In simple terms, it's the loss of film speed when doing long exposures, and the film does not obey the law of reciprocity. (you know, the open up 1 stop of aperture, you need to 1/2 the shutter speed, etc).
So let's say you have an exposure of 1s at f/4.
At f/16, it would be 4 stops down, so exposure time would be 16s. But becoz of possible reciprocity failure, this combination might result in underexposure. To counter this, you would have to open up the aperture to say f/11 (or whatever determined by experience and experiment, depending on the film).
This is a slightly more technical explaination than CK's.....
First off you need to understand what teh Reciprocity Law actually is.
The law was first devised by Bunsen (of Bunsen Burner fame) and Roscoe in 1862 and states that for any photosensitive material the photochemical effects are directly proportional to the incident light energy. That is the product of a given light level (illuminance) and exposure duration as expressed in the formula H = Et where E = illuminance and t = time. H = product
In photographic terms as long as H is kept constant the values of E and t can be changed by manipulating the aperture and shutter of the camera (as per CK's post).
However, someone (Abney I think it was) demonstrated that the photographic effect depends on the acutal values of H and t and not solely on their product. This failure of the law of reciprocity is caused because the effect of exposure on a photograhic material depends on the rate at which energy is supplied.
Reciprocity failure is a fact of life with all photographic emulsions, and it occurs not only with low lighting levels but also with extremely high illuminence levels (bright light).
In the real world the high light levels aren't often encountered, unless you are photographing high powered laser pulses, atomic bomb detonations or the sun with very large telescopes. However, the low level reciprocity failure is often encountered and depending on film the time for reciprocity law failure can be as little as 1 second or less. With low illuminence level (eg at night) failures a loss of contrast and colour shifts are the two most common indicators of reciprocity failure. However the good news is most films have published compensation charts that allow the photographer to compensate for reciprocity failure.
say for example for a perticular image:
shutter speed @ 1/125 and aperture f8 - (Name this part 1)
- If you move shutter one speed higher,
then the aperture needs to be larger by one stop.
(to get the same exposure as part 1)
- if you move the shutter one speed slower,than the aperture will need to be be smaller by one stop
(to get the same exposure as part 1)
When this type of compensation fails, then you will have reciprocity failure - thus image turning over or under exposed.
The exposure adjustment you need to make for different films is available on the websites ( www.kodak.com etc ). There's no hard and fast rule of 10 sec for Kodak or 16 sec for Fuji films. It varies on a per film basis. For example, Velvia needs compensation after 4sec, but Provia 100F can handle 120 seconds without failure.
Originally posted by ckiang
That's NOT reciprocity failure, that's a colour balance issue. The slide is daylight balanced, and the greenish cast you get is probably from some artificial lighting, notably flourescent.[/B]
Sorry that's not right. It is a reciprocity failure issue. Velvia's spec sheet states that you need to use magenta filtration when shooting longer than 4 sec. I read that it's because of the uneven response of the different colour layers beyond 4sec.