The correction
—
corrected = meteredp, the Schwarzschild approximation · fits of manufacturer data, good to about ⅓ stop · under 1s no correction is needed
Past one second, film stops honouring the meter — long exposures come out thin unless you give extra. Metered time in, your stock's corrected time out. The live readings happen on the meter page; this is the long-exposure arithmetic.
corrected = meteredp, the Schwarzschild approximation · fits of manufacturer data, good to about ⅓ stop · under 1s no correction is needed
Film's whole deal is a fair trade: half the light, double the time, same negative. Photographers call that bargain reciprocity, and for every shutter speed you'd use handheld it holds perfectly. Past about a second it starts to fail — the silver-halide crystal needs several photons in quick succession to form a stable latent-image speck, and in dim light the first photon's work fades before reinforcements arrive. The film is quietly forgetting light while you expose it.
So a metered 30 seconds is not enough seconds. How much extra depends on the emulsion: T-Max 100 barely flinches, HP5+ wants roughly double, Fomapan wants your evening. The correction here is the Schwarzschild approximation — corrected = meteredp, one exponent per stock, fitted to the maker's published curves. Ilford prints its exponents in the datasheets; the rest are the fits the long-exposure community has converged on.
metered 10s and 60s shown corrected, so you can feel each stock's appetite
| film | p | 10s → | 60s → |
|---|
← back to the light meter — where the film dial applies this correction automatically when the solved shutter runs long.