ecka said:
I'm not talking about crop vs FF

. I'm talking about fast primes on crop vs fast zooms on crop. Why would I pay $500 for the 28/1.8 or 35/2 or 24 or 20, while there is 18-35/1.8?
1) Weight.
2) Sharpness. Primes of the same price/quality/'age' (when they were designed) will outresolve zooms. Comparing a 2015 zoom to an old beater from 1995 isn't exactly a fair comparison.
3) Low-profile appearance. Not everyone wants to go around with a huge pickle jar bolted on to their rig.
4) Less moving parts or things that can fail.
5) Price.
6) First-party Canon AF routines.
Primes aren't always better than zooms, but implying that a sharp faster-than-previously-ever-offered-before zoom obsoletes the need for primes is, quite simply, madness. There is so much more to a lens than its speed, max aperture and sharpness.
- A