Features
The lens is constructed from 19 elements in 14 groups, including two Ultra-low Dispersion (UD) lenses to combat chromatic aberration. The aperture is created from eight blades, as on the latest 70-200mm models, for a smoother bokeh in the out-of-focus areas of your pictures. Focusing is achieved through a USM (Ultra Sonic Motor) for fast and silent operation, while allowing manual override at any time.
The zoom extends the length of the lens by up to 6cm at full extension, and so a lens lock is present to prevent the zoom creeping. A focus window sits towards the back of the unit, with focal distances in feet and metres, and markers for the various focal-length positions.
Three switches on the side control auto/manual focus selection, stabilisation and stabiliser mode, between mode 1 (full stabilisation) and mode 2 (vertical only) for panning. The system offers up to 4EV in added stability, which will make a considerable difference at full extension.
Unlike the 70-200mm lens there is no focus limiter, which seems unusual considering the added length. The focus ring sits in the centre with the zoom ring at the front, suggesting that the zoom will be used most often.
The front optic is just 67mm in diameter thanks to the smaller apertures offered. The aperture range is not unusual for a lens of this length, but I feel users would have accepted a slightly larger unit, perhaps 77mm, if it had a straight f/4 aperture instead of the variable f/4-5.6.
