Two different ways:

1. With the "Brightness Control" knob, always use the lowest possible setting, becasue it increases the resolution on the dark side of illumination.

2. With the "Iris Diaphragm Lever" turn the lever to the point towards the darkest setting where you can still see the image clearly.

3. The combination of the both the brightness control knob and the iris diaphragm lever will work together to give you the perfect resolution.