I've been shooting with my Industar-50 on my Leica IIIc. (When this roll is done the Industar-50 goes on my Zorki 3M.) It's a sweet little lens that collapses making it easy to put in a coat pocket.
It's a Tessar design. The Tessar optical design goes back to 1902. The Leitz Elmar 50/3.5 on the early Leicas was a Tessar. Actually, a version of the Elmar 50 is still in production.
It has four elements in three groups. When the Soviets copied the Leica II they also copied the Elmar in a series of lenses that ended with the Industar-50. The Elmar had the diaphragm between the first two elements while the Industars had the diaphragm in front of the third group. It's a little slow at f3.5 but that's an OK price to play for its small size. Most shooting isn't done below f3.5 anyway. For a faster lens I have a Jupiter 8 or a Leitz Summitar. Both need repair work, though. Later Industar-50s had a rigid body and there was a rigid body version for the Zenit SLRs, first in M39 mount and then in the still popular M42 mount. I've not done much shooting with my Pentax SLRs because they won't fit in my coat pocket with a modern lens. I've been noticing the M42 Industar-50s on eBay. Tiny little things. They can be had for between $10 and $15. I had my son's Zorki 4 at Oleg's for a CLA and new shutter curtains when I noticed he had new Industar-50s for $15. Oleg sent one back with the Zorki with no extra shipping.
It's not the prettiest lens but it makes the Pentax H1a small enough to fit in my coat pocket.
That's a Super Takumar 55/2 on the Spotmatic. The Spotmatic won't fit in my coat pocket. The I-50 is derived from a rangefinder lens and doesn't have automatic stop down or presets. You focus wide open and then stop it down the old fashioned way. The I-50 aperture ring is on the front. This would probably cause long time SLR shooter's heads to explode but I'm used to that sort of thing. It does make the viewfinder dim but it's been working for me. It also works to preset everything like many do on rangefinders. The H1a doesn't even have a built in meter so using this combination is a lot like using my rangefinders except this one will focus down to .3 meters and you can frame accurately at that distance. This is not a lens for low light conditions. For that I have the 55/2 and maybe someday a Super Takumar 50/1.4. So far I've enjoyed shooting this combination. The prints will tell if it's worthwhile. But a camera with a slow lens in your coat pocket always takes better pictures than the camera with the fast lens that is left home.
But wait! There's more! The I-50, with an adapter, will fit on our digital Pentax *ist DL making it a 75mm equivalent. How is that for a compact long focus lens?