Now, look at the way of shooting. (Shutter-speed-priority for this time.)

1. First, set the shutter speed, 2. and then by turning the dial, the shutter speed is coupled with the aperture value.
3. The metering needles are in the viewfinder and on the top cover at the side of pentaprism.

When the needle indicates the circle, the light value is determined and after this changing the shutter speed also changes the aperture value still remains coupling the light value with it.