OK, so now I am going to ask a really fundamental question which will make some of you say "it's obvious" and others will say "Hmmm?".
Here's a couple of quotes from another thread.
Originally posted by Igilligan For me, on the digital bodies the 135 is just a bit too long for most of my snap portraits. The Jupiter 9 is even a little long but I use it a lot for head and shoulder shots. ... Since you are looking at those two lenses, also consider a Helios 44M, the version with the A/M switch on it. It is a 58MM f2, so the length is good for portraits and it is sharper at F2 than the Jupiter 9 is.
and
Originally posted by kuuan For portraits I'd also recommend a 50 or 55 mm lens as they make roughly 75-80mm lenses on a Pentax dSLR.
I have often heard the "around 80mm for full frame" quoted for the perfect portrait lens. And it seems that the consensus is that focal length is the main criteria, with a minor criteria of the lens being not so sharp as to show up skin imperfections. Softness can be done in post processing, or if you really insist, you can use a softon filter or take a picture slightly out of focus. So using a lens that is purposely soft seems a little unecessary.
Surely the behaviour of the lens to skin tones is more important, and two lenses of the same focal length will not necessarily be the same in this respect. For example, my M50/1.4 gives a much warmer colour rendition than my Pentacon 50/1.8 where the images are quite cool.
But why the emphasis on focal length? I mean, in simple terms, if you fill your frame with head and shoulders, the focal length is just a matter of how close you are to your subject, right? If you have a short focal length (and rectilinear) you have to get closer, if you have a tele then you have to get further away.
The other difference between focal lengths is, of course, the effect of perspective, and this is really where my question lies. A long focal length will give a flatter rendition, so there is less apparent depth to the image. I can imagine that you don't want a portrait to be "flat", but going on that, why consider anything longer than "normal" focal length for the frame size (so for APS-C why consider anything longer than 40mm)? The classic "about 80mm" for full frame is certainly longer than the "normal" length for the format, so a certain "flattening" is considered good, but what determines how much is good? I have seen images of outdoor fashion shoots where the photographer is 15m away using a long tele because that was considered the right focal length (and the curious sight of the photographer's assistant with the reflective board next to model taking instruction over a walkie-talkie) and it certainly made me wonder why.
For the record, on film (full frame) the best portraits I took were with a CZJ 135/3.5 wide open: the depth of field was just right, but what made the images was the colour rendition. On digital (APS-C) the best portraits I have taken have been with my SMC Takumar 55/1.8 (stopped down a bit), for the same reasons as the Sonnar: the skin colours looked exactly right. Incidentally both are sharp lenses and if the images are sightly soft, it is my fault and nothing to do with the lens.
Richard