Anyone actually compared results.
I have a Sigma AF lens, and I’ve never been totally satisfied with the results compared to my manual Nikon lens.
Has anyone else noticed a difference, or am I just being old-fashioned?
Mediocre is an applicable term here. Good enough for holiday snaps, but not exactly pro material. I bet the Nikon wins hands down though.
I’m not much of an expert, maybe others can help you better. Personally, I own a Zeiss-Ikon Contaflex (SLR, 1954) for the more elaborate pics (also, only with clear weather) and a Rollei Prego 90 P&S (new, Schneider Kreuznach P&S lense that outperforms 90% of all SLR lenses 
).
Coldfire
“You know how complex women are”
- Neil Peart, Rush (1993)
Back in my PENTAX days, I had a little SIGMA 35-70 push/pull zoom. It was possibly my favorite lens for my ME-SUPER (great camera!).
Now I’m a NIKON person and have a SIGMA AF 400mm tele and a 90 AF macro. I bought the 400 because it was on sale (even if it wasn’t on sale, it would have been a hell of alot less expensive than a Nikon lens). I think its picture quality is much better than any of the mirror lenses out there (including the Nikkor 500mm CAT).
The Sigma macro is not as good as my ancient Micro-Nikkor 55mm manual lens, but it’s a little faster even at the bigger focal length.
Rumor has it that some of the PROMASTER brand lenses are in fact make by SIGMA .