One thing I try to stress in my class, which is sometimes ignored, is the value of what we call in the biz “fast glass,” meaning lenses that feature big apertures. In general, the gold standard in my line of work is f/2.8. It represents the point at which most camera/lighting/lens combinations can get the job done.
Most of the “kit” lenses sold with SLRs today start at about 18mm, an adequately wide angle focal length (with APS-sized sensors), and zoom to about 55 or 70mm. To keep cost and weight down, these lenses have maximum f stops that shift when you zoom them, from f/3.5 at the wide setting to f/4.5 or f/5.6 at the long end. In many of the limited lighting situations that I shoot, f/3.5 is barely adequate, and f/5.6 is out of the question. Though I often recommend to my students that they consider a “prime” lens (one that doesn’t zoom), since they are cheaper, lighter, and sport nice, big maximum apertures, these lenses lack the entertainment of zooming, and I don’t see very many in my classes.
One of my favorite lenses is the big, heavy, expensive AFS-Nikkor 28-70mm f/2.8. It’s sharp through its whole zoom range and focuses amazingly fast. It’s a problem-solver for me, and without it I would have to rethink what I shoot, and sometimes miss out. So much of photography, especially in my work, is on the margins of light and shutter speed, that it would be much more difficult to shoot at smaller apertures.