Classic Sonnar forumula, based on the Wartime Carl Zeiss Jena Sonnar. Not all J-3's are created equal, only way to state it. Most are good at F1.5, much sharper at F2.8. Most benefit from changing the shim to optimize for close-up/wide-open work. This one is from 1950, required a complete rebuild- including changing the focal length and setting into a new focus mount. It was worth the effort. These are at F1.5, with a Red filter.