Reciprocity and Reciprocity Failure - an Explanation
Reciprocity is the interchange of shutter speed and aperture.
This rather posh word just means that a combination of a shutter
speed of 1/125th of a second with an aperture of f8, which is
referred to as an exposure setting of '1/125th at f8', will give
the same exposure to the film or digital sensor as 1/250th at
f5.6, which is the same as 1/500th at f4 etc. This reciprocity
works well, up to a point, but film users will find that, when
you have very long exopsure times, several seconds or so, the
reciprocity breaks down and extra exposure time is neccessary
to compensate. This is known as reciprocity failure.
Reciprocity Failure (film users only)
All colour films suffer from reciprocity failure when exposed
for longer than a few seconds. This results in a colour shift
as the three layers of the film respond to a different degree.
The exact colour will depend on the brand of film you are using.
The colour cast can often be corrected at the printing stage so
is not a major problem unless you are using slide film. The only
real way to avoid the problem is to use a faster shutter speed,
which will mean also using a larger aperture and sacrificing some
depth of field. If you don't understand the connection between
aperture and depth of field read my article on shutter
speeds and apertures.
Reciprocity failure will also be a problem with black and white
film but in this case all that is required to put things right
is to make an even longer exposure. How much extra you need to
give is hard to tell, the best solution is to make several exposures
at different times. After all your subject is not going to run
away, if you are hoping to get a good shot of it with a two or
three second exposure, it has to be something that is not going
to move.
All you really need to be aware of is that if you are shooting
night scenes for instance at very small apertures, you would be
well advised to shoot a series of photos at varying exposures,
this is called 'bracketing' and is a very useful technique even
in these days of digital cameras and instant replay.
Good news for digital camera users
Reciprocity failure is not a problem with digital cameras, however
noise can be.
Bad news for digital camera users
Long exposures on digital cameras can produce visible noise, which
looks a bit like the grain you see in a fast film image. This
noise or grain is usually most noticeable in plain areas of the
picture. See my article on ISO
Film Speed for some pictures and more information.
|
