Illuminate Research - Reciprocity Law Failure
Reciprocity law is a law in photography which refers to the relationship between shutter speed and aperture. The shutter speed should be the same as the focal length. If you are shooting with a 50mm lens, your shutter speed should be at 1/50. The longer the lens, the more exaggerated the movement becomes.
Understanding this law allows photographers to be able to over or under expose their shots in a controlled manner. This then will help you be in control of the motion blur and depth of field.
Captured hand-held following reciprocal rule
(Sony A7R + FE 35mm f/2.8 ZA @ 35mm, ISO 100, 1/40, f/11)
It would be ideal to shoot at faster shutter speeds to minimise movement. This will avoid camera shake. Humans cannot be completely still so this law helps to avoid camera shake. Camera shake is not to be of fused with motion blur.
Digital camera sensors do not suffer from reciprocity failure.
If you decrease the light intensity, exposure time must be increased proportionally to produce the same total exposure. The same as if you increase the light intensity then you need to decrease the exposure time.
Reciprocity failure tends to be when film is exposed to light longer than a few seconds. Which means, when you are in a darker environment and you’re needing to use a longer exposure the silver halides within the film don’t get in contact with the photons evenly enough. Thus then makes the density of the film decrease and very little to no shadows.
A compensation when developing is, for example pulling back the developing time will help with the contrast problem. With long exposures the contrast seems to increase highly, therefore not enough details in the shadows, furthermore declaring for less amount of time as normal could fix this problem slightly, but it does depend on the light levels within your image. Another way to help with this problem you can decrease the exposure time, if you lower the shutter speed then you let more light in, which is probably the best option as if you were to alter the aperture then you will be messing with the sharpness of the image and a shallow depth of field might not be what you wanting in your shot.
The formula is the amount of seconds the exposure time is metered for eg( 5 seconds) then the Xy button on a scientific calculator, then get the factor for the roll of film you used (shown in the table below) then it will give you the compensated time to use instead. Example: 5 Xy (hp5+) 1.31= 8.23 seconds.