Glossary
Sonnar
A Sonnar is a lens design that packs its elements into thick cemented groups, minimizing air-to-glass surfaces to gain contrast in the era before anti-reflective coatings.
Also called Sonnar formula
How it works
Fewer glass-to-air boundaries means fewer internal reflections, which is why Sonnars delivered high contrast on uncoated or single-coated glass while competing designs of the day were flarier.
The trade is correction at the widest aperture. A fast Sonnar typically renders low contrast and soft wide open, then firms up quickly on stopping down. Portrait shooters treat that as the appeal rather than a flaw.
The Soviet Jupiter-9 85mm f/2 is a Sonnar derivative and the accessible way into the look. Its many aperture blades keep out-of-focus highlights round even when stopped down, unlike designs with five or six blades that turn them into polygons.
Why it matters when buying
Buy a Sonnar for how it draws at f/2 to f/2.8, not for corner sharpness. Sample variation among Soviet examples is high, so condition and factory matter more than they do for Japanese glass.