PSA: A hot tip for enabling your Canon DSLR to shoot with a speedlight in AV mode
If you shoot with Canon DSLRs and want to use a speedlight with your camera, it may be worthwhile checking this setting in your Canon menu. In the recent "Intro to Speedlights" workshop I ran, a number of participants were perplexed when their Canon DLSRs would not adjust the shutter speed in Av mode when they had their speedlights mounted in the hotshoe.
All of this boils down to this thing called "Flash Sync Speed" -- which is the fastest shutter speed that the Canon DSLR can shoot at while still synchronising with the burst of flash from the speedlight. In many Canons (depending on model), this Flash Sync Speed can range from 1/160 - 1/250. If your DSLR has a pop up flash, then you may notice that your shutter speed locks at the Flash Sync Speed the moment you pop up the flash when shooting in Av or P modes. This is because your camera won't synchronise the shutter with the flash at speeds above this.
Give this a go. Put your external speedlight on your Canon DSLR and then set your shooting mode to Av (Aperture Priority). Turn on your speedlight and look through the viewfinder as you test shoot in different lighting situations (eg. in full sun, in shade). If you notice that your shutter speed in Av mode doesn't change, then Houston, we have a problem. The reason for this is because your camera has been set to lock the shutter speed to a particular speed (or speed range) when you are shooting in Av with a speedlight on.
It's an easy fix, but the option to fix this is not easy to find. Here's how you do it.
- Go to your MENU.
- Select the SECOND camera tab (see image below).
- Select the "External Speedlight Control" option. 
- Move down to select the "Flash Sync Speed in AV Mode" option. 
- That's it!
If you have this set to the second or third option, it's the cause of your headache. Change this to "Auto" -- which means the camera will automatically select the shutter speed to suit the Aperture you set in Av mode. 
Now you should see your camera changing the shutter speed to adapt for lighting conditions when you shoot in Av, even when you have your speedlight mounted on the hotshoe.