In terms of overall build quality, Minolta’s xi power zoom lenses from the early 1990s may represent a high water mark. Minolta’s lenses, like the cameras and lenses from all makers, have gotten cheaper over the years, in more ways than one. The upholstery in 1930’s Mercedes cars was made to last 100 years. Today a car is doing exceptionally well if it makes it to fifteen, and the upholstery certainly will not feel new! In the 1970’s and 1980’s cameras–and especially lenses–were made to last a half century. They were also relatively more expensive in inflation-adjusted dollars. The famous Beercan (Minolta’s 70-210mm f/4 lens from 1985) cost about $700 new in today’s money. The 28-105mm xi powerzoom lens was also sold for $400 new, or about $900 in today’s money. But everybody wants things for less money, so camera makers obliged (as did Mercedes in the 1990’s) and shaved costs in production. Quality declined.
By the late 1990’s the kit lenses for the cheaper (consumer) cameras were so cheap that when Minolta made something to the old standards they needed a way to explain why it cost so much. So Minolta introduced the “G” line of lenses to indicate the quality was still very high. And G lenses sold in the $800-2000 range, new. Today Sony still makes lenses as good as the best Minolta glass. You just have to pay for it. Consider the Sony Alpha “Carl Zeiss” and “G” series lenses. Pricy, yes. But they are as good, and often better, than anything Minolta ever made. I’ve heard nothing but praise for the image quality of the 16-80mm DT Carl Zeiss zoom lens (Sony model number SAL-1680Z, not to be confused with the SAL-1680, no Z on the end!). The build quality is criticized as plasticy, but no one ever disparages the image quality. That lens sells new for $750 today, or about what a 28-105mm xi lens cost new, when adjusted for inflation.
In photography, image quality usually comes at a high price. There are exceptions (see my article on lens bargains for Sony alpha digital camera). Some of the best lenses in the last 20 years are available at greatly reduced prices on eBay. Owners of Sony alpha digital cameras have a bit of advantage. Because the lens motor is inside the camera body, even the very oldest Minolta Maxxum mount lenses will perform quite well on a modern digital camera body. Owners of Nikon and Canon digital cameras have to be very careful not to purchase and lens that contains an outdated motor and therefore focuses unacceptably slow.