How to Change the Flash-Sync Speed in Av Mode on Your Canon EOS 6D
When you’re shooting in Aperture Priority (Av) mode, by default your EOS 6D shutter speed is set between 30 seconds and 1/180 of a second when the flash is enabled. The flash duration is very short and fires when the shutter opens, which gives you a sharp image. If the shutter speed is slow, you see motion trails if your subject moves during long exposure. This can be very artistic.
However, if you want to eliminate the possibility of motion trails, you can use a custom function to change the shutter speed used when a flash unit fires. Choosing one of the options that uses a higher shutter speed prevents motion trails, but the background will be dark. To change the flash-sync speed when shooting in Av mode:
Navigate to the External Speedlite control menu.
Use the Multi-controller or Quick Control dial to highlight Flash Sync in Av Mode, and then press Set.
The Flash Sync in Av mode options are displayed.
Use the Multi-controller or the Quick Control dial to highlight one of the following:
Auto: The camera chooses a shutter speed between 30 seconds and 1/180 of a second when a flash unit is used in Av mode.
1/180–1/60Sec. Auto: The camera automatically chooses a shutter speed between 1/60 of a second and 1/180 of a second when a flash unit is used in Av mode.
1/180Sec. (Fixed): The camera sets the shutter speed to 1/180 of a second when a flash is used in Av mode.
After choosing an option, press Set.
Your flash unit is set to the desired flash-sync speed.