Reciprocity Failure in film by

Long exposures

phototechs2 — 2025-06-25 10:57:07 UTC

When doing exposures longer than 1 second with Black and White or Colour film the metered exposure time will result in an underexposed image. To get the correct exposure, you need to increase the exposure time.

Your light meter can't work out the compensation for you, so you will need to know how much compensation your chosen film stock needs for a correctly exposed photo.

phototechs2 — 2025-06-25 11:05:55 UTC

Reciprocity Failure is a characteristic of the film stock. Some films do worse than others, so you have to account for reciprocity failure differently for different film stocks.

The short version

phototechs2 — 2025-06-25 11:29:15 UTC

With exposures of more than 1-2 seconds, your film stops being able to gather light effectively.

Up to 1 second of exposure time, the film's exposure plot is linear. ie increasing the exposure time by 1 stop from 60th of a second to 30th of a second will double the exposure time and double the amount of light reaching the film. This rule is the same every time we increase the exposure time by one stop, up to around 1 second. Then the film loses its ability to capture light in this predictable way.

phototechs2 — 2025-06-25 14:28:38 UTC

With long exposures you will often get more contrast and more visible film grain.

Colour negative film will start to get colour shifts with very long exposures. This is because the three colour layers (yellow, magenta and cyan) have varying sensitivities to light.

phototechs2 — 2025-06-25 14:35:12 UTC

Every film stock has a different rate of Reciprocity Failure. Some will need a much longer compensation time than others. So, choose a film stock that needs minimal compensation.

For example -

With Ilford HP5 if the light meter is saying 10 seconds you will need to expose for 20 seconds.

But with Kodak Tri-x if the light meter is saying 10 seconds you will need to expose for 50 seconds!

There are various charts and tables that photographers have made using the information manufacturers provide. Ilford have made it easy to work out by giving their film stocks a 'P factor' and calculation that is relatively easy.

See this PDF

phototechs2 — 2025-07-09 11:41:29 UTC

Below are some apps and websites that do the maths for you. They seem to be quite accurate, but it is always good to do a test shoot in the same conditions you are planning your shoot in to make sure you are getting the results you want

Website with LOADS of films

phototechs2 — 2025-07-09 11:43:03 UTC

Light Meter Apps with reciprocity calculator

phototechs2 — 2025-07-10 09:25:24 UTC

Google Play - LINK