Can you explain why the AF-ON button overrides my chosen AF settings as well as operates focus, but the shutter button doesn’t on my Canon EOS camera?
BRIAN SAYS… EOS cameras that have the AF-ON button are configured by default so that both a half-press of the shutter button and the AF-ON button will initiate focus. If the AF-ON button is pressed and held down, then the shutter button is pressed to take a photo, the AF function assigned to the AF-ON button will take priority. If the AF-ON button is pressed and then released, the shutter button controls the AF. It’s common to stop the shutter from initiating focus by changing it to a metering or metering lock function only.
The AF-ON button can be configured for a wide range of capabilities, and even when used to initiate AF, there can be a lot of choices beyond simply initiating AF. The choices on the EOS mirrorless cameras are extensive and can be used to force the camera to Eye detection mode and start focusing, or just switch Eye detection on and off. Depending on the camera there may be three to seven sub options to change the AF area, switch to Servo or One Shot, and for some cameras change the subject to detect. A number of photographers have made settings in this sub-level of menus and then forgotten they had done so which leads to unexpected operation. Recent camera manuals do not cover the settings in this extra level of menus. For the EOS 7D Mark II there are four settings that singly (or in combination) will override the camera settings when the button is pressed; AF point location, focus mode, focus area and AF case. For the EOS R6 Mark II there are seven options.