While doing some testing of the flash on the EOS 40D I came upon what I though rather strange behavior.
I was using an external studio strobe connected to the PC connector on the camera in conjunction with either the built in flash or a 550EX speedlite mounted on the camera. The external studio strobe was on 1/4 power, which meant that it didn't need time to recharge between two successive flashes (actually it will do at least 3 rapid flashes when set to 1/4 power). I was intending to use the studio flash as the main light, and use the built in flash set to -2 stops FEC (Flash Exposure Compensation) as a slight fill. What I found was the following.
• If I used the built in flash with FEL (Flash Exposure Lock), then the light from the studio strobe contributed to the image illumination, but if I didn't use FEL, then the studio strobe did not add to the image illumination.
• If I used the 550EX, then the light from the studio strobe contributed to the illumination whether I used FEL or not.
This is illustrated by the images below:
I thought this was a bit odd, so I tried the same tests with my 20D. Whether I used the built in flash or the 500EX and whether I used FEL or not, in every case the studio strobe contributed to the image illumination.
So what's going on? Well, I looked at the flashed from the various components using a digital storage oscilloscope. I was wondering if the time between the metering pre-flash (E-TTL) and main flash was perhaps different between the cameras and flashes and that perhaps the studio strobe wouldn't respond if they were too close together.
I found that in all cases with either camera and either the built in or 550EX, the time delay between the metering pre-flash and main flash was around 60 ms (0.06s), so that couldn't explain the difference.
However I did find that only the 40D using its built in flash triggered the strobe via the PC socket when the metering pre-flash fired. When that happened, the studio strobe was unable to respond to a second trigger only 60ms after the metering pre-flash. When FEL was used of course, the gap between the metering flash and main flash was much longer and the studio strobe had time to recover and respond.
The conclusion of all this (if you haven't been following, and I can forgive you for that), is that the EOS 40D using the built-in flash triggers a strobe attached via the PC socket during the metering pre-flash. The EOS 20D doesn't do this and neither the EOS 20D nor EOS 40D do it when an external speedlight is mounted on the hot shoe.
Is this important? Well, it is if you're using an externally attached studio strobe and you're using the built in flash for a little fill. In that case, you probably won't see the main studio flash. I'm not sure if this is a bug in the firmware or if Canon meant it to be that way, but at least be aware of it.
Note also that when the 40D uses it's built in flash as a AutoFocus assist light, it also triggers an external strobe attached to the PC connection. I'd assume this is a bug not a feature!