Pinhole and exposure

Work from a measured hole and film distance; keep the optical exposure separate from a material-specific reciprocity correction.

Pinhole-to-film distance (mm)Measured pinhole diameter (mm)Film width (mm)Metered exposure (s)Meter f-numberReciprocity exponent from your material’s data (1 = no correction)Design wavelength (nm)Pinhole design constant (state your convention)

Calculate

Enter your values and calculate. Read the assumptions below before using a result.

Method and assumptions

The exposure calculation uses the measured hole. Effective f-number is film distance divided by hole diameter. Exposure at otherwise identical illumination scales as the square of the ratio of f-numbers. The horizontal field calculation assumes a centred pinhole and a flat film plane; a curved-plane camera needs its own geometry.

For a 50 mm distance and a 0.25 mm hole, the aperture is f/200. A one-second reading at f/100 becomes four seconds optically. That result does not yet include reciprocity failure. The optional exponent implements only the power law you explicitly supply, with time expressed in seconds relative to one second. An exponent of 1 makes no correction; it does not assert that your film obeys reciprocity. Use a manufacturer-supported law in its stated time range, or keep the optical result and consult your own measured correction curve.

The separate design calculation is d = k√(fλ), with wavelength converted from nanometres to millimetres. Its constant is an explicit convention, not a universal optimum. The default k = 2 is a design example; changing the wavelength or sharpness criterion changes the result. Do not let the theoretical diameter replace a measured aperture in the exposure log.

Record film distance, actual diameter, wavelength and constant for the design, metered exposure, correction source, predicted exposure and the final result. Compare sharpness and illumination across the whole image. A good central image does not prove adequate corner coverage or light-tightness.

Evidence and record

Read the teaching method and its sources. Keep your inputs, the displayed assumptions and the result together; a result detached from its conditions cannot be checked.