As seen in PetaPixel, photographer Mathieu Stern mounted the 21M to his Sony A7 II mirrorless camera to test it in a variety of photographic situations, including portraits, wildlife, and close-ups. He also shot some video footage with it. His conclusion is that the lens has held up very well over the years, producing sharp results with strong detail, but it’s also worth noticing the beautiful aesthetic quality of the images that came out of it. F/4 lenses aren’t usually known for their smooth out-of-focus (or bokeh) patterns, but the Jupiter glass is absolutely gorgeous, even from a purely subjective point of view.
No camera manufacturers are using the M42 mount anymore, although at one time it was so popular that it was known as the “universal screw mount.” Fortunately, with adapters, it’s possible to use M42 lenses on just about any mirrorless camera and most modern DSLRs with full manual control, although Nikon cameras will require a refocusing adapter, as the flange distance for M42 is shorter than that of the Nikon F mount.
Also, as it is rather old, the 21M doesn’t offer any modern conveniences, like autofocus or autoexposure, and it weights over two pounds. However, it’s currently going for as low as $60 on eBay (while supplies last, of course) so it’s hard not to recommend one to any photographer curious enough to give it a try. For comparison, Nikon’s 200mm f/4 sells for $1,300, although it does have autofocus and macro capabilities.