I've had the Sigma APO 300mm f4 Tele Macro for a few months and love it but wanted to compare it to the Canon and Nikon offerings. I found the Canon EF 300mm f4 L USM and Nikon AF-S 300mm f4 ED IF on Fleabay and tried them out for a week. Here is my experience. The Nikon is heavier and longer than both. And contrary to most review sites, I found it to be the sharpest lens at f4 of all three. At f8 they are all equally sharp. But at F4, the Nikon was king. Colors and contrast were superb. But, the tripod mount is needlessly complicated. Its impossible to get it off when the lens is mounted to the camera. Even unmounted, it has rivets on the lens and you have to unscrew the mount from the lens and back it off. It is also the heaviest mount of all and it has some flex in it. The lens also has no glass behind the diaphram, meaning dust can get pretty easily. Manual focus is terrific on this lens, but you are limited on the aperture ring to f4, 5.6, 8, 11, 16, 22, 32. No in between stops. Minimum focus is at 1.45m with no macro capability. And there is some CA at f4. I got the Canon with the Kipon AF adapter. Doesn't work that well. It will autofocus, and you get a green box confirmation. But it backfocuses most of the time and the shots you get are not sharp. This lens also has CA at f4. It is lighter than the Nikon, but longer than the Sigma. Minimum focus is at 2.5m, which is LONG. No macro. With manual focus, the lens is sharper than the sigma but I liked the colors on the Nikon better. The Sigma has a great tripod mount, is light enough to walk around with, is the shortest of the bunch, and is so close in sharpness at f4 with the other two I don't see the point in keeping the Canon or the Nikon. Back they go. I wanted to love the Canon because I had heard so much about it. But so much better it isn't. I wanted to love the Nikon because the copy I got is in MINT condition, and the build quality is insane. My Sigma copy is beat up on the outside, but super optics on the inside. It doesn't MF as well as the big boys in terms of how the focus ring feels, but it focuses down to 1.2m, has macro capabilities, nice colors, and it costs 1/3 of the Nikon and Canon. I'm sure the Olympus is sharper and focuses better than any of these. But in real world applications, the differences between any of these brands at 300mm are small and down to personal preference and budget. I'm sticking with the Sigma and will send the rest back.