early 35 mm slr cameras had similar functionality to larger models, with a waist-level ground-glass viewfinder and a mirror which remained in the taking position—blacking out the viewfinder—after an exposure, returning when the film was wound on. innovations which transformed the slr were the pentaprism eye-level viewfinder and the instant-return mirror—the mirror flipped briefly up during exposure, immediately returning to the viewfinding position. the half-silvered fixed pellicle mirror, without even the brief blackout of the instant-return mirror, was innovative but did not become standard. through-the-lens light metering was an important advance. as electronics advanced, new functionality, discussed below, became available.