I have a Tamron 17-50 mm, f/2.8 non-VC lens and a Canon 50 mm, f/1.4, and decided to compare the two, because I don't need more clutter in my life and I was thinking to sell the Canon. I have read the numerous lens reviews and was thinking to see if I could see the sometimes mentioned benefit of a prime lens.
So, I took a series with my camera mounted on a tripod, same subject (a cluster of orchids), in the same light (indoors, using window light). In aperture priority mode, I shot at ISO 100, f/4, 5.6, 8, and 11. Anyway, the result was that all of the zoom lens photos were slightly smaller (I didn't move the tripod) at 50 mm than were the prime--I am thinking that the Tamron is really about 45 mm at 50 mm. And, all of the Canon photos were about 1/2 ev darker, as a result of the shutter speed being consistently less on the prime. Not sure why this is. Otherwise I couldn't tell you, in a double blind comparison, which lens took which photos.
So, I guess my question is--in general, is there an appreciable difference photogenically between the images produced by a decent prime and an equally decent zoom? I can't imagine that there is. If the manufacturer has good QC of the complexities of moving the lens groups about inside the lens, it seems to me that they must produce images indistinguishable, whether they are from a zoom or a prime. But, maybe I am missing something.