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I gave a Canon 40D and a 580EXII flash a tough challenge yesterday, and it didn't do well.
There's a hummingbird feeder just outside my patio door; I'm in a warm place. I put the camera on a monopod, attached a 24-70 F:2.8 lens and a 580EXII flash with a DIY flash diffuser, and walked slowly toward the feeder until I was no more than 18 inches from it. I stood in position for a long time, and eventually enough curious and hungry birds started coming around. Before they arrived, I set the flash mode to manual and the power level to 1/4; I have found that the ETTL preflash spooks the birds.
By the time I got into position, the ambient light was pretty low, and I found that the camera's autofocus system was unable to focus on the birds when they hovered briefly above the feeder's plastic flower. Eventually I set the lens to manual focus and, using live view, set the focal point slightly behind the flower at a point where I guessed the birds might be in their hovering flight. I tried to focus manually on each bird, but they would hold positions only very briefly, and my results, although colorful, were not sharp, even with the lens set to F:11 to increase the depth of field. I realize that by being so close to my target, I multiplied my chances for focusing errors because the relative distances between the flash and the various focusing points were quite large.
Today, I remembered that Custom Function 8 on the 580EXII activates an AF-assist beam in low light. That feature is enabled on my two 580s, and I wondered why the beam wouldn't have helped me focus on the boards. I brought the camera fitted with a flash into a dark room and tried to see any evidence of that beam. I couldn't see it after several tests, though the photos looked pretty sharp.
I'm wondering if the AF-assist beam just wasn't working or whether there was anything I could have done with this camera and flash combination to produce more usable results. I'd appreciate your thoughts. Thanks!