This high-performance lens was designed with portability and handling ease in mind. One Super UD-glass element, whose characteristics are similar to fluorite, and one UD-glass element result in sharp pictures from corner-to-corner.
The EF lens is compatible with both full-frame and cropped-frame Canon SLR cameras, though on the latter the lens will have an effective field of view of approximately 640mm.
Canon's L-class lenses typically offer excellent resistance to chromatic aberration, and the 400mm ƒ/5.6L is no exception. On both the sub-frame 20D and full-frame 5D, CA performance is practically identical: at no time do we detect more than 3/100ths of a percent of frame height of chromatic aberration. If you are really concerned about CA, performance is better with the lens wide open at ƒ/5.6 and gets very slightly poorer as you stop down.