Originally posted by Larrymc I would think you would want a lens to accurately (clinically) portray the scene or subject.....then there is processing. Someone explain "Pixie Dust" and how can you determine the "Pixie Dust" quality of a lens with a RAW file?
I don't subscribe to the notion of "pixie dust", per se. It's largely subjective and over-used. In general, though, I think it describes the unusually-pleasing character that a lens brings to images.
If I were to take an artistic portrait with my Sigma 17-50 f/2.8 at 50mm f/4, then take the same shot with my Zenitar-M 50mm f/1.7, also at f/4, I can tell you now that - for my own tastes - I would prefer the shot from the Zenitar lens. I can observe some obvious factors at play, but couldn't begin to tell you every contributing factor that makes the Zenitar image more appealing to me.
Optical formula, sharpness at various points of the image frame, quality of out-of-focus rendering (both in front of, and behind, the focus distance), the transition from in-focus to out-of-focus areas, colour, contrast, various aberrations, field curvature, vignetting, geomeric distortion, flare resistance etc. all contribute to the rendering characteristics of a lens. Some of these can be partially emulated in post-processing, but some of them are almost impossible to recreate substantially.