Film loses sensitivity in long exposures, so a metered time over one second must be lengthened. Ilford models this as corrected time = metered time raised to a film exponent P. For HP5 Plus, P is 1.31, so a metered 10 seconds becomes 10^1.31 ≈ 20.4 seconds, about one stop of extra exposure.
Black-and-white film photographers shooting at night, in dim interiors or with dense ND filters run into reciprocity failure: past about one second, doubling the exposure no longer doubles the density on the negative. A light meter or a digital camera reading does not know this, so its suggested time underexposes the film. This calculator takes the metered time and the film stock, applies the manufacturer's power-law exponent, and returns the corrected time as a count of seconds and as minutes, the extra exposure in stops and the multiplier over the reading. It shows the working for each figure. Exponents are taken from Ilford's published compensation sheet and cover Ilford and Kentmere films; for other stocks, enter an exponent from the maker's data. The model does not correct colour shifts in slide or colour negative films, which need filtration as well as time.
Choose your stock from the list, or Custom exponent to type a P value from another data sheet.
Type the exposure time your meter or camera suggests, as a number of seconds.
Use the corrected time, given both as seconds and as minutes, and note the extra stops if you prefer to open the aperture instead.
Result Corrected exposure (seconds): 20.4
Corrected time: 20.4 s
Extra exposure (stops): 1.03
Multiplier on metered time: 2.04
A night street scene metering at 10 seconds needs about 20 seconds on HP5 Plus, roughly one extra stop.
Result Corrected exposure (seconds): 174
Corrected time: 2 min 54 s
Extra exposure (stops): 1.54
Multiplier on metered time: 2.9
A one-minute reading on FP4 Plus stretches to just under three minutes once reciprocity failure is allowed for.
Result Corrected exposure (seconds): 8
Corrected time: 8 s
Extra exposure (stops): 1
Multiplier on metered time: 2
With an exponent of 1.5, a 4 second reading becomes 4^1.5 = 8 seconds, exactly one stop more.
Tc = Tm^P, extra stops = log2(Tc ÷ Tm)
TmTcPIlford publishes a power law for its black-and-white films: the corrected time is the metered time raised to a film-specific exponent. It only applies when the metered time exceeds one second; shorter exposures keep the reciprocity law and need no change. The extra stops figure is the base-2 logarithm of the ratio between corrected and metered time, which is useful if you would rather widen the aperture than wait longer.
The reciprocity law says that halving the light and doubling the time gives the same exposure. Film emulsions follow it between roughly 1/1000 s and 1 s, but at long times the latent image forms less efficiently, so the film behaves as if it received less light than the meter assumed.
Ilford's compensation formula is defined for metered times above one second. Below that, its films follow the reciprocity law closely enough that the metered time is used unchanged, so the calculator returns your input and adds a note.
They come from Ilford's reciprocity law failure compensation sheet, which lists a P value for each current Ilford and Kentmere black-and-white film. For example FP4 Plus is 1.26, HP5 Plus 1.31 and Delta 400 1.41.
Yes, with the Custom exponent option, if the maker publishes a power-law value. Kodak and Foma usually publish tables or curves instead; in that case read their table directly, or fit an exponent from two of their points and enter it here.
Opening up by the extra stops shown shortens the exposure, but reciprocity failure then changes because the metered time itself changes. Recalculate with the new metered time, as the correction shrinks rapidly as times get shorter.
Apply filter and bellows factors to the metered time first, then run the corrected total through the formula. Correcting first and multiplying afterwards gives too short an exposure on long times.
Using this tool in an assignment, article or report? Copy a citation, or a link for your website.
Tools Directory Editorial Team. (2026, October 16). Reciprocity Calculator. Tools Directory. Retrieved October 27, 2026, from https://tools.milunir.com/photography/exposure-and-camera-settings/reciprocity-calculator/
Tools Directory Editorial Team. “Reciprocity Calculator.” Tools Directory, 16 Oct. 2026, https://tools.milunir.com/photography/exposure-and-camera-settings/reciprocity-calculator/. Accessed 27 Oct. 2026.
Tools Directory Editorial Team. “Reciprocity Calculator.” Tools Directory. Last modified October 16, 2026. Accessed October 27, 2026. https://tools.milunir.com/photography/exposure-and-camera-settings/reciprocity-calculator/.
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