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Reciprocity failure

There is a proportionate relationship – a reciprocal relationship - between shutter speeds and f-stops (apertures); 1/8 sec at f16 is equivalent to ¼ sec at f22, and ½ sec at f32, and so on. When exposures get beyond 1 second[1] as a shutter speed, the proportionate relationship breaks down and you no longer get equivalent exposures – you get less exposure. I don’t know why. In Hero-speak, this is called “failure of the reciprocity law.” Impressive, eh? Most just call it reciprocity failure.


There used to be all sorts of elaborate and mind-deadening guides and charts for this. None are necessary. Photo-science nurds used to enjoy discussing this stuff endlessly in the RIT hallways. I don’t think any of them ever took any pictures.


Here’s a simple guide. For exposures over 1 second, use this. (We assume a constant aperture.)


If the meter indicates


·    2 seconds, give 3 seconds;

·    4 seconds – give 8 seconds;

·    8 seconds – give 16 to 20 seconds;

·    for 16 seconds and up, I always give at least a minute. From here you can wing it. Err by giving more exposure;

·    if the meter indicates 32 seconds, give at least 2-1/2 minutes (150 seconds);  5 minutes is better (300 seconds).

·    If you’re uncomfortable, give extra time.


This always works. Even for the shorter times – 3 seconds, 8 seconds, etc – give more exposure if the subject itself is very dark. Any question – give more exposure. I’ve been doing it for more years than I care to say and it always works.


Another point – light meters, all of them, are inaccurate at the extremes. Chances are, if the meter indicates ½ c/ft2,[2] the light level is actually less than what it’s telling you. There’s another reason to err on the side of more exposure.


Even if the meter is accurate, reciprocity failure always occurs in dark conditions. In dark conditions it’s nearly impossible to overexpose. The difference between 5 minutes and 10 minutes exposure is very slight, yeah? Use these recommendations and you’ll get a negative you can print.


This always works for me. I’ve used it with everything: Foma, Tri-X, Panatomic-X, HP-4 and 5, FP 4, even dear old Isopan and Versapan, and other films. It always works.[3]


I once saw Kodak reciprocity tables recommending reduced development for very long exposures. Don’t do this – just use your normal development. There’s never enough light present to require it – not even close. (I wonder what Ridge Road genius dreamed that up? . . . probably another RIT-trained photo-chemist wandering around with half a job in those cash-rich days.)

With very long exposures, the local contrast - tones within a tone - increases. This is because the higher values break the film threshold immediately, and the lower values take longer. Visually this results in a strange, otherworldly quality. It isn't enough contrast increase to require a "mechanical" adjustment like reduced development or lower contrast paper, but the effect is striking. I often find an eerie atmosphere pervading very early morning photographs in swamps or deep woods.

Rules of thumb for long exposures: in doubt? - give more exposure. If indicated exposure is 16 seconds or higher, one minute is not excessive. If the indicated exposure is 1 minute or higher, you ‘ll need to give 10 or 20 minutes . Go home. It’s too dark.



[1] It also occurs at extremely fast shutter speeds. I’ve never encountered this so I don’t know anything about it. I suppose Langfords Basic Photography textbook may explain it.

[2] EV 6 at ISO 200.

[3] I’m not some Photographic Champion. I’ve just been around a long time. Even a dummy learns a few things after decades doing it.

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