For full length portraits and small family groups (2-3 people) the EF
50mm f/1.4 would be a superb choice; on the 20D it would have almost the
same field of view as an 85mm on 35mm film bodies. The EF 50mm f/1.8
would probably be almost as good at a much lower price (I don't have one
of these so I can only go on others' recommendations, but it's almost a
giveaway).

For tighter head and shoulders, the 85mm f/1.8 would be excellent; it
has a field of view approximately the same as a 135mm lens on 35mm.

For real tight head shots, the EF 100mm f/2.8 macro will give shots so
sharp your relatives may complain about the wrinkles; it will also be
superb for flowers and insects.

I have all 3 of the recommended lenses and can endorse them from
personal experience; all should give results good enough to give the
maximum enlargement the camera body is capable of. You should think of
using a tripod to ensure the ultimate in sharpness though, unless using
flash. If you are using natural light, then take care to focus
correctly; the above lenses (especially the first 2) will have very
small depth of field if used wide open, and AF can often misjudge the
point you want to be sharp (which should be the eyes, or the nearer eye
if oblique).

BTW, I assume "lense" is some US eccentricity; it is unknown over here.

David

   I would go for a f 1.4  55mm.   You don't need anything better.

   Having said that, that would be my choice.  Photography is an art and
portraits have much to do about style.  I tend towards a rather tradition
style.  Other people might want to use something much different.

   I chose a f 1.4 so I could reduce the DOF.  I might prefer a f 1.2 but
then there would be some give in other areas.  I would like a lens that was
sharp in the center with a fall off toward the edges, but there are other
ways of getting what I want there.

   I chose a 55 mm because that on a 20d will put me at the right distance
for the perspective I like and gets just enough room between me and the
subject that I can be personal but not overwhelming.

   BTW those 1.4 lenses are very available and not expensive.  The 1.2
would be a bit more expensive.

If you want to "go nuts".. the EF 85 f/1.2 would be the most desirable
portrait lens..   If you just want good results, the 50mm f/1.4 would
be perfectly adequate..

 Money is no object?  85mm f/1.2, first and foremost, then a 135mm f/2.0.
Then 50mm 2nd 35mm f/1.4.

  Buy a 50mm f/1.8 (or, if you really want to spend the money, a 50mm
f/1.4), and take your normal photos, and when you need it, rent the 85mm
f/1.2.

steve

I use three different lences for portraits:

* EF 28-70 f2.8 L USM
* EF 135 mm soft focus
* EF 70-200 f2.8 L IS USM

The 135 mm is the cheapest one, while the 70-200 is the most expensive one.