D5 & D8
On the screen of the Digital Pointfinder on top of the crosshair you see a symbol existing of a “Sun” and a “+” and “-“. By pressing the “Plus” and “Minus” key you are able to adjust the brightness in 5 steps.
E7500i & D810
When turning the Digital Pointfinder on it is indicated in the display that you can change the brightness with the left and right arrow keys on the keypad.