This was the first Canon wide angle interchangeable lens for SLR camera that had the floating mechanism in its optical system. The floating mechanism moves a part of the optical system separated from the focusing lens group to minimize fluctuation of aberrations caused by focusing distance change. A inverted telephoto lens type wide angle lens like this lens, having negative power like a concave lens for front lens group and positive power like a convex lens for rear lens group, tends to have more astigmatism at shorter focusing distance with more extension. The floating mechanism corrects this astigmatism well. This mechanism is named “floating”, because an optical part moves like a float, not in union with the rest of the focusing group. S.S.C., an abbreviation of Super Spectra Coating, is a coating technology developed by Canon to achieve harmonized color balance for all lenses.