A documented long exposure correction
Apply the published factor for 11 ILFORD and KENTMERE black and white films
Compare the meter reading with the time the emulsion actually needs
See the correction as extra seconds, stop lift and a visual film path
Keep the result useful in the field with three darkroom presets
# Why film reciprocity failure needs a correction
At ordinary shutter speeds, the reciprocity law makes exposure easy to treat as light intensity multiplied by time. During a long exposure, the emulsion becomes less efficient at forming stable development centres. The meter can therefore indicate a time that is physically correct but chemically too short for the intended density.# The manufacturer equation behind the result
| Film | Published factor | Example at 10 seconds |
|---|---|---|
| ILFORD HP5+ | 1.31 | 20.4 seconds |
| ILFORD FP4+ | 1.26 | 18.2 seconds |
| ILFORD SFX 200 | 1.43 | 26.9 seconds |
| KENTMERE 400 | 1.30 | 20.0 seconds |
Use the answer as a starting exposure
Long exposures are sensitive to more than reciprocity. Metering error, scene contrast and development can all affect the negative. For an important frame, bracket around the calculated time and keep notes about the process.# When to trust a film reciprocity correction
- Use a published factor: Match the selected film and format to the manufacturer information you are following.
- Keep the process consistent: Development, temperature and agitation influence the final negative.
- Bracket very long frames: The longer the exposure, the more useful a practical test becomes.