Get a correct exposure

As shown in the left figure, there are numbers from 1 to 8 and the aperture figures above them in the viewfinder and the numbers have shades in order.
These shades of the numbers are changed and also the aperture figures move when the shutter speed is changed.
A correct exposure is obtained by selecting the shutter speed in order that the aperture figure selected moves above the dimmest number that can be seen.