Doing portraits are fun. I do portraits of my friends, relatives and other people. Basically when doing portraits a lot of people usually ask what is the best portrait lens to use. From my opinion it's best to use a lens which is 50mm above. Most of the times a lens that is below 50mm will give you undesired distortion...not good if you are doing headshots. Also it depends on what kind of camera body you are using. I use a 1.6 crop body so it means that a 50mm isn't really a 50mm. you multiply it by 1.6 and you get 80...which means my 50mm becomes an 80mm, still good for portrait photography. If you are using a full frame body such as a 5d or a 1d then your 50mm is REALLY a 50mm. So basically you can still get away by using a 35mm lens on a crop body and it becomes a 56mm.
My main portrait lenses for portraits are my Canon 50mm 1.8 and my 70-200 IS USM lens. Both lenses deliver ultimate results for this type of work.
My Canon 50mm 1.8 mk2 lens. A very cheap lens which delivers results. If I ever lose it, I'll buy another one. C'mon, it's not bad for a $100. The cheapest prime lens you can buy. Plus it's got a 1.8 maximum aperture to give you that depth of field you always want for creating really blurred backgrounds to isolate your subjects. Of course the 1.4 version is better for 3 times the price...but for now this works for me.
Another lens that I use is the 70-200mm IS USM which is perfect for outdoor portraits. For a 200mm lens you can really blur out that background especially with that 2.8 maximum aperture.
Sample shot from my 50mm lens.
Sharpness is easily obtained with this lens because it's a prime lens...meaning it's a fixed lens so there are no moving parts inside except for the focus mechanism. So basically, prime lenses are sharper that zoom lenses in principle. That is why the sharpest macro lenses are prime lenses.
This is shot using my 70-200 IS USM lens. Notice the soft creamy background due to the high quality glass plus the zoom range of this lens. At 200mm you can really isolate your subjects from the background.
More sample portraits at my pbase site at: