 1)², and instructs photographers to multiply exposure time by the factor. Schneider-Kreuznach’s optical imaging formulas describe the focal-length, extension, and magnification relationship.

Measurement accuracy and limits

Field-calculation limit: this is a simple-lens planning model, not a camera or lens calibration. Use manufacturer data, TTL metering, a calibrated meter, or a test exposure when precision matters.

The extension in the formula is an optical image distance, not automatically the outside length of the camera. Measuring to the lens board or diaphragm works reasonably for many conventional lenses because the rear nodal point is nearby, but it can be misleading for telephoto, retrofocus, strongly asymmetric, or internally focusing designs.

The calculator does not model pupil magnification, lens transmission, filters, flash falloff, film processing, or film reciprocity failure. Reciprocity correction is separate and can make a long corrected film exposure longer still. If a through-the-lens meter reads after the lens and extension, it may already observe the light loss; applying a second correction could overexpose the image.

Bellows exposure compensation FAQ

What is the bellows exposure factor?

It is the multiplier applied to metered exposure time to offset light lost as the lens moves farther from the film or sensor. In the simple extension method, factor = (total optical extension ÷ focal length)².

How do I convert the factor to stops?

Take log₂ of the factor. Factor 2 is one stop, factor 4 is two stops, and factor 8 is three stops. A factor of 2.25 is about 1.17 stops.

Is bellows extension the extra draw beyond infinity?

No. Enter the total optical image distance after focusing, not only the added movement beyond the infinity position. At infinity, a simple lens has E/f = 1 and factor 1.

Where should I measure the extension?

Measure from the film or sensor plane to the rear nodal point. The diaphragm or lens-board plane is commonly used as a practical approximation for conventional large-format lenses, but it is not universal.

Does TTL metering already compensate?

Usually a meter reading through the taking lens includes the light actually reaching the camera, but systems differ. Consult the camera instructions and avoid applying the same loss twice.

Why might a telephoto lens give a misleading result?

A telephoto lens can place its rear nodal point well away from the lens board, so physical board-to-film distance is not the optical extension required by the formula.

Is reciprocity failure included?

No. Bellows compensation and reciprocity failure are separate. After calculating the bellows-corrected time, apply the film maker’s reciprocity data when relevant.

Are my settings uploaded?

No. The page calculates, copies, and creates its CSV entirely in your browser.