What is reciprocity?
We control the aperture and shutter speed to obtain the proper exposure on film and the relationship between aperture and shutter speed is called reciprocity law.
Total Exposure = Light Intensity x Exposure TimeReciprocity Law
When we open up the aperture by one stop (double the light intensity going through the lens), the shutter speed should reduce by half (reduce the exposure time by half) in order to obtain the same total exposure on film.
In general, the reciprocity law holds very well, except in extreme bright or extreme low light intensity condition. This is due to the imperfection of silver halide crytal under extreme conditions.
When forming a latent image, specks will form on the surface of silver halide crystals. In too strong light intensity, a large number of electrons will be knocked out from the halide ion and may become “too crowded” to find the silver ion and combine to form the silver atom. So some electrons may just meet the halide and revert back to the halide ion. In too weak light intensity, too few free electron to form the silver atom and too few silver atom is not enough to form the speck and therefore, the silver halide grain may result as unexposed.
To make thing simple, let me use an analogy of water bucket and silver halide crystal. A water bucket is used to capture water and silver halide is used to capture image.
Since there is no perfect in nature, assume that the water bucket (silver halide) has its own imperfection when holding water (recording image). The water bucket volume is 10 litre and it is leaking water at 0.1 litre/sec. You can fill the bucket under water tap at 100 litre/sec (aperture) for 1/10 sec (shutter speed), or 1000 litre/sec for 1/100 sec, while only 0.01 L (1/10 sec x 0.1L/sec)and 0.001 L (1/100 sec x 0.1L/sec) will leak respectively. So in both cases, you will have 9.99L and 9.999L of water in the bucket which is not far from the 10L (correct exposure).
Reciprocity Failure in Low Light Condition
However, if the bucket is filled by a slow water tap (low lighting scene), says at 1 litre/sec, reciprocity failure will happen. According to reciprocity rule, the bucket supposes to be filled (normal exposure) at 1 L/sec for 10 sec. but it also leaks 1 litre of water (0.1L/sec x 10 sec), so the 10L bucket will contain only 9 litre of water which is a noticeable 10% less(underexposure). In this case, you have to increase the filling time (shutter speed) by 10% to compensate. This is exactly the case for low light situation, the exposure time (shutter speed) will need to be adjusted.
The above graph shows that the effective film speed reduces with increase in exposure time. You can see the 3 color curves response differently and therefore, color filters may also need to add to correct the color bias in long exposure.
Below is a table of reciprocity failure adjustment for some common films I use (copied from manufacturer’s sites). I omit the color correction recommendation as I usually just let it be.
|Ilford Delta 100||No adjustment needed for shutter speed of 1 sec or less||For longer than 1 sec, Ta = Tm1.26|
Ta = adjusted time
Tm = metered time
|Ilford Delta 400||No adjustment needed for shutter speed of 1 sec or less||For longer than 1 sec, Ta = Tm1.41|
|Ilford Delta 3200||No adjustment needed for shutter speed of 1 sec or less||For longer than 1 sec, Ta = Tm1.33|
|Ilford Pan F+||No adjustment needed for shutter speed of 1 sec or less||For longer than 1 sec, Ta = Tm1.33|
|Ilford FP4||No adjustment needed for shutter speed of 1 sec or less||For longer than 1 sec, Ta = Tm1.26|
|Ilford HP5||No adjustment needed for shutter speed of 1 sec or less||For longer than 1 sec, Ta = Tm1.31|
|Fujichrome Provia RDPIII||No adjustment needed for shutter speed of 2 min or less||For over 4 minutes, +1/3 stop|
|Kodak E100||No adjustment needed for shutter speed of 2 min or less||No information provided for over 2 min|
|Kodak TMax 100||No adjustment needed for shutter speed less than 1 sec||For 1 sec, +1/3 stop|
For 10 sec, +1/2 stop
For 100 sec, + 1 stop
|Kodak TMax 400||No adjustment needed for shutter speed 1 sec or less||For 10 sec, +1/3 stop|
For 100 sec, + 1.5 stop
Reciprocity Failure in Strong Light Condition
When the light intensity is too strong to expose the film, reciprocity rule may still fail and causing underexposure on the film. A very famous example is the “Dark Sun” by Ansel Adam.
You can see the sun in the photo is a grey spot instead of a white spot. It is because the light intensity of the sum is too strong and caused underexposure of the sun.
The reason for reciprocity failure is also due to imperfection of silver halide (water bucket). When the tap is too wide opened (light intensity very strong), the water is coming in too fast to the bucket and causing a lot of water splash out. Therefore, the bucket (silver halide) just simply cannot collect all the water (underexposure).
Nowadays in modern film, the reciprocity law holds very well in the high light intensity so the “Dark Sun” effect happening is almost gone. So most of the time we just need to care about the shutter speed adjustment in low light intensity and long exposure.