The Mamiya Sekor SX 2.8/135mm shares its basic optical construction with the Sekor SX 2.8/105mm - and with many other 2.5/135mm, 2.8/135mm and 3.5/135mm lenses: Its Ernostar formula was standard for most 135mm in the 1970s. Most 135mm lenses of the time were constructed and built trying to keep costs as low as possible. While their build quality is excellent compared to todays standards, their optical construction usually was as simple as possible - the Mamiya SX 2.8/135mm being no exception. That said, the performance of these 135mm lenses is surprisingly good, and again the SX is no exception. In fact, it belongs cleary to the best vintage 2.87135mm I know. 

At infinity and on a 24 MP FF camera, the SX 2.8/135mm is as good as the Zeiss CY Sonnar 2.8/135mm. If we are pixel peeping, the Zeiss may have slightly less lateral CAs, but that's completely negligible in real world shooting.