I must be missing something with my 40D with 580EX II

you will have to forgive my flash ignorance as I rarely use a flash. Anyway, I need some assistance in why the camera is choosing shutter speeds below 1/60 when in AV mode and the flash on. I thought it would always stay at 1/60th? How do I ensure that I will be at at least 1/60th using AV mode?

No. Av flash resorts to 'fill-flash' mode, where a similar shutter speed to what you would get without flash must be maintained to properly expose the background too. If you forced the shutter speed to be atleast 1/60th all the time, it wouldn't be fill-flash anymore.

You can force Av mode to push the shutter speed to 1/200th during flash shoots by going into C.Fns, although I personally wouldn't do that. Fill flash can be very useful and if you prefer the flash to be the primary source of light instead (with a higher emphasis on sufficient shutter speed), just shoot in Manual.

Try bumping your ISO. If you need to ensure that the speed is above 1/60, use TV mode.

Yes. The flash works in two mode. When operating the camera in any of the basic modes, the flash uses 1/60 and fires to be the primary light source. This is the way point and shoot cameras work. When operating in Av, Tv and M the ambient light is the primary source and the flash is just for fill in.

carpenter wrote in post #4156086
why the camera is choosing shutter speeds below 1/60 when in AV mode and the flash on.

Because Av mode is your "slow sync" mode. See page 99 of the 40D manual.

You don't. By using Av mode you're telling the camera "I don't care about the shutter speed, set it however you like". If you care about both the aperture and shutter speed, you should be in M mode.

Well, there is C.Fn.I-7 (page 155), where you can tell the camera to forget about slow sync and fill flash and always use 1/250 for flash shots in Av mode.

thanks for the comments guys. I guess for some reason I thought that its also would stay at 1/60th in AV.

I use M mode for flash if environment is dark and flash is required. I will use AV (fill) mode for daylight situation with flash.

carpenter wrote in post #4156217
thanks for the comments guys. I guess for some reason I thought that its also would stay at 1/60th in AV.

It's not just the 580EX II either. Even your onboard flash should operate under this paradigm, so you've perhaps overlooked this detail in the past. Like already mentioned, you should just be using Manual if shutter speed was that important to you. That way you can still leave Av in it's fill-flash mode for situations which require that.

P mode will set SS to 1/60 and may be what you are looking for; flah will be fill in Av. Check the links in the lighting forum esp. this one https://photography-on-the.net/forum/showthre​ad.php?t=171657

lungdoc wrote in post #4157343
P mode will set SS to 1/60 and may be what you are looking for; flah will be fill in Av. Check the links in the lighting forum esp. this one https://photography-on-the.net/forum/showthre​ad.php?t=171657

thanks. Actually I just always thought that when I'd used flash before on AV it would remain at 1/60. I was mistaken is all. I was able to set the high speed sync and lock it at 1/250 but I think I will just use M.