Photoclubalpha

What exactly is a Minolta AC CLR Filter?

I'm guessing clear/coated.

Trying to decide if I should replace with a newer UV (72mm) as a lens protector.

'AC' stands for Achromatic Coating which traditionally is the name for Minolta's multi-layer anti-reflex coating in both filters and lenses ... most other manufacturers would call the same thing 'multi-coating.' It has nothing to do with chromatic aberrations or achromatic lens design; it's just an ill-chosen name.

'CLR' indeed stands for Clear, i. e. a neutral filter with no filter effect. Used before a lens' front element, it serves as a protective filter for lenses with front elements made from unusually soft materials, as in some apo lenses. Used in a filter drawer or as a rear filter, it acts as an optical element because the glass of filters inside lenses is part of the optical design so rather than just removing a filter when no filter effect is required, you have to replace it with a neutral filter of the same refraction index and thickness.

-- Olaf