Mr Pinhole Calculator
Use this tool to estimate the optimal pinhole diameter, resulting f-number, and an adjusted exposure time from your meter reading.
What this pinhole calculator does
The Mr Pinhole Calculator is built to help you move quickly from idea to image. Pinhole photography is simple in concept, but exposure and sharpness are not always intuitive. A tiny change in hole size or camera depth can radically change brightness and detail. This calculator gives you a practical baseline so you can spend more time shooting and less time guessing.
It estimates three key values:
- Optimal pinhole diameter based on focal distance and wavelength.
- Effective f-number for your pinhole camera setup.
- Exposure time scaled from a metered exposure, with optional reciprocity correction.
The core formulas
1) Optimal pinhole diameter
A commonly used approximation is:
Where:
- d = pinhole diameter
- f = pinhole-to-film distance (focal length equivalent)
- λ = wavelength of light
This formula balances geometric blur (too large a hole) and diffraction blur (too small a hole). It is not magic, but it is an excellent starting point for real-world builds.
2) Effective f-number
Once diameter is known, the aperture is:
Pinhole apertures are often very small in light-gathering terms, commonly around f/120 to f/300 depending on camera depth and hole size.
3) Exposure conversion
To convert from a meter reading:
If your material suffers reciprocity failure, the corrected time can be approximated using a power factor:
Set k = 1 when no correction is needed, or increase it slightly for film that under-responds at long exposures.
How to use this in practice
Simple workflow
- Measure camera depth from pinhole to film/sensor plane.
- Enter that value in millimeters.
- Keep wavelength at 550nm unless you have a specific reason to change it.
- Take a meter reading (aperture + shutter pair).
- Enter reciprocity exponent if your film data sheet suggests correction.
- Shoot, then bracket around the suggested time (for example, 0.7× and 1.4×).
Why bracketing still matters
Even great formulas cannot account for every variable: pinhole edge quality, light leaks, spectral shifts, development differences, and scene contrast. The calculator gives a mathematically sound center point; your test shots fine-tune it for your specific camera and material.
Example scenario
Suppose your camera depth is 50mm. Using 550nm light, the calculator may return a pinhole around 0.32mm and an effective aperture near f/156. If your meter says 1/125s at f/16, the pinhole exposure may land around one second or longer, depending on final values and reciprocity setting.
That jump surprises many photographers at first, but it is normal in pinhole work. Long exposures are part of the aesthetic.
Tips for better results
- Use thin metal (like shim brass or aluminum) for cleaner pinhole edges.
- Deburr gently and inspect with magnification.
- Keep interior camera surfaces matte black to reduce flare.
- Record every shot: scene, meter reading, calculated time, actual time, and result.
- Build your own correction habit for each film stock and developer combination.
Final note
The best pinhole image quality comes from a mix of math, craft, and iteration. This calculator handles the math quickly so you can focus on composition, timing, and creative experimentation. Use it as your launch point, then refine with your own field notes.