Many years ago Pop Photo had a testing lab that was used to review new lens performance and publish results.
I seem to remember that 35mm lenses, like enlarging lenses, were optimized in their designs by manufacturers to produce the best images at something like 20X or 100X the lens' focal length or something. I seem to remember that Pop tested at these distances when testing lenses. I can resurrect in my mind how that lab appeared and remember aerial image testing taking place at distances around 20X or so focal lengths.
Does anyone remember or know if, 1.) lenses are indeed optimized at a particular (maybe standard) focal length, and, 2.) what is that number?