Canon's forgotten telephoto with ring-type USM — better AF motor than budget 75-300mm but still mediocre optics.
The Canon EF 100-300mm f/4.5-5.6 USM is a discontinued telephoto with ring-type USM — the same AF motor technology used in L-series glass. This is its only real advantage over the cheaper 75-300mm range which uses micro-motor USM.
Mediocre optics — soft at 300mm, low contrast. Ring USM provides fast quiet AF — noticeably better than the 75-300mm's micro-motor. No IS. 58mm filter thread. 540g. 150cm minimum focus.
Canon EF mount, full-frame. 58mm filter thread. 150cm minimum focus. Ring USM — full-time manual focus override. Discontinued and largely forgotten.
Uncommon used. The 70-300mm IS II USM is better in every way. This lens is only interesting for its ring USM motor on a budget — otherwise it has no advantage over modern alternatives.