50 lumens, one candle power is 12 so 4 candles. Better be pitch black or this will disappoint..

#1, that "better be pitch black" comment is true for a lot of projectors.

The conversion [actually one candela = 12.57 lumens] assumes that the light is emitted over a solid angle of 4*pi steradians, or a solid sphere. Most projectors don't emit their light in all directions simultaneously.

Put another way, "lumen" is total light output. "candela" is a spot measurement. The two do not directly compare.