Helios-44 Lens

One of the early preset versions with a 49mm filter ring. Photographed with an AF Nikkor 50mm f/1.4D lens on a Nikon D2Xs.

The 42mm screw mount was introduced in 1947 with the Zeiss Ikon Contax S 35mm SLR. Other manufacturers adopted it: Praktica, Pentax, Zenit. But bayonet mounts are better for changing lenses, and M42 was eclipsed. With some makes of digital SLR cameras, it is possible to use the M42 lenses via an adapter ring. Canon and Pentax are probably the most suitable for old M42 lenses.