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.
|Glass by Hobbyfotograaf|
from High speed photography
|Northern water thrush by APenza|
from A Big Year - birds
|4th of July Outfit by Jack Sofield|
from - Old Age - (Black and White only + A Border)