Sigma 28-105mm f/4-5.6 UC — compact manual-focus standard zoom from Sigma's Ultra Compact line.
The Sigma 28-105mm f/4-5.6 UC belonged to Sigma's Ultra Compact line of the mid-1980s, a manual-focus standard zoom built to pair a wide-angle start with useful short-telephoto reach in a lens smaller than earlier designs.
Covering 28-105mm with a one-touch zoom and manual focus ring, the UC badge signalled Sigma's push toward more compact barrels for the period. It was made in several SLR fittings, including Pentax K, Canon FD, Nikon F, Minolta MD, Olympus OM, Konica AR and Contax/Yashica mounts.
As a modest-aperture all-rounder it trades speed for range and portability, with softer rendering at both ends of the zoom and at maximum aperture, sharpening up usefully once stopped down to the middle of its range.
Used copies are affordable and plentiful. Check the zoom action for stiffness or looseness, confirm aperture blades are free of oil, and inspect glass for the fungus and haze that affect many stored 1980s Sigma zooms.