Image quality: Even at full aperture can convince the Sigma. It is extremely sharp in the image center and drops only very slightly towards the edges off, but remains at very high levels. Only on the wide angle there is weakening a little at maximum aperture. Stopping down to f4 brings visible improvement. Also, the Bokeh is commendable to mention. For a zoom lens, it provides a very soft and harmonious bokeh and is very suitable for cropping.
Flexibility: One of the biggest plus points in addition to the picture quality. The zoom range is ideal. The wide-angle covers most situations and is well suited for landscape photography and interiors. The focal length of 70mm is ideal for portraits and in conjunction with the minimum focusing distance is also good in the macro area well equipped and it saves one or the other objective change.
Quality and processing: Comparing it with the kit lens one talks of worlds. Although it weighs just twice, but is not too heavy for the D5100 and gives you the feeling to hold a high-quality lens in hands. The materials are very positive and also in the processing there is nothing to complain about. The zoom ring run tight pleasant and prevents unintentional adjustment. Only the focus ring could be a bit more resistance tolerated and a slightly longer path between minimum focus distance and "infinite".
Flares: The only real gripe I have about this lens, the flare in backlit scenes. You can sometimes be quite large and are then only hard to come by EBV out. But you know this problem, can enjoy the view angle or the focal length change something that usually gives the problem Remedy. Sometimes they are the image and positively beneficial and can be used as a design element as they are never ugly with their characteristic green color and can harmonize well with landscapes.
Autofocus: Pleasantly quiet and very fast. I can find no front or back focus.
Image Stabilization: Does what it should and is quite effective, 3-4 stops more are no problem with the OS. Only in quiet environments, he can be heard clearly in part, but none if disturbing.
Conclusion: In my opinion, the perfect "Always top" for DX format cameras. Image quality, workmanship and good light intensity are unbeatable for the price. I have a lot of fun with this glass. Thank Sigma and thank Amazon for the quick delivery.