Specifications

Auto-exposure Flash Confirmation Signal (During its
introduction, it works with the Speedlite 188A and 166A only,
but other similar dedicated speedlites like the 299A or the
TTL's 300TL should works (TTL mode is not possible). Please
check with your local Canon Distributor for the compatibility
listing.
After the shutter is released, continue to press the shutter button halfway; the green
will flash on and off for two seconds if the shooting distance provided correct
exposure.
Note: The auto-exposure confirmation signal displayed in the viewfinder is for use with the Speedlite
188A and 166A (or similar dedicated flash units). When using other flash units, the
may flash on and
off after the shutter is released. In such cases, however, it does not confirm auto-exposure and should
be disregarded.
The AE-1 PROGRAM has two flash terminals.