Canon EOS 5 - Manual (Page 98)

10. Multiple Exposure
When taking multiple exposures on a single
frame,
you should decrease the exposure
value for each exposure using exposure com­
pensation (refer to page 33).
Number of multiple Compensation
exposures amount
2 exposures -1.0
3 exposures -1.5
4 exposures -2.0
The above values should be used only as a
guide.
The actual compensation amount re­
quired depends on the shooting conditions
and should be determined by prior testing.