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.
|Kinderdijk by PEB|
from Best Landscape With at Least One Wind Mill.
|Lights of Manhattan by cand1d|
from Your City - Night Skyline
|Mornin Dew by Abbasi46|
from Macro world
|Crash and Boom by qhenson|
from My Best Photo of the Week