pinhole exposure calculator

What this pinhole exposure calculator does

Pinhole photography usually means very small apertures like f/138, f/180, or even f/250. Your handheld meter (or phone app) gives exposure for a much larger aperture, so the shutter time you actually need with a pinhole can be dramatically longer. This calculator converts your metered exposure to a pinhole exposure and then applies optional reciprocity-failure correction for film.

In short, it helps you avoid guesswork and underexposed negatives.

How the calculation works

1) Aperture conversion

Exposure time scales with the square of the f-number ratio. If your meter says t_meter at f_meter, and your camera is actually at f_pinhole, then:

Base pinhole time = t_meter × (f_pinhole / f_meter)²

This alone can push a fraction-of-a-second meter reading into several seconds or minutes.

2) Reciprocity correction (film only)

Film becomes less efficient during long exposures. This is called reciprocity failure. A common practical approximation is to raise the base exposure to an exponent greater than 1:

Corrected time = (Base time)^p (usually for base times longer than ~1 second)

Different films have different behavior, so the result is an estimate. Always bracket if the shot matters.

Suggested reciprocity exponents

Profile	Approximation	Use case
No correction	p = 1.00	Digital sensors, direct-positive paper tests, or when correction handled elsewhere
Ilford HP5+	p ≈ 1.31	General black-and-white pinhole workflows
Ilford FP4+	p ≈ 1.33	Fine-grain B&W with moderate-long exposures
Kodak Tri-X 400	p ≈ 1.45	Stronger correction for longer times

Worked example

Suppose your meter reads 1/30 s at f/8, and your pinhole camera is f/180.

• Aperture ratio: 180 / 8 = 22.5
• Squared ratio: 22.5² = 506.25
• Base pinhole time: 0.0333 × 506.25 ≈ 16.9 s
• With HP5+ reciprocity (p = 1.31): 16.9^1.31 ≈ 40.6 s

So you would expose for about 40 seconds, then consider a bracket around that value (for example 30s, 40s, 55s).

Practical field workflow

1. Meter the scene at any convenient aperture/time pair.
2. Enter the meter time and meter f-number into the calculator.
3. Enter your camera's pinhole f-number.
4. Select a reciprocity profile that matches your film.
5. Shoot the calculated time and bracket one stop around it.

Tips for more consistent negatives

Know your true f-number

Nominal pinhole sizes can differ from actual sizes due to burrs and manufacturing variation. If possible, measure the pinhole diameter with a loupe/microscope and compute f-number from focal length ÷ diameter.

Use a stable shutter routine

For exposures above a few seconds, operator timing error matters less. For short times (1–3 seconds), practice opening/closing the shutter smoothly to reduce vibration and timing variance.

Bracket whenever light is tricky

Backlit scenes, high contrast, and uncertain reciprocity behavior are perfect times to bracket. One extra frame can save a whole trip.

About focal length and pinhole size

If you provide focal length in the calculator, it also displays your effective pinhole diameter from the entered f-number. This helps check if your camera geometry and expected aperture match reality.

Rule of thumb for an “optimum” pinhole diameter is based on wavelength and focal length. In practice, image character is often more important than strict sharpness, so treat optimum formulas as a starting point, not a hard rule.