Quick answer
A 50 mm pinhole-to-image distance with a 0.26 mm hole has an equivalent f-number of about f/192.31. Compared with f/8 at 0.008 seconds, the square-law estimate is about 4.62 seconds at the same ISO and scene brightness.
The suggested 0.259 mm hole is a visible-light starting point. Pinhole exposures are sensitive to diffraction, reciprocity failure, material thickness, stray light, and the actual shape of the opening.
How to use the calculator
- Measure the distance from the pinhole to the film, photographic paper, or sensor in millimetres.
- Measure or obtain the pinhole diameter in millimetres. A nominal needle size is not always the actual opening.
- Enter a reference f-number and shutter time for the same ISO and scene brightness.
- Use the estimated time as a starting point, then bracket exposures because long-exposure materials do not respond perfectly to the simple square-law comparison.
What each result means
- Equivalent f-number: image distance divided by pinhole diameter. A larger number means a dimmer opening.
- Exposure multiplier: the square of the pinhole f-number divided by the reference f-number.
- Estimated shutter time: reference shutter time multiplied by that exposure multiplier.
- Suggested pinhole diameter: a visible-light compromise estimate; it is not a substitute for trying the material and camera.
- 1/100 guideline: a simple check on hole size relative to image distance. It does not account for every diffraction or vignetting condition.
Decisions this calculator helps with
Choose a starting exposure
Translate a normal-lens reference exposure into a first pinhole shutter-time estimate.
Check a handmade hole
Compare the measured diameter with a visible-light starting point and the simple focal-length guideline.
Compare camera distances
See how moving the film or paper changes both the f-number and suggested opening.
Plan a bracket
Save the calculated time, then try shorter and longer exposures around it for the actual material and scene.
Pinhole exposure formulas
For image distance f, pinhole diameter d, reference f-number Nref, and reference exposure time tref:
- Equivalent f-number = f ÷ d.
- Exposure multiplier = (equivalent f-number ÷ Nref)².
- Estimated pinhole time = tref × exposure multiplier.
- Suggested diameter ≈ 0.0366 × √f, with f and diameter in millimetres for a 550 nm visible-light approximation.
The shutter-time comparison assumes unchanged ISO and scene luminance. It does not model the film or paper reciprocity curve, development, flare, or the detailed diffraction pattern.
Assumptions and limitations
- The reference exposure is valid for the same subject brightness, ISO, and recording material sensitivity.
- The pinhole is treated as a round opening with an effective diameter equal to the entered measurement.
- Long exposures can suffer reciprocity failure: doubling the calculated time is not always enough to double the recorded exposure.
- A hole that is too small can lose sharpness to diffraction; a hole that is too large can increase geometric blur and vignetting.
- Paper, film, and digital sensors need different practical workflows. Follow the recording material's instructions and protect sensitive equipment from stray light.
Worked pinhole exposure examples
| Inputs | Calculation | Estimated result |
|---|---|---|
| 50 mm, 0.26 mm hole; f/8 at 0.008 s | 50 ÷ 0.26 = f/192.31; (192.31 ÷ 8)² | 577.85×; about 4.62 s |
| 100 mm, 0.37 mm hole; f/5.6 at 0.0167 s | 100 ÷ 0.37 = f/270.27; square the f-number ratio | 2,329.27×; about 38.90 s |
| 25 mm, 0.18 mm hole; f/4 at 0.004 s | 25 ÷ 0.18 = f/138.89; square the f-number ratio | 1,205.63×; about 4.82 s |
Common mistakes when estimating pinhole exposure
- Using the pinhole-to-subject distance instead of the pinhole-to-image distance in the f-number calculation.
- Comparing a reference exposure made at a different ISO, scene brightness, or metering point.
- Assuming the suggested diameter guarantees the sharpest result in a thick or irregular piece of material.
- Ignoring reciprocity failure during exposures longer than the recording material's linear-response range.
- Treating the calculator's shutter time as a guaranteed final exposure instead of a bracket starting point.
Pinhole camera exposure FAQ
How is a pinhole camera f-number calculated?
Divide the distance from the pinhole to the film, paper, or sensor by the pinhole diameter, using the same units. A 50 mm distance and 0.25 mm hole gives an equivalent f-number of f/200.
How does the calculator estimate shutter time?
It compares the pinhole equivalent f-number with a reference aperture. For the same ISO and scene brightness, exposure time scales with the square of the f-number ratio.
What is the suggested pinhole diameter?
The visible-light starting point shown here is Young's simplified approximation d ≈ 0.0366√f, with d and f in millimetres. It is a diffraction-versus-geometric-blur compromise, not a guarantee of maximum sharpness for every material or wavelength.
Why can the real exposure differ from the estimate?
Film and photographic paper can show reciprocity failure during long exposures. Light leaks, material thickness, diffraction, development, scene contrast, and the actual hole shape also matter.
Can I use a normal camera light meter?
A meter can help establish the reference exposure, but many pinhole setups are far slower than the camera meter range. Use the result as a starting point and bracket exposures.
Does the calculator account for ISO?
No separate ISO input is needed because the reference exposure is assumed to use the same ISO as the pinhole exposure. Changing ISO changes the reference exposure you should enter.
Sources and method notes
References used for the f-number, exposure comparison, and pinhole-size method:
- The equivalent f-number follows the standard image-distance ÷ opening-diameter relationship.
- The exposure estimate uses the square of the f-number ratio while holding ISO and scene luminance constant.
- The suggested diameter uses the visible-light simplified approximation d ≈ 0.0366√f, with f and d measured in millimetres.
- Long-exposure reciprocity failure and diffraction are intentionally presented as limitations rather than hidden in the result.
- Calculation method: PINHOLE_EXPOSURE-1.0.
Sensitivity hint: the exposure multiplier grows with the square of the f-number ratio, so a smaller hole or longer image distance can move the estimated time quickly.
Updated: July 25, 2026. Last reviewed: July 25, 2026.
What to do next
Use the estimated time to make a first exposure, then bracket around it and record the actual hole, material, scene, ISO, and development process. For another practical hobby planning tool, see the wrapping paper calculator. If the result looks inconsistent with the displayed formulas, report an issue with the inputs and expected result.