When shooting long exposures on film it’s necessary to consider what’s called reciprocity failure. This is when the efficiency of the film reduces at lower light levels.
If there’s less light then the film is less effective.
What this means is that as longer exposure times are required, greater than 1 second exposures need to have more exposure time added to compensate for Low Intensity Reciprocity Failure. Exposure times of one second or less will not require any compensation. Ilford films have made an equation to calculate Reciprocity Failure for their films.
Tc = Tmp
Where Tm is the metered (indicated) time and Tc is the corrected time. P is a factor calculated following a range of exposure times.
So, for example, using HP5+ at a metered time of 10 seconds gives us: Tc = 10 exp 1.31 = 20.4 seconds Round this result off to give an exposure of 20 seconds.
FACTORS FOR ILFORD FILMS
| Film | Factor |
| SFX | 1.43 |
| Pan F+ | 1.33 |
| D100 | 1.26 |
| D400 | 1.41 |
| D3200 | 1.33 |
| FP4+ | 1.26 |
| HP5+ | 1.31 |
| XP2 | 1.31 |
| Ortho+ | 1.25 |
| K100 | 1.26 |
| K400 | 1.30 |
For very long exposures at very low light levels then some other variables come in to play such as the accuracy of the light measurement. This means that some trial and error may be required. You may also find that the contrast is increased with long exposures. This is because of the light level difference in light levels between highlights and shadows in the image in effect giving different reciprocity failure within the image. If this occurs, then pulling the development may be required (reducing development time) but will depend on the range of light levels in the image.