Excellent choice for a 'walking about' telephoto lens

I chose this lens as a compromise to get the maximum focal length against weight (and cost) as I tend to walk around when shooting wildlife rather than use a tripod or monopod. The 500mm f/4 is just too heavy for me to walk round with or hand-hold at eye-level for long periods.The 400mm f/4 DO can...

Relatively lightweight; f/4 max aperture allows a x2 Extender to be used

Expensive; Focus not the fastest with a x2 extender

The EF 400mm f/4 DO IS II USM lens delivers brilliant high-speed performance for professionals with minimal glare and superb image quality. Taking advantage of the best in lens technology, it features gapless dual-layer diffractive optical elements for improved diffraction efficiency with reduced...

After reading countless blogs, tutorials and reviews ...

I am a huge fan of my Canon 600mm f/4. The sharpness of that lens continues to blow me away. I realized that over the past few years all my wildlife pictures were taken between 400-800mm. My next in line lens fixed tele-lens was an old 300mm Sigma f/2.8 without IS.

Close but no cigar!

Would prefer a f3.5 version of the DO lens. Because of price, plus size and weight, It would soon become the most popular telephoto for wildlife, just as it was for many years with Nikon.