Vivitar's 100mm f/2.8 — an affordable third-party M42 portrait telephoto prime.
The Vivitar 100mm f/2.8 is a short-telephoto prime sold under the Vivitar brand in M42 mount. Vivitar sourced lenses from various Japanese makers, so build and optics can vary by production batch, but this was a portrait-length telephoto in their screw-mount range.
This is a manual-focus M42 screw-mount lens with a 100mm focal length and a maximum aperture of f/2.8. Focus and aperture are set by hand on the barrel, with an automatic diaphragm. It is an all-metal short telephoto. As with Vivitar-branded lenses the maker varied, and detailed element figures are not asserted here beyond the verified focal length and aperture.
At 100mm f/2.8 the lens is a portrait and short-telephoto tool, giving flattering compressed perspective and good subject isolation. Vivitar telephotos of this type render cleanly with pleasant out-of-focus areas, suiting portraits and distant detail as a versatile everyday long lens for adapted use.
Used copies are affordable, with quality varying by maker. Inspect for haze, fungus and cleaning marks, confirm the aperture blades are clean and oil-free, and check the aperture-ring and focus feel over the longer travel. Verify the M42 mount before purchase. An M42 adapter makes it an affordable portrait telephoto on mirrorless.