Conclusion: Best lens I've ever used for aviation photography. All of the versatility needed to capture images in any setting, and the built-in converter loses almost nothing I can detect in terms of image quality or focusing speed. Extremely sharp, extremely versatile, and built ruggedly for travel to rugged places.
Pros: Fast auto-focus, Superior build quality, Super-sharp images, very verstaile telephoto range, built-in teleconverter
Conclusion: This is the finest lens I have ever owned! It is lightning fast and crisp and the 1.4 extemder transition works brilliantly. It is heavy so I recommend a GITZO 2541 travellers tripod with the lightweight Acratech Ballhead. It is such a gem that I carry it in a Think Tank Glass Limo backpack! The uses are nature and sport photography but I have used handheld for neighborhood shooting!
Conclusion: I took delivery of the new EF 200-400mm f/4L IS USM Extender 1.4X three days ago. First impressions out of the box were exactly as I expected. From the workmanship of the lens to the case and accessories, the quality is Canon’s finest. Having worked with the 600mm, the size and weight were as expected. Though I have taken a couple dozen handheld shots resulting in tack sharp images, you will need to use this lens with a tripod or monopod. I am using the 5D Mark III body.
Pros: Fast auto-focus, Superior build quality, Super-sharp images, versitility