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Art Deco Cameras

Reciprocity Failure

Camera f stops and shutter speed are normally linked in a simple way. However, at shutter speeds slower than about 1 second, this relationship breaks down and there is a need to compensate for that failure. This is especially important if you use your camera in 'T' or 'B' mode.

For most photographic materials, reciprocity is valid with good accuracy over a range of values of exposure duration. This means for correct exposure, if you increase the aperture by 1 stop (double the light entering), then the speed should be halved. But as the light level decreases the increase in duration, and hence of total exposure required to produce an equivalent response, becomes higher than the expected. For instance, at half of the light required for a normal exposure, the duration must be more than doubled for the same result. Multipliers used to correct for this effect are called reciprocity factors and are needed when exposure time become greater than about 1 second.

Most traditional films like Tri-X, HP5+, FP4+, etc. require roughly the same reciprocity correction. T-grain films require different compensation that doesn't follow the same pattern. Here is a table giving corrected exposure times for some popular films. They are:-

For intermediate times, a bit of extrapolation is required. e.g. FP4+ nominal time 6 seconds, use multiplier 2.5; so exposure time will be 15sec. Always go for slight overexposure if estimating.

Minimum Exposure Times [multiplier]
 Nominal Exposure Time (sec)
Film Type248153060
FP4+
125
3[1.5]8[2]21[3]51[4]2m 23s[5]6m 49s[7]
HP5+
125
3[1.5]8[2]22[3]56[4]2m 36s[5]7m 27s[7]
Delta
100/400
3[1.5]8[2]22[3]56[4]2m 36s[5]7m 27s[7]
SFX
200
3[1.5]8[2]21[3]51[4]2m 21s[5]6m 41s[7]
Acros
100
2[1]4[1]8[1]15[1]30[1]60[1]
Portra
100/400
3[1.5]6[1.5]15[2]35[2]1m 29s[3]3m 47s[4]
Ektar
100
2[1]6[1.5]14[2]30[2]1m 08s[2]2m 29s[2.5]

For other films, you will have to search out the date sheet or use an App. The easiest solution is to just use a film like Acros that requires no correction out to 2 min and then only +1/2 stop after that.