Are lenses made specifically for crop sensor vs. full frame cameras or can I buy lenses for my crop sensor camera and use them if I ever upgrade to full frame? Say, I bought a Canon 50d and was planning to get a 5d in 3yrs or something. Could I buy good lenses now and upgrade to full frame later keeping the lenses. Or is money spent on lenses for a 50D lost money when/if I upgrade? Obviously I can sell them so it wouldn't be completely lost but you get what I'm saying?