You can't expect the same performance from a superzoom lens like the Sigma 18-300mm F3.5-6.3 DC Macro OS HSM Contemporary ($579) as you can from a prime lens or a shorter high-quality zoom like Sigma's own 17-70mm F2.8-4 DC Macro OS HSM .
16.7x zoom range; 1:3 macro reproduction; Image stabilization; Available for multiple APS-C systems; Compact
Inconsistent sharpness; Noticeable image distortion; Dim corners