Just posted: Our quick review of the Canon EF 200-400mm f/4L IS USM Extender 1.4x: the world's first SLR zoom with a built-in switchable teleconverter. It's been two years in the making, but when Canon offered to show us a production version of its professional super-telezoom, we couldn't help but be intrigued. In this review we've teamed up with DxOMark to bring you full optical test data of this unique lens, backed up by some quick real-world examples. So how does it perform? Click below to find out.