Sigma 18-50mm F3.5-5.6 DC Asp.

The Sigma AF 18-50mm f/3.5-5.6 DC is an budget lens which comes with a couple of weaknesses but also surprising strengths. At 18mm the level of distortions and CAs is quite extreme. Vignetting is also an issue at 18mm and 50mm - the latter is quite unusual especially because it doesn't really help much to stop down here. However, if you limit your aperture settings to f/5.6 and smaller the resolution figures are pretty impressive and comparable to much more pricey zooms out there. Regarding the extremely low price tag Sigma had to implement some compromises on the mechanical side like a rotating front element and a rotating focus ring in AF mode but other than that the build quality of this little lens is actually quite decent. Regarding its weaknesses the corresponding Nikkor AF-S 18-55mm f/3.5-5.6G ED DX may be a better choice though.

The Sigma AF 18-50mm f/3.5-5.6 DC is not a lens without flaws - specifically quite extreme distortions and very high CAs at 18mm as well as soft borders at wide-open aperture at 18mm and 28mm. Nonetheless you can get lots of bangs for your (few) bucks with this lens. If you limit your aperture settings to f/5.6 and smaller the resolution figures are pretty impressive and quite comparable to much more pricey zooms out there. Sigma was smart enough to avoid more ambitious max. apertures where optical problems are a magnitude more pronounced. Naturally Sigma had to implement some compromises on the mechanical side like a rotating front element and a rotating focus ring in AF mode but other than that the build quality of this little lens is actually quite decent. So if you can live with its shortcomings and you just can't spend more for a faster zoom this Sigma is a viable option and all-in-all a slightly better bet than e.g. the cheapo Canon EF-S 18-55mm f/4.5-5.6 II.