The EOS R3 has an autofocus system that’s different to the R5 and R6. It also allows you to customize buttons to activate autofocus features. The most common customization is commonly called back-button focus. This allows the photographer to separate the release of the shutter from the activating the autofocus. The thinking behind this is to give the photographer more control over when to start and stop the camera from attempting to acquire focus. To do this, Custom Functions 4 menu has the customize buttons option and setting the shutter button half-press to Metering start only stop this button from activating the focus function. Then setting the AF-ON button to Metering and AF start will enable that button to perform AF.
However, the R3 allows you to do a lot more. Pressing the INFO button will get you to another screen where specific AF features can be set. The R5/R6 also has this ability, but the settings are much more limited and explained more clearly. There isn’t any documentation about this detail screen in the current R3 Advanced User Guide.
On this screen, the AF settings you can affect are on the right. For example, you can set AF Operation to Servo or One-Shot. But to enable that, you then have to click on the left so there’s a tick against that setting. This will override any AF setting you might have. But if you don’t enable that function, it will use whatever setting you have set in the camera. On the R5/R6, it’s explicitly labelled “Maintain the current setting”.
So in this screen, the AF area is whatever has been set using the menu system. However, if it was enabled, the camera would use the large AF area.
Note: For Eye detect to be enabled, Subject tracking must also be enabled. Having a tick against Eye detect alone will not enable it unless Subject tracking is enabled. The Eye detect setting should really be greyed-out when Subject tracking is off.
And, if you have a R5 / R6 – this is a great way to set up eye detect https://youtu.be/fWpuF6tGVTc