Canon, like most Japanese lens makers, use the optical specifications of a lens to distinguish one model from another. (European makers have traditionally come up with fun names resembling Star Trek planets, like “Tessar,” “Biogon” or “Super Angulon,” to describe their lens designs) But sometimes a maker will produce new models later on which happen to have the same basic specifications as previously-made ones. To distinguish these lenses one from another Canon will add a Roman numeral to the end of the lens specification, starting with II. For this reason you will never see a lens marked with a I, though people often refer to “mark I’” lenses when subsequent models are released. You will, however, see lenses marked with II, III, IV, etc. These are commonly referred to as mark II, mark III, etc, lenses in common parlance.

Sometimes these later lenses are an improvement over the original, sometimes they’re worse and sometimes they are almost identical bar some cosmetic changes. For example, the 50mm 1.8 II is markedly inferior to its predecessor in build quality but has the same great optics, the 28-80 3.5-5.6 USM II is entirely worse in every respect to the mark I edition, but the 28-105 3.5-4.5 and 28-105 3.5-4.5 II lenses are basically the same with slight cosmetic differences. There’s unfortunately no way to tell from the Roman numeral designation itself whether or not a later lens is better or worse. I try to identify these differences between versions in my PhotoNotes lookup page, so that’s a good starting point.

Canon also release updated versions of lens hoods using Roman numeral designations, as described above.