The use of top-quality fluorite elements aims to ensure great image quality with high resolution and contrast, along with minimal colour fringing.
Canon's SubWavelength Coating (SWC) is also employed to reduce ghosting and flare, while the outer surfaces of the front and rear elements have fluorine coatings for easy cleaning.
Optical quality is impeccable in every respect. We were massively impressed at the sharpness and contrast, even when shooting at the maximum aperture of f/2.8 under gloomy lighting.
However, you do need to watch out for depth of field. At a focus distance of 10m with an aperture of f/2.8, DOF is only 10.2cm on a full-frame DSLR camera and a mere 6.4cm on an APS-C-sized body.
Distortion is practically zero, chromatic aberrations are negligible and peripheral illumination is excellent. The lens is also very resistant to ghosting and flare.
Given the long 400mm focal length, or an effective 640mm on Canon APS-C cameras, the 4-stop stabiliser really does have its work cut out.
In our tests, it performed extremely well, not only enabling sharp handheld shots when needed, but working brilliantly when using the lens on a monopod.
Autofocus is similarly top-drawer, with spectacularly fast and supremely accurate performance.