"The selection of half-stops does make a difference in the amount of light each opening allows through a lens."

Half-stops are the aperture adjustments midway between the standard f-stops. These are useful when just a little more or less light is needed to make the correct exposure.
An Aperture ring is used to adjust the diaphragm for the various f-stops. The f-stops are clearly marked, generally in white, on the ring. This makes them easy to see the black color of the lens barrel. A diamond shaped line, usually red, is marked on the stationary area of the lens barrel next to the aperture ring. An f-stop is set by aligning the selected f –stop number and the red line.
Exact f-stops, whether full or half, are easy to adjust. The aperture ring contains "clicks" or slight notches that can be felt through the fingers when the ring is moved. The clicks alco can be heard with most lenses.