Canon's 500mm f/4L EF IS II USM Lens has all the upgrades needed to make it the go-to long telephoto lens for professional photographers and anyone who has stringent standards for the gear they use.
First and foremost, image sharpness has been enhanced with completely redesigned fluorite optics that deliver reduced chromatic aberration. The lighter weight (1.5 lbs lighter due to magnesium and titanium construction elements) is always welcome in large telephoto lenses. Using 3 IS (Image Stabilization) modes allows you to shoot at shutter speeds 4 stops slower, and the advanced third IS mode activates only when the shutter button is fully pressed, so you can pan with fast-moving subjects without the IS function interfering with composition and framing.
Buttons and switches have been redesigned to give you more intuitive operation, and dust and water sealing keeps the lens functioning even in less than ideal conditions. A Power Focus mode allows smooth focus change while shooting video.