I have the D90 and the 18-200 and 50 1.8. On the D90 the 50mm is really like 72mm right? I find it reachy enough for most situations. It's a great lens and better than the 18-200 in low light for sure. I have not shot with the 50 1.4 but I can't see the extra money being worth it. I'm very happy with the 1.8. The 18-200 is a great lens but if I could afford a bag full of prime lenses that's what I'd have.