Canon A-1 - Manual (Page 47)

Metering at Low Light Levels
With AE coupling all the way down to EV -2
(with ASA 100 film), the A-1 is capable of
metering and producing a natural rendition
of the light in very dim lighting conditions.
Since EV -2 is practically the lowest light
level at which you can even perceive your
subject through the viewfinder, you can
basically perform AE photography with the
A-1 as long as you can see your subject in
the viewfinder when using a film with appro-
priate sensitivity. This means that the A-1
will usually give you a meter reading even in
the most adverse of lighting conditions when
performing photomacrography and photo-
micrography in the stopped-down AE mode.
What this also means is that you will be
using very slow shutter speeds and may be
subject to failure of the reciprocity law. The
reciprocity law is that relationship between
apertures and shutter speeds discussed earlier
in which several combinations of the two
will give the same exposure value. The
trouble with this law is that it may not hold
true at slow shutter speeds. More exposure
may be necessary. You will find information
concerning reciprocity law failure and how
to correct it in the data sheet that comes
with your film. Correction of reciprocity
failure may involve increasing the light
level so that higher shutter speeds can be
used. Even when following the instructions
of the film data sheet, it is advisable to
slightly adjust the exposure over several
shots to get at least one perfectly exposed
image.
Be especially on the alert for reciprocity
failure when using color reversal (slide) film
which has a smaller exposure latitude than
black and white or color negative film.
Having a smaller exposure latitude means
that it is more touchy about exposure error.
Incorrect exposure by only 1/2 exposure
step may make a noticeable difference in the
image results. Reciprocity failure with color
film will result in color shifts as well as
underexposure and may require corrective
filtration as recommended by the film manu-
facturer.
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