At almost £2000, this pricey L series optic puts in as good a performance as expected
The rearmost part of the barrel contains a focused-distance window, to the left of which sits the slider switch to change between manual focusing and combined AF/MF. Focusing is carried out internally, without disturbing the focus ring in AF mode yet always allowing manual intervention. The distance window includes depth-of-field markings and an infrared focusing index.
One problem ultra-wide lenses have is an inability to attach filters at the front owing to the fact that this would cause vignetting: This lens overcomes this by providing a large gelatine-filter holder behind the rear element. This facility would previously have been useful for contrast control in mono film photography (as would the IR focusing index) and digital users are unlikely to value these features as highly but will appreciate the rubber flange that provides a dust- and water-resistant seal between the lens and host camera body.
Other features include aspherical profiles and ultra-low dispersion elements (which both should enable top-notch images); plus ultrasonic-motor focusing for quick, quiet, and accurate focusing.
Technical testing reveals a very impressive peak resolution just short of 0.3 cycles-per-pixel at f/8 but a slightly weak performance wide-open and, even more so, with the aperture fully closed down. Distortion is very well controlled: straight lines remain straight even close to the frame edge.
Overall this is a very impressive lens. For those who would not use such a lens often enough to justify its purchase price, hiring or choosing a zoom alternative might be the better option.