For crop sensor, in the case of Sony, there is a 'multiplier' effect, so 50mm has an effective focal length of 75mm (50 x 1.5), whereas on FF, 50mm = 50mm.What is a 1.5 multiplexer?
Canon / Nikon is slightly different at 1.6 or something or other...not so sure. Same concept though but harder to do mental arithmetic.