general purpose - general practitioner
i.e. be able to handle any subject competently!
I'm not convinced that it is necessary to know how to shoot macro, astro, motorsports, equestrian, indoor sports, landscapes, smoke and water droplets, products, BIF etc. etc. in order to be able to shoot portraits. While general photographic competence will obviously provide a head start, if one already has it, if starting from scratch I don't see how it is helpful to go off and master the art of shooting football if what you actually want to do is to shoot portraits. Would you go and play cricket for a season in order to prepare you for a career in tennis? You might as well just go ahead and learn to play tennis in the first place.
What is needed is a basic appreciation of light, shade, contrast, DOF, exposure, perspective, focusing, posing, building a rapport with the subject, and specific lighting techniques and tools for portraiture. Outside the studio there is the additional need to know how to make best use of available light, shade and backgrounds and how to balance flash and ambient both in quantity and colour.
While I do have a decent grounding in general photography I only started shooting portraits this year. I spent a considerable time learning from YouTube and other videos, a portrait lighting DVD, plenty of reading and attending a couple of TP "studio" meets for practice and experience before investing in studio lighting and backgrounds. I'm fairly sure that the best way to learn portrait photography is to study and practice portrait photography. However, I'm not so sure that buying a bunch of gear (lights, backgrounds, specialist lenses) specifically to do studio portraiture, before you have sufficient understanding to know what you need, is a good approach.
Moving back to the topic of lens choice....
In the studio I'm not sure the need for shallow DOF is as great as outside it. With a plain background it's not as though you need it to be blurred into oblivion and I think that having only one eye in focus, or the ears soft is probably a bit of a niche technique that takes second place to having your subject properly sharp. Nothing wrong with it, but not bread and butter. This was shot at 195mm and
f/5.6. Would it have been better at f/4, f/2.8, f/2? Personally I think the DOF is more than shallow enough.
Lighting was from a couple of Speedlites firing into brollies. This was taken on my first ever "studio shoot", which was actually a TP meetup arranged in a church hall in Rivenhall, Essex.
I do have a 50/1.4 and an 85/1.8, but it's the 70-200/2.8 I turn to for portraits on my 5D2, and then I stop down in the studio.
This is from my second TP "studio" meet (in a community hall) and was taken with my 7D at 100mm and
f/8, with studio lighting providing plenty of power (500Ws into huge softbox on camera left, Hilite acting as a reflector for fill on camera right, a studio head and Speedlite for the background).....
An 85/1.8 or 100/2 would have done just as well, but there's no need for the fast apertures for this type of shooting.
May as well throw in a couple from my third TP "studio" meet (in a hotel). The first is with a single Speedlite (Canon) into a brolly at 100mm and
f/7.1 (hair/rim light courtesy of mother nature). The second is with a couple of Speedlights (Nikon!), I think through brollies (one for subject, one for background), plus a hand held white reflector, at 130mm and
f/6.3....