Industar 50-2: the tiny Tessar that could
The Industar 50-2 is a 50mm f/3.5 pancake based on the Tessar formula — four elements in three groups, the same design that made Zeiss famous a century ago. It was kitted with Zenit cameras for decades, which means it's everywhere, and it means nobody respects it.
They should. From f/5.6 this lens is simply sharp, corner to corner, with the gentle, slightly low-contrast micro-rendering that makes faces look like faces instead of dermatology reports. At f/3.5 it glows a little. Call it a flaw or call it a feature; portrait sitters call it flattering.
The build is comedy: an aperture ring the size of a shirt button, placed where your fingers never are. You adapt by feel within a week. At 70 grams it lives on the camera as a body cap that takes pictures.
Best use case: the lens you bring when you're not "going out to shoot". It has produced more keepers per gram than anything else I own.