Reciprocity Failure

Reciprocity Failure is the breakdown of the normal reciprocity behaviour of a film, causing (for long exposures) underexposed shadow areas and an increase in contrast.

Another form, short-exposure reciprocity failure, occurs when light arrives too rapidly for the film to register it; this is generally not seen with currently available films, even at the very short (200us to 3ms) exposures seen with flash photography.


Good reciprocity performance means that a film responds only to the total quantity of light hitting it without regard to how rapidly the light is arriving. If operating within the normal reciprocity region of the film, you can halve the brightness (stop the aperture down one stop) and double the length of exposure (one stop slower shutter) to get the same number of photons arriving at the film and the same exposure (ignoring depth of field changes) in your final image.

Likewise you can in theory add a 3-stop neutral density filter, make your exposure 8 times longer and get the same final result except for motion blur.

Reciprocity Failure, a simplified physical explanation

The problem with silver halide film is that most emulsions require the absorption of several photons to activate the halide into a stable higher-energy state. If an insufficient number of photons is received, the halide will transition to a higher energy state that isn't stable and then after some (very) short period of time, decay back down to the unexposed, minimal-energy state.

If the rate of photon arrival is high, a halide molecule absorbing only a single photon is likely to absorb another photon before it decays again, so the rate of arrival doesn't really matter and the film exhibits reciprocity.

If the rate of photon arrival is low, there is a finite probability that a partially energised halide will decay before absorbing the next photon. As the illumination decreases (and therefore the photon arrival rate slows), the probability of decay increases accordingly and the effective sensitivity of the film is reduced because more of the incoming photons are effectively lost to the decay process.

Effect on Photos

The net effect is that the film is less sensitive at lower levels of illumination. At a minimum, this means that for a long exposure, you must increase the exposure duration even further to obtain a decent exposure without loss of shadow detail.

Because the sensitivity loss is illumination-dependent, it is worst in the shadow areas of an image and less bad in the highlight images. This means that reciprocity failure will increase the contrast of an image because the exposure is reduced more in the shadows than the highlights; clearly this is a problem for many night photos which are often already of high contrast.

Colour films contain multiple layers of emulsion, each sensitive to a different spectrum (red, green and blue, typically). These layers can fail at different rates, causing colour shifts with long exposures in addition to loss of overall sensitivity. This effect can be particularly ugly because it causes colour shifts that vary across an image (crossover), e.g. red shadows and cyan highlights.

For example, Velvia 50 goes progressively green with exposures over 4s and Portra 160 has red shadows. Velvia also has huge green response to fluorescent and mercury-vapour lighting so (independent of the reciprocity issues) is generally unsuitably for night photography by artificial light.

Correcting for Reciprocity Failure

To make a good long exposure:

It should be noted that the reciprocity-failure datasheets of many modern films are completely out of date and that the films perform much better than the datasheets would indicate; Ilford is a particularly bad offender here. If you apply the correction factor from the datasheet on a very long exposure, you may end up with (very) overexposed film.

Film Choice

Films are not made equal in regard to their reciprocity performance. The following films have excellent performance and are therefore suitable for night photography:

Acros and Provia require no correction until 220s, and approx 1/3 stop correction between 220s and 1000s. TMY2 has a little more reciprocity failure than Acros but still retains greater film speed at exposures of up to 8 hours, i.e. it requires less than 2 stops of correction.

And the following films have very poor reciprocity performance:

The datasheet for each film should list its reciprocity performance using a table or graph of required exposure extensions as a function of exposure time and (for colour films) the colour filter.

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