The “Industar 61”and lanthane glass.

The “Industar” lenses are constructed following the “Tessar”design, elaborated in the year 1902 by an employee of the C.Zeiss company, Dr.P.Rudolph. The original design appears in many shapes in the history of the photographic lens  in many countries, under diifferent names such as Ektar,Elmar and Xenar.
The Soviet Union at the time was labouring under its 10-year plan  “Industar”. Hence the perhaps impopular name “Industar 61”
The optical calculations were performed by the soviet optical scientists G.G.Sliusarev and W.Sokolov.
The lens was issued as the standard lens for the FED, and as an exchangable lens for the “Helios”.
It cost 16 rubles.
The excellent optical characteristics of the “Industar 61”were obtained by the use of a positive element made of crown glass with added lanthanum, STK-6. ( See article about the Helios.)
Ou can find very many examples of working solutions in the litterature on lenses. ……
When designing lenses, exept the most simple ones, you have to accurately satisfy Petzval’s condition, which corrects the curvature of field and also to take into account the correction of the chromatic aberration. To decrease the latter, in general you need at least two different kinds of optical glass or similar materials. In general, a positive lens made of crown glass is used with a negative element made of flint glass. (Positive means that the thickness in the center is larger than at the edge, negative is the opposite. So called crown glass has a small index of refraction and a large dispersion, (=difference between the refractive indices for light of  different colour), flint glass is the opposite.  
Simultaneous correction of the chromatic aberration and the curvature of field can possibly put too much strain on the design of the lens surfaces. And, it may lead to an increase of higher-order aberrations. To avoid that, a combination of so-called anomalous glasses is used- heavy crown glass 
(with for instance added lanthane) and light flint glass, where in stead of the usual-refractive-index crown a higher index value is combined with Flint.
The refractive power of a lens made from STK is about double that of a normal glass lens.
Patent applications for recipes for lanthane glass have been appearing for some 30 years now, but an industrially made half-product appeared in the USA in 1941 from Eastman-Kodak.
The application in lenses of the glass type STK with high index of refraction allows the correction of the aberrations that were discussed above. Although in general, this creates increased freedom to correct the astigmatism, and spherical and spherochromatic aberration, it does not do so for all lens formulas. In multi-element designs the application of extra-heavy crown glass may not give an improvement over the more usual glass types. Some 40 years ago, Kodak supplied the “Aero-Ektar” lens which was mounted in the cameras for aerial photography in the so-called “Flying Fortresses”.
One half of the lens  containing positive elements was realised using lanthane glass, and this is superfluous in this application. Professor D.S.Volosov invented a version of the Aero-Ektar made-up of the usual glass. He made an experimental lens, that in quality was no worse than the Aero-Ektar.
There is an example to the contrary: the multi-element lens “Geogon”, designed by the American optical designer Baker. Not an attempt to imitate soviet optics, the use of  lanthane glass in the Geogon allowed the preservation of the image quality.
Conluding for now: The four-element lens “Industar-61”has almost the same resolving power as the six-element “Helios-81”.  So is it worth the trouble to put in 6 lenses where 4 will do? There is ample evidence, how the Helios-81 reproduces small details with a substantially better contrast. Naturally, six lenses will produce better results.

Type                                                Industar 61 L/3
Focal length                                    52  mm
Angle of view                                  45 degrees
Relative opening                            1:2,8
Smallest Diaphragm                       16
Closest focussing distance             0,3 m
Elements/groups                            4/3
                                      Centre       46
Resolution (lines/mm)    av field      37
                                      Edge          27
Vignetting                                        48 %
Dimensions                                     59x57 mm
Weight                                             225  g.