image by Stonepicker (Image rights)
The Helios-44 is one of the most mass produced Soviet SLR lenses, a 58mm f/2 normal.
It is essentially a Zeiss Biotar clone-- this was a fast normal for the Exakta mount. The Helios-44 was made for the Zenit m39 mount and then for the m42 mount among others. Early versions have 13-blade diaphragm and later ones have 8 or 6 blades.
As a Biotar copy it is a modified Double Gauss design with two groups of three elements each, relatively symmetric, and well corrected for aberrations. This puts it in a family with a wide variety of common SLR normals.
All were SLR lenses. Most versions were not automatic, so the aperture control was by preset, aka. dual ring: one ring with click stops and marks for the f/stops controls the position of the preset stop, while one free-moving ring actually controls the aperture, but stops when it gets to the preset stop. Thus, by twisting the latter ring until it stops, the lens can be conveniently closed down from wide open to a selected f/stop without taking one's eye away from the viewfinder, and vice versa. This continued to be Soviet camera industry practice except on the most basic lenses well after West German and Japanese makers went more or less exclusively to automatic apertures.
The Helios-44's are generally noted as good lenses, the common allegation of poor quality control notwithstanding. Bokeh is the lens's most commonly-mentioned attribute, with a medium-fast max aperture of f/2 giving plenty of opportunity to blur the background. The bokeh, as on many early SLR lenses, is "swirly--" that is, the light disks formed by out-of-focus points of light distort away from the center of the image, and when there are many of them, they can form a sort of "vortex" or "swirl." Many photographers seek out this effect due to its evocative appearance.
Optical formula from OPAL database
Cистема-Гелиос-44 f'=59 2w=40 1:2 Вариант-ST01FB06 21/ 8/2017 0:57 OPAL-PC КОНСТРУКТИВНЫЕ ПАРАМЕТРЫ ОПТИЧЕСКОЙ СИСТЕМЫ N Радиусы Осевые Световые Марки Показатели преломления пов. кривизны расст. высоты стекол L= .589300 .434050 .656280 ВОЗДУХ 1 38.070 4.810 14.750 ТК14 1.613002 1.625839 1.610006 2 136.365 2.260 14.750 ВОЗДУХ 3 25.330 9.070 13.200 ТК14 1.613002 1.625839 1.610006 4 -124.225 1.310 11.600 ЛФ7 1.578297 1.596755 1.574256 5 15.995 9.330 9.750 ВОЗДУХ 6 -16.620 1.320 9.350 ЛФ7 1.578297 1.596755 1.574256 7 66.085 6.250 10.200 ТК14 1.613002 1.625839 1.610006 8 -22.210 .500 10.600 ВОЗДУХ 9 191.540 4.940 12.350 БФ16 1.670899 1.689317 1.666790 10 -52.725 12.350 ВОЗДУХ ПРЕДМЕТ: Удал. размер Y = -20.00000гр.мнсек S0= .000дптр от вх.зрачка ИЗОБРАЖ: Близ. размер Y'= .00000 плоск. SI= .000мм от пл.Гаусса ДИАФР. : ND= 5 плоскость SD= 4.7000 мм задняя апертура= .2500(sin) Относительные величины предмета : 1.000 .707 Относительное виньетирование верхнее: .250 .000 Относительное виньетирование нижнее : .250 .000