Vivitar's 400mm f/5.6 — an affordable third-party M42 long-telephoto prime.
The Vivitar 400mm f/5.6 is a long 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 long reach lens in their screw-mount range for distant subjects.
This is a manual-focus M42 screw-mount lens with a 400mm focal length and a maximum aperture of f/5.6. Focus is by hand over a long helicoid, with a preset or automatic diaphragm depending on version. It is a large all-metal telephoto, often supplied with a tripod mount. As with Vivitar-branded lenses the maker varied, and detailed element figures are not asserted here beyond the verified focal length and aperture.
At 400mm f/5.6 the lens gives long reach for wildlife, sport and distant landscape detail, with the moderate aperture keeping it manageable for its length. As a refractive design it renders cleanly stopped down, and it is best used on a tripod or monopod given its size and weight for sharp results.
Used copies are affordable relative to Western long lenses, with quality varying by maker. Inspect the long barrel for internal haze and fungus, confirm the aperture and any preset ring work, and check the focus over its long travel and the tripod mount. Verify the M42 mount. An M42 adapter allows long-lens use on mirrorless with a firm support.