DxOMark published their test results for the Sigma 18-300mm f/3.5-6.3 DC Macro HSM C Pentax mount lens ($499). Their conclusion:
Sigma’s latest 18-300mm super zoom boasts a greater focal range compared to its predecessor, the Sigma 18-200mm, and manages to improve overall image quality, too. Lens metric scores are towards the lower end of the performance spectrum compared to bigger, heavier, more expensive and less versatile options, however, which is the price you pay for the convenience of a lightweight super-zoom. With greater sharpness, less distortion, and fewer chromatic aberrations, the Sigma 18-300mm is the best-performing lens of its type on the Pentax K-5 IIs, slightly outperforming Pentax’s own 18-270mm alternative. What’s more, its 1:3 magnification capabilities and compatibility with Sigma’s AML72-01 filter for closer 1:2 macro photography open up some creative possibilities. If you’re in the market for a Pentax KAF lens mount super-zoom, put the Sigma 18-300mm at the top of your list.