Depth of field for a digital camera is a hell of a lot greater than with
film SLR. That having been said, the focal length previously considered
in 35 mm circles to be a "portrait lens" is too long for digital with a
1.6 crop factor. This is 75mm to 90mm, depending on the maker.

I use a 50mm 1.4 lens for most portraits. This is roughly equal to the
75mm of a true portrait lens however... The lens is too sharp for mature
aged portraits. I partly overcome this with the use of a "Duto" filter.
These are hard to come by now but are clear glass with circles etched on
the glass. They soften an image - ironing out wrinkles and other skin
imperfections. You can get a similar effect by lightly sandblasting a
filter. I have also used the body oil from scraping where your nose
joins your face and smearing it on a filter. This provides a soft look.

I wouldn't bother too much about the eyes/ears focus depth. Many of my
best portraits concentrate on the so called 'features' of a person's
face. It is only in reportage and travel shots, sharpness is beneficial
to a portrait. Some of the most highly regarded portraits from well know
photographers are softly focused. The digital brigade would probably
call them out of focus due to their lack of detail.

There is such a thing described in almost every book on the subject of
portraits as "the perspective of a portrait". Some techno geek from the
frozen wastes of Canada told me perspective is not tied to focal length.
For a portrait it is. He is yet to show me his "traditional portrait"
taken with a 14mm lens. (Douglas sniggers quietly)

For the purpose of discussing the artist merits of images taken with a
camera, I will offer you the suggestion that you get a better
perspective to your portraits when you use a telephoto lens. You will
need to move farther away than with a normal lens.

The 85mm Canon Portrait lens is a really nice lens but if you have 1.6
crop factor, you'll get too much depth of field and it will be too much
length, for studio portraits. Why not try the (relatively) inexpensive
50mm f1.8?

There are portraits and there are portraits.  Most people who have their
portrait done for "family" or "corporate" style want themselves to be in
focus from the nose to the ears.  For full figure (or head to thighs),
more DOF usually looks better.  an f/2.8 or better lens in 50mm to 100mm
(for cropped sensors) is ample, but such a lens will begin to peak on
sharpness at about f/5.6 to f/8.

Photographers and art department editors may have need for photos where
there is feature isolation (eye in tack sharp focus, other eye soft,
nose soft, etc...).  So a fatter (f/1.4) lens is best.

The f/1.2 and f/1.0 lenses are more bragging right than anything else,
and are not noted as particularly sharp compared to the f/1.4 lenses in
the portrait range.

Yes, FL is proportional to distance for the same framing.

(note that a lens that says "50mm" might be 48 or 49; and a lens that
says 135 might actually be 131 or so).

Cheers,
Alan

Its an artistic choice, but it seems a hallmark high volume low cost to have
everything in focus, including the texture of the background, and many
portrait photogs use the shallowest f/no they can.

I used to shoot a 4x5 wide open with a DOF so shallow that only one eye was
in focus.   Lots of old hollywood images were like this.

Thanks everyone for the good advice - I think photography must be one of
those disciplines in life where "the more you learn the more you realise how
much you have to learn!"

Perhaps I should give up on portraits and go for nudes instead - Hmmm -
would probably need a super-wide-angle lens for that!