Reciprocity Law Compensation for Long Exposures on Black and White Film: ILFORD Method and Applicable Boundaries

Reciprocity Failure Affects Image Density

ILFORD describes low-light reciprocity failure as: if the same total exposure is given over a longer period, the resulting image density may be lower, and the effective sensitivity reduced. The manufacturer explains that this is due to a decreased efficiency in forming stable developing centers at low light levels. Metering results still depend on scene brightness and measurement conditions.〔G-RF-01〕

Using Manufacturer Coefficients by Film Type

ILFORD provides a product coefficient P for its black-and-white films and calculates compensation time as Tc = Tm^P; the page states that exposure of one second or less does not require compensation. Since the coefficient is provided for specific products, it should only be applied according to the corresponding model data and cannot be directly used for color film or other brands.〔G-RF-02〕

HP5+ Calculation Example

According to the ILFORD table, HP5+ has a P of 1.31. If the meter indicates 10 seconds, then 10^1.31 ≈ 20.4 seconds, and the ILFORD example rounds this to 20 seconds. The page also reminds that extremely long exposures at very low light may require test shots: measurement accuracy, scene brightness range can affect results, and whether to reduce development for changes in contrast also needs to be judged according to the specific scene.〔G-RF-03, G-RF-04〕