I was reading my manual and I came across the Manual Settings section. It says "Aperture can be set to values between the minimum and maximum values for the lens." I'm using a 18-70mm 1:3.5-4.5G lens, if you could explain it using those numbers to me. How come on my camera body, aperature can go from 3.5 to 22 (on my D50). What does that have to do with my lens' aperture range?