If you are doing studio work, a 135mm may have some advantages.
At 85mm you will only be about 3 feet from your subjects. This is pretty close, and you may risk throwing a shadow on your subject.
With a 135mm, you are about 6 feet away (for head shots), which is a much more comfortable distance.
My preference is for very tight, sometimes confrontational, headshots. I use 50's (Nikkors, Makro-Planar, Planar) and 85's, (2.0/85 Nikkor, 1.4/85 Planar C/Y or 2.8/85 Sonnar C/Y), all at wide openings (although up very close f4.0 and 2.8 work best.
I don't mind a bit of distortion. I like my portraits to be very intimate. Male subjects only.
For some examples go to my Flickr, where I'm Miked700. http://www.flickr.com/photos/6636032...7628580833673/
I disagree that wide-angle distortion is only noticeable if prints are viewed from too far away. Our minds are very aware that the print is flat so any wide-angle distortions are very noticeable regardless of viewing distance. Even if viewed very closely wide-angle distortion is quite plain to see because, again, our minds know the image is planar. Whether or not any type of distortion is acceptable or preferable is up to the viewer.
Depth of Field
To me the most important thing to consider is depth of field, and I like to make portraits using an aperture around 18-25mm.
25mm aperture with a 50mm lens is f/2 (50/2=25). With a 35mm lens it's f/1.4 (35/1.4=25). Magically, 50mm at f/2.8 = 18mm, and 35mm at f/2 = 17.5mm. Using a 100mm lens I need f/5.6 or f/4. Etc.
This is completely regardless of format. If I were using my 5x7, which has a 210mm lens, I'd be using f/8 or f/11. 210/8=25mm and change. Etc...
Attached are a couple of portraits, all with different lenses, but similar aperture. To me the lens focal length doesn't matter so much.
For 35mm I like my 105, as it's the longest lens for 35 I currently have. When I had a 135, I liked that. I've used a 180 with great results outdoors, might be a bit long for most studios.
I'll be getting an 85, and I'll like that too. 50 is too short. If you're restricted to a 135 or longer, try using a half or three-quarter profile for less apparent compression if you don't like compression.
Very interesting how, in the 'real world', this focal length determinant is not mired in absolutes. Different lenses create different effects. That attitude, more than anything else, is refreshing, as it segues into the creative element. Some things are worse today but this is an example of thinking that is truly better. Benjiboy's suggestion that one use a 70-150 (assuming for the ability to determine 'on the fly') is potent. Also, agnosticnikon's assertion that even a 500mm (!) portrait was not only acceptable but WANTED by the client (over the others) was interesting.
The feedback here is great and telling, not only from the standpoint of technique, but also from the standpoint of human nature. More opinions are welcome (if not already mentioned). - David Lyga
"even a 500mm (!) portrait was not only acceptable but ..."
Beyond about 180 - 200mm on 35, there is very little change of perspective ("compression").
If you double the standard focal length, there is a very noticeable difference. Doubling again, to 180 - 200, there is a less-but-still noticeable difference, and so on. One advantage of longer lenses is that the background fades into an abstract mush.
The point is to use what you like, what works for you and your subject.
My first lens was a 57/1.4. I used a Soligor 2X teleconverter which gave me, effectively, a 114/2.8. Getting close was not a problem but sharpness was not too good until at least f/8 (effective f/16). My next lens was a 28/2.8 and was not used for portraits. The next lens was a 135/2.8. It was a decent lens but only got down to about 5 feet. It was later traded in for a 135/3.2 which focused down to 3 feet. I found that adequate for tight portraits. Years later I got a Vivitar 135/2.8 Close Focusing lens. That lens goes down to 1:2 which is much too close for a portrait but I find it useful for anything from the near macro range to infinity and very good for portraits. I have this lens now in many mounts.
Over time I accumulated many other lenses suitable for portraits. My favorite may be the 100/2.5 Minolta MC Rokkor. I have taken nice portraits with the 85/1.8 Konica Hexanon, 85/1.8 Canon New FD, 85/1.8 Canon FL, 85/2 AI Nikkor, 100/2.8 Canon FD SSC/New FD, 105/2.8 Super Takumar, 100/2.8 Zuiko etc. They are all good and how far you want tp be from your subject when shooting really is a subjective thing. Medium telephoto macro lenses like the 90/2.5 Tamron or 90/2.5 Vivitar Series 1 are also good for portraits but can sometimes be a little too sharp. I find both older and newer 105/2.5 Nikkors good for portraits. There are times when I will try a portrait with a 200/2.8 Canon New FD or a 200/3 Vivitar Series 1. A lens in the 50mm range can work when shooting a small group of people or 2-3 people standing close to each other but I do not like its look for a tightly cropped photo of one person.
Nothing stopping you from using a 20mm or even a 17mm if people are whacky enough to want a portrait shot on 500mm....
What did they use 120 years ago? For all intents and purposes, a pinhole will do fine too.
agreed, my rule of thumb is double the normal focal length or longer.