Helios-44 is a series of standard lenses for SLR cameras, produced in Soviet Union by KMZ, MMZ, Jupiter from 1958 until 1999; it is considered to be one of most mass produced lenses in the world. Based on the lens Biotar 2 / 58 , at the beginning was called "БТК" - "БиоТар Красногорский" (BioTar Krasnogorski). A fast 6-element anastigmat, it was available in both single-coated and multi-coated versions.
The name of Helios lenses are usually writen as Helios-44X-N, where X -is the index of lens mount (M for m42 thread, K for Pentax K bayonet, Д for Zenit-D bayonet) and N is a optical resolution index 2, 3, 4, 5, 6, 7 (the higher number the higher is an optical resolution of lens).