To measure something you have to effect it. If 100% of the light passed through a sensor it could measure nothing. If 50% of the light passed through the sensor would only be half as sensitive as an opaque sensor and the viewfinder would only be half as bright as one that uses a mirror.
|Hot Air Balloons Over Bagan by User9320321874|
|Yellow Warbler by LeeS|
from A Big Year - birds
|Waiting for the Parade by tcoker1103|
from - La Vida Loca - (Black and White Street Photography+ A Border)