Sony NEX-5n vs Nikon D5100

I realized that I might not have been clear in my question.

Say I have a Leica 35mm f/1.8 lens. It is a 35mm on a full frame sensor, but on a APS-C sensor, it is 35mm x (if I recall correctly) 1.5 = 52.5mm lens, but does it still have a f/1.8 aperture with a f/1.8 depth of field? Or does that change to a f/2.7?

On a 4/3 system, the crop factor is 2 vs 1.5. So the same Leica 35mm f/1.8 is now a 70mm lens (and does it then have a f/3.6 maximum aperture size?).

Thanks!