Canon's budget EF telephoto with USM motor — marginally improved over the Mark II but still basic optics.
The Canon EF 75-300mm f/4-5.6 III USM was released in 1999 as the third-generation update to Canon's budget telephoto zoom, featuring micro USM for faster, quieter AF than the DC motor versions — note that despite the 'USM' designation the III uses micro USM, not ring USM. At 480g and 58mm filter thread it is compact for a full-frame telephoto zoom. On APS-C the 120-480mm equivalent extends the effective reach.
The optical design uses 13 elements in 9 groups. Seven aperture blades. The 58mm filter thread is consistent across all versions of the 75-300mm family. At 480g the lens is lightweight. Minimum focus distance of 1.5m. Micro USM motor provides AF — unlike ring USM, full-time manual focus override requires switching to MF mode. No Image Stabiliser. Variable aperture from f/4 at 75mm to f/5.6 at 300mm.
The III USM's micro USM motor is faster and quieter than the DC motors of the original and II versions — making it the preferred 75-300mm variant for photographers who want reliable AF at minimal cost. Without IS, minimum shutter speeds of 1/300s at 300mm on full-frame or 1/480s on APS-C are required for sharp handheld results. The 75-300mm III USM is among the most common entry Canon telephoto zooms in the used market.
On the used market the EF 75-300mm III USM is very affordable as an entry Canon telephoto zoom. Condition checks: micro USM AF response, zoom ring smoothness, aperture blades for oil contamination, and front element for marks. The EF 70-300mm IS II USM provides IS and Nano USM at a substantial premium. Compatible with all Canon EOS DSLR and R bodies via EF-RF adapter.