Comparison of 300mm L vs 100-400mm L

I hired a Canon 300mm f/4 L IS USM prime lens from Lenses For Hire a few weeks ago, and took it to Chester Zoo to put it through its paces. I was also interested to see how it performed against my existing Canon 100-400mm f/4-5.6 L IS USM, with the assumption that the prime would be sharper as well as having a greater maximum aperture at 300mm. I’ve heard stories about bad 100-400mm lenses, and while it should be obvious if a lens is really bad, it’s harder to say if it’s just a little soft without having an equivalent lens to compare to. The below shots are full size output from each lens, taken with a Canon 7D. They have been converted from RAW using Bibble with the standard settings and no other colour/sharpness/contrast adjustments. As these were taken with an APS-C sensor, it’s important to note that the images are using the central part of the lens, which is normally sharper than the far extents. Using these lenses on a full-frame camera will yield different results. The images are of York Minster and are cropped – the left hand side is the far left centre of the