If By reciprocity you mean..the relationship between different choices of aperture and shutter speed that result in identical exposure.
For example..In most situations there is an relationship between aperture and shutter speed, with a wider aperture requiring a faster shutter speed for the same exposure. For an exposure value of 10 may be achieved with an aperture of f/2.8 and a shutter speed of 1/125 s. The same exposure can also be achieved by doubling the aperture to f/2 and halving the shutter speed to f/250 s; or by halving the aperture to f/4.0 and doubling the shutter speed to 1/60 s.
However, during very long exposures, sensors can respond much more slowly than usual, resulting in underexposure. It is said then, that reciprocity failure has occurred, i.e. the conventional relationship between aperture and shutter speed has broken down. Most film manufacturers publish data on the latitude of their films (how much they can be pushed or pulled beyond their normal exposure range), and also reciprocity corrections. For example, if a light meter indicates a required EV of 5 and the photographer sets the aperture to f/11, then ordinarily a 4 second exposure would be required; a reciprocity correction factor of 1.5 means that the actual exposure must be extended to 6 seconds.
With digital it’s a little different and the best way is to check your results as you shoot using the histogram.
For this shot is was unnecessary as It was just inside the limits of the camera light readings and I also shot this in RAW which gave me extra headroom to recover the picture if required (this was not required in the end as the exposure was spot on though.
However if it had been outside the range when I was shooting I would have done as below…btw this is one of the tips and tricks that can be read in article that I wrote a while ago and is posted on the front of the site
here
Quote:
The Easiest way I know is to set the camera on manual and the aperture wide open (small F stop), then check to see what the meter says. If necessary increase the ISO up too, until you get an exposure of less than 30 seconds. Then start adjusting back to the aperture and ISO you really want, doubling the time for every full stop. Here is an example for clarity..
If you have to set the aperture to f/4 and ISO 400 to get a reading under 30 seconds - for simplicity, say that the camera picks exactly 30 seconds but you really want f/11 for good depth of field, and of course you want ISO 100 to keep noise to a minimum.
f/4 to f/11 is 3 stops (f/5.6, f/8, f/11). So you have to double 30 seconds 3 times (1 minute, 2 minutes, 4 minutes)
ISO 400 to 100 is 2 stops (200, 100). So double 2 more times (8 minutes, 16 minutes).
In this example you would set the camera to f/11, ISO 100 and an exposure of 16 minutes.
Since 16 minutes is a long time to wait I'd cut that in half by underexposing 1 full stop and do a picture at 8 minutes and then check the histogram. Judging the histogram will give you an idea for the next try. Sure, the 8-minute shot might be an extra, unnecessary step, but why wait 16 minutes to find out it's messed up? Also remember that most photographs can easily be recovered from being underexposed by 1 stop in most image editing software packages, so if time is a premium, sometimes this can be a life saver.
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