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.
|AF4_2483 Surfing the Serengeti by DaveInHouston|
from Hot Air Balloon view
|Peregrine Falcon by Psychic1|
from Best Wildlife Photo of the Week - 4