you'll technically be exposing it "normally", you're just cutting down the light down if you want to slow your shutter speed as if exposing for the shutter speed(1 more stop over a 100asa film) req'd by a 50asa film.

Shoot Ektar 100, or use the Gold 100, its good stuff too, not as fine grained, but I've used it @ ASA64, and the reds/yellows(warmer colors) POPPED. Like E100VS poppy.