[WayneF]

The lens surely depends on your lights. If trying to use continuous hot lights, then a f/1.8 lens is probably pretty important, probably at ISO 400 to achieve a 1/60 second shutter. But if using decent flash units, like Alienbees, light is no problem, and you will be up around f/8 or f/11, and a f/1.8 lens is about the last thing you need. F/8 or f/11 also improves the inexpensive f/4 or f/5.6 zooms.

We hear of 85 to 105mm "portrait lenses" historically, but of course, that was for full frame 35 mm film, in the days before the 1.5x lens factors. For most of todays DSLR, that would be 50 to 70mm now, to be equivalent.

Focal length depends a bit on how much space you have, but you do always want the camera to be at least 5 or 6 feet from subject for proper perspective, to prevent making noses look larger when up too close. So for the DSLR 1.5x lens factor, you want 50mm to 70mm to get proper framing for a portrait (at the same distance as the 85-105mm full frame lenses for film - at least 5 or 6 feet).

You need 5 or 6 feet behind the subject too, in front of background, so we're talking a minimum of 15 or 16 feet for the total setup. With an 85mm lens today, you'll need quite a bit more space, to back away far enough from the subject. Which is fine if you have the space, but such space is a bit rare sometimes.