1 Quality threshold. Above a certain level, for a given print size, all lenses above the threshold are likely to be indistinguishable from one another. Lenses below the threshold are likely to show different degrees of different kinds of defects: lack of sharpness, low contrast, distortion... The threshold is not very high even for an 8x10 inch print, and on a monitor, it's very low indeed.
2 'Magic'. Some lenses suit some people so well that they are prepared to pay whatever they can afford. My 75 Summicron is such a lens for me. The 75/2.5 Voigtländer is very good, but not 'magic', and besides, as I rarely carry an SLR any more, it's useful to have one lens that focuses very close (by RF standards).
3 Some people see more differences than others. A friend of mine, an ex-editor of a photo magazine, invariably comments when he sees even an 8x10 from my 38 Biogon, "That's got to be a Biogon shot." I like the Biogon, but I don't see as much 'magic' as he does.
4 With a reasonable-sized wet print (i.e. not strained through a scanner and computer) differences are often much clearer.
5 Most of the time we are indeed looking at the picture, not for varying kinds of technical quality, but I'd lay odds that many people can see quite significant differences in technical quality between different prints if they bother to look, even in a blind test. This doesn't mean they can guess which lens was used, except perhaps in a very few cases such as a Thambar or 50/1.2 Canon, and then only at some apertures. Even then, I can't always tell the difference between my Thambar shots ($3000 ultra-rare Leitz) and my Dreamagon shots (100 euros from Monchrom).
6 There may be no alternatives for a particular camera, as with the Thambar, so unless you want to use/carry two systems, you may have to buy the expensive lens if you want a particular look.
Cheers,
R.
