A tremendously flexible standard zoom for APS-C-format DSLRs, the EF-S 17-55mm f/2.8 IS USM from Canon pairs the 27.2-88mm equivalent focal length range with a bright f/2.8 constant maximum aperture. Benefitting this design is a sophisticated optical layout, which includes both Ultra-Low Dispersion glass and aspherical elements to significantly reduce chromatic and spherical aberrations for improved clarity and sharpness throughout the zoom range.