The Sigma 100-400mm f/5-6.3 DG DN OS Contemporary is a subtle improvement over the older DSLR version - both optically as well as mechanically. The image quality is primarily better at the wide end of the zoom range with a much higher sharpness in the outer image field. The high quality is maintained in the middle range. At 400mm, the center sharpness remains very good but there's a drop in corner performance. Stopping down helps to broaden the really sharp zone. Keep in mind that the resolution has its limits because of the inevitable diffraction at f/6.3. Lateral CAs are low. Uncorrected image distortions are fairly high for a tele-zoom lens with a peak of 2% (pincushion style) at longer settings. This is, of course, not a drama with activated auto-correction. This also helps with vignetting which otherwise would be very visible at the long end as well. The quality of the bokeh is decent for a zoom lens albeit primes still rule the scene in this respect.

The build quality of the Sigma lens is quite impressive for a consumer-grade lens. Plastics have been replaced by metal near the mount and there's at least a mount-gasket providing some shielding against dust and moisture. Few will probably take advantage of this but it's nice that Sigma provides the option of attaching a tripod mount. The AF is snappy and combined with the effective image stabilizer, it's fun to use in the field - also because it's relatively compact and lightweight for such a lens reaching 400mm.

The decent optical- and mechanical qualities don't even cost a fortune either and, as so often with Sigma lenses, the 100-400mm f/5-6.3 DG DN OS Contemporary is an obvious value king. If you want higher quality, you'll have to invest quite a bit more.