It’s common for super-zooms to exhibit similar behavior with only average sharpness (particularly at the longer end of the range), strong distortion, vignetting and chromatic aberrations as well as other deficiencies. Such large zoom ranges stretch technology to their limits, and it’s this group that’s also one of the most price sensitive. Nevertheless, they are compelling for their flexibility and can’t be ignored. When viewed against rivals the Sigma performs very well indeed. For Canon users, it’s an excellent alternative to both the cheaper Tamron 18-270mm F/3.5-6.3 Di II VC PZD and the more costly Canon EF-S 18-200mm f/3.5-5.6 IS. The same can pretty much be said for Nikon users, though they have the advantage of a much newer Nikkor-branded optic. That has an extended range and slightly higher sharpness but its control of fringing is poor and at nearly double the price (in the USA at least) the Sigma is an attractive proposition and would make a solid choice.