I photograph for almost a year in full format with the EOS 5D III. I am very much and also like traveling professionally and personally, my focus has been for many years the travel photography, with all the compromises, which are thereby required. Main criteria: Solid yet light! A few weeks ago I had the opportunity at the photo shop around the corner the 70-200 f4.0 and the 70-200 f2.8 for a day borrowed to get. It then became very strenuous 24 hours. :-) Anticipation may I say that I am not a line counter and not much thought about it for hours to photograph graph paper with countless focal lengths / aperture combinations and the results widerum hours 1: 1 to compare It was important debug both lenses outdoor under similar conditions as when traveling. I did that on a slightly rainy day in my hometown Ulm. Incidentally, I did not have rain gear for this equipment. The Camera and both lenses are splashproof. As long as it does not distribute down with rain ... no problem. To get to the point: "The 2.8-it must not be!" It is bulky, heavy as lead, and forms, in my view at full aperture slightly weaker than the 4.0 from it. In particular, on the edge. It also shows at open aperture significantly more tendency to Vignette as its little brother. An attached UV filter with a socket thickness of only 3 mm reinforced this tendency. The 4.0-he will be spared as much as possible with and without filters at maximum aperture vignetting. You'd have a very monochrome, bright object with diffuse light photograph (in rainy weather no problem) to determine a slight darkening of the image corners to one können.Fotografiert with two lenses in common Aperture range 5.6 - 11, I can with the best will in the practical Application notice any performance differences. The light gray "lads" give up nothing. The distortion is negligible in my opinion at all focal lengths, except the starting focal length, both lenses. Susceptible subjects (eg architecture) are represented largely uncurved. Who but mainly operates architectural photography, is anyway better off with fixed focal lengths. Both L-lenses are processed excellently. One has to keep the feeling of something made for eternity in his hand. All rings run extremely supple, raced the sun visors fill in bayonet lock, the AF motors are virtually noiseless, quickly and precisely. The image stabilizers work very efficiently. Somewhat rested I can do so at Endbrennweite easily exposure times all the way down to 30-STEL. With the 4.0-it even safer, because my arms do not get tired so quickly due to lack objective weight. The stabilization in the 4.0-he mumbles somewhat louder than the the 2.8-er. Anyway, with my lens pair. For the 2.8-I recommend it anyway regular dumbbell training. :-) Conclusion: If you often have to take pictures in low light conditions, sports or wildlife photography operates, is for the the 2.8-He certainly the better choice, especially since, according to test reports it works properly together with the 2x extender III, since the light intensity still then is 1: 5.6. With the 4.0-he no longer works that way. Anyway, you have to give then to the AF. I need to travel possible light and handy equipment and can get over the loss of a diaphragm well, especially since I often work with a tripod. In addition, the sensor of the EOS 5D III is a light giant who also feels fully in his element at 3200 ASA. Noise almost undetectable. Not to compare with the sensor of my predecessors camera. The EF 70-200 4.0L IS USM is certainly among the best of Canon's zoom lens line. Perhaps it is even the Best. From me 5 stars happy. In fairness, I have the lens this time not ordered from Amazon, but the friendly Ulmer photo dealers bought my test model. The Extender 1.4x III comes then from Amazon as soon as I'm liquid again. I hope this review was informative enough, despite the brevity. Some reviews I find simply too lengthy.