Quote: I very much want my potraits to look natural and in the right perspective ,it would seem therefore that as long as I steer clear of wideangles most good lenses between 50m and say 135m should produce satisfactory results.
Agreed, wide angles are generally best suited to environmental portraiture though I have seen examples where they have been successfully used for more 'formal' portraits - though you need a subject who really doesn't mind the lens practically being stuffed up their nose.......
This was shot on a 20mm lens on a 1.5x crop D300
In a shot like this, I feel the distortion of the wideangle adds to the image but it does depend on your subject.
To add another recommendation, I've just ordered the Sigma 50-150 f/2.8 zoom as a portrait lens. This will give me a similar angle of view as a 70-200mm on an full frame DSLR/35mm body. I should receive it before noon tomorrow so will have the weekend to try it out and report back before I head off to Edinburgh for the Festival.