Canon EF - Manual (Page 64)

mines the proper exposure needed for a single frame.
When making multiple exposures, less exposure must
be given to each shot. One simple way to do this is
to reset the ASA ring to a higher setting, which will
fool the camera into decreasing the exposure read­
ing. Multiply the normal ASA of the film by 2 for a
double exposure, by 3 for a triple exposure, by 4 for
a quadruple exposure, and so forth. However, when
making multiple exposures of night lights or a
subject against a black background, do not reset the
ASA ring, because the black areas in the scene will
become overexposed.