>>1113270Similar to sensors.... f/stop or "speed".
Lower f-stop number = bigger aperture hole = more light.
That blurred-background effect called "bokeh" comes from those big-ass apertures, and their associated depth-of-field.
With glass, you do get what you pay for. Shoot for f2.8 zooms or f1.8 primes if you want good optics. The lower-end f3.5+ glass isn't terrible, but when you've shot with good f2.8 stuff, it's hard to go back to the cheaper/slower stuff.
Some lower-level DSLR's don't have autofocus motors in the body, so you have to go with a lens with an internal AF motor. Most modern lenses have them, but there's some real good old stuff that doesn't.
Nikon's used their F-mount for lenses for decades, almost any F-mount lens can mount to even a new/modern body.
>70-200 VRII at 92mm, 1sec f/2.8 ISO500 