No other info other than that it's well-known that MG (and CP for that matter) filters fade; the result with MG filters will generally be that what should be a #4 gives only maybe #2.5 contrast, for example.(posted 8909 days ago)The concept of speed matching uses middle grey as the tone that's held constant; imho that's rather less than useful. Ideally it'd be just under paper-base white.