Quote Originally Posted by Colin
1.6x applies to the effective equivalent aperture.... So f/4 on full frame is 'faster' in effect, than f/2.8 on a crop sensor.

The 'crop factor' applies to aperture in terms of effective depth of field, but not light-gathering ability - in other words, you'll have the same exposure settings at f/4 on full frame and a 1.6x crop body. But, the full frame sensor will have less noise, so performance in low light will be better.