When you place shutter switch to “A” and lens to”A”, you are in the P mode. The “Professional mode”. This means that the camera will select both the shutter speed and the aperture with a bias toward the higher shutter speed.

Do the half press on the shutter button and you will see the settings for both the aperture and the shutter speed.

If you keep your finger “half-pressing” the shutter and rotate the rear control wheel, you will adjust the aperture of the lens.

This means that you have overridden the P mode and switched to aperture priority mode (while the lens switch is still in A position).

The camera will stay in aperture priority until you change the lens or turn off the camera off.