This is a lens which can truly be described as a professional workhorse, with robust build (including dust and moisture resistance), wide F2.8 maximum aperture, fast and silent ultrasonic autofocus motor, and optical image stabilization for hand-holding at slow shutter speeds. The optical design is somewhere on the far side of complex; it features 23 elements in 18 groups, with 4 UD elements to provide compensation for chromatic aberration. According to Canon, this gives a 'high-resolution, high-contrast optical capability', as demanded from a lens which needs to perform all day, every day in the hands of professional photographers across a wide range of subjects and conditions.