Having watched reviews on Youtube about this lens, read many other reviews mostly all positive I decided to give this a go, only had a week tried it out in the garden using high contrast articles such as bright cleaning fluid bottles at a range of about 15mts using 400mm, 200iso i was pleasantly amazed at the sharpness, even enlarged with just touch of definition in Iphoto you could quite cleary read the small print on the rear of bottles, I have used it once also to shoot red kites on a very bright day again very pleased with results. I use this with a canon7d mk1 & find it fast to focus on moving objects, not too heavy to hold with a very smooth zoom action, i used to own the Canon 100-400 mk1 & would consider the image quality to be every bit as good & focuses just as fast on 7D, there has been a lot of comments about the lack of a tripod collar, i tried mine on 7D with a velbon sherpa tripod & found it rock solid when zoomed out at 400mm with no downward movement, could be different on a smaller body, but this lens is light enough to carry on a shoulder strap all day. all in all i would consider the Sigma 100-400 OS C to be very good value for money