Film sensitivity to light is not linear. This means that as an exposure time increases, the film is less sensitive. So to get a proper exposure, you need even more time. This is called film reciprocity.
The Reciprocity Charts below show the adjustments that should be made for different film types–each film has different sensitivity.
An example using the chart, if the indicated light meter or estimated time is 1 minute for Kodak Tri-X, the adjusted exposure should be 8 mins.
Exposure Calculator web app is one way of estimating exposures, light meters don’t work too well at night. You can see my Night Photography for examples of my work. Charts
Table 1: Films: Fuji 64T, Fuji Velvia
Indicated: 4s 8s 10s 16s 20s 30s 40s 1m 2m 4m 6m 10m 15m 20m 30m 1 hr Adjusted: 5s 12s 16s 28s 39s 1m 1m 28s 2m 30s 4m 50s 10m 15m 25m 37m 50m 1m 14m 2 hr 30m Table 2: Films: Kodak Tri-X, Kodak Plus-X
Indicated: 2s 3s 4s 8s 10s 15s 20s 30s 40s 1m 2m 3m 4m 6m 10m 15m 20m 30m 1 hr Adjusted: 4s 8s 12s 36s 50s 1m 35s 2m 3m 30s 4m 40s 8m 18m 25m 40m 1 hr 2 hr 3 hr 20m 4 hr 6 hr 20m 12hr 30m Table 3: Films: Kodak 160t
Indicated: 4s 8s 16s 30s 1m 1m 30s 2m 3m 4m 6m 8m 12m 16m 30m 1 hr Adjusted: 5s 12s 26s 54s 2m 6s 3m 26s 5m 8m 11m 22s 18m 39s 26m 29s 43m 1 hr 2 hr 3hr 50m Table 4: Films: Kodak T-MAX
Indicated: 2s 5s 10s 15s 20s 30s 1m 2m 4m 10m 20m 1 hr Adjusted: 3s 7s 15s 25s 35s 50s 2m 4m 30s 10m 30m 1 hr 10m 3 hr Table 5: Films: Ilford HP5, Ilford FP4
Indicated: 5s 10s 15s 20s 25s 30s 1m 2m 4m 6m 10m 20m 1 hr Adjusted: 13s 31s 55s 1m 23s 1m 57s 2m 35s 7m 8s 17m 21s 36m 55m 1hr 32m 3 hr 9hr 20m Table 6: Films: Fuji Provia 100F
Indicated: 2m 5m 20s 6m 40s 8m 20s 10m 40s 13m 20s 16m 40s 21m Adjusted: 2m 5m 32s 7m 10s 9m 30s 13m 31s 19m 27s 29m 33s 50m Sources
Table 1 Velvia data from Michael Davis. Fuji 64T equivalence from Olympus Reciprocity Table Table 2 Tri-X data from The Nocturnes, includes Kodak Plus-X equivalence from Kodak reciprocity data sheets. Table 3 Kodak 160T data from Lunar Light Table 4 T-MAX data from The Nocturnes Table 5 Ilford FP4 and HP5 data from Ilford Data Sheets and HueCandela.com [pdf] data was not available for the higher indicated exposure values, the adjusted values were extrapolated out.
Table 6 Fuji Provia 100F data from Lunar Light