When shooting negative film, it is always better to err on the side of over-exposure and in low light, this means long exposures. However, when shooting film at exposures greater than 1 second, film is prone to something called “reciprocity failure” resulting in under-exposed images. Whilst your light meter is giving you the correct exposure reading, your film’s ability to record a useful image at that reading is diminished so you have to compensate by giving some extra exposure time to record a correctly exposed image.
Each film is different and each has different and each will require varying degrees of correction. I have tested the films using various starting points from other sources and arrived at the times in the chart below. These work for me and my equipment so you may need to do some fine tuning. If these are accurate and work for you then please do drop me a line.
Please note: Giving additional exposure by itself does not cure reciprocity failure and you will need to adjust your development time to account for any increases in contrast. For example, at longer exposures, shadows fail whereas highlights don’t resulting in an increase in contrast. You may need to reduce your development time in this instance, unless extra contrast is your desired result.
Converting film rolls to 5×4 for processing
1 x 36 exposure 35mm = 1 x 120 roll film = 4 x (5×4) sheets = 1 x (8×10) sheet
Reciprocity chart Ilford FP4+ 125 iso
|Meter reading||Corrected exposure for 5×4 sheet||Stops||Development correction|
|1 sec||1.5 sec||+0.5||85%|
|2 secs||3 secs||+0.5||85%|
|4 secs||7 secs||+0.5||85%|
|8 secs||16 secs||+1||70%|
|15 secs||45 secs||+1.5||60%|
|30 secs||2 min||+2||45%|
|60 secs||5 min||+3||40%|
|120 secs||24 min||+3.5||35%|