A meter that reads correctly in daylight can underexpose badly once the indicated shutter time stretches into seconds. The cause is reciprocity failure: at low light levels the film stops responding in proportion to exposure, and the negative comes back thin unless the time is extended. Understanding the effect, and the data manufacturers publish for it, is what separates a usable night or pinhole negative from a wasted one.

Why sensitivity falls off

The reciprocity law states that exposure is the product of illuminance and time, so halving the light and doubling the time should yield the same density. The relationship holds across the normal range of shutter speeds but breaks down at its extremes. During very long, dim exposures the effect is described as low-intensity reciprocity failure, and it is the form that concerns long-exposure work.

The mechanism is photochemical. A developable image requires that several photons strike a silver halide grain within a short interval to build a stable latent-image centre. At low light levels the photons arrive too sparsely, and partially formed centres decay before they are completed. Ilford’s technical information on reciprocity failure attributes the loss to this reduced efficiency in forming stable development centres, with the result that effective film speed drops as exposure time grows. Because the shortfall compounds, the correction is not linear: doubling an already long metered time is not enough.

Reading a stock’s reciprocity data

Manufacturers express the correction in two common ways. Kodak publishes a discrete table. Its datasheet F-4016 for T-MAX 100 specifies no adjustment for indicated times between roughly 1/1000 and 1/10 second; at 1 second it calls for plus one-third stop, at a metered 10 seconds for plus one-half stop (an adjusted time of 15 seconds), and at 100 seconds for a full stop, giving an adjusted exposure of 200 seconds.

Ilford instead publishes a single exponent per emulsion and a formula. Its compensation method uses Tc = Tm^P, where Tm is the metered time in seconds, Tc the corrected time, and P a factor determined for each film. Ilford lists P as 1.31 for HP5 Plus, 1.26 for FP4 Plus and Delta 100, 1.41 for Delta 400, and 1.33 for Pan F Plus. A higher exponent means a steeper penalty at long times. Both makers note that exposures of one second or shorter need no compensation.

Applying the correction

Worked against the Ilford formula, a metered 10 seconds on HP5 Plus becomes 10^1.31, or about 20 seconds — the example Ilford’s own document uses. The same exponent turns a metered 30 seconds into roughly 90, and a metered minute into about three and a half. The Kodak table is interpolated between its listed points and rounded to convenient values.

Two secondary effects accompany the correction. Contrast tends to rise, because shadows sit deeper in the reciprocity region than highlights and lose proportionally more density; Ilford suggests reducing development to compensate. And at very low light levels metering accuracy itself degrades, so manufacturers concede that extreme exposures may require bracketing rather than calculation alone. The published figures are a reliable starting point, not a guarantee of a perfect negative.