Review by Klaus Schroiff, published December 2021

Introduction

It may come as a little surprise but the Sigma 100-400mm f/5.6-6.3 DG DN OS is actually slightly longer than the DSLR design. At longer focal lengths there isn't much you can do about the size unless you start using e.g. fresnel lenses which come which their own challenges. The build quality seems to have improved a bit specifically near the mount where Sigma has replaced the plastic rear tube with a metal one. The weather sealing is still rather basic with just a gasket at the mount. As usual, the lens uses an extending zoom mechanism towards longer focal lengths. Besides the zoom ring, you can also push/pull the inner tube at the front. Sigma calls this a "Dual-action" zoom mechanism. A transport lock is also available at the 100mm setting. The focus ring operates smoothly. A deep barrel-shaped hood is part of the package. 

The stepping motor used for auto-focusing is pretty fast and noiseless. You can also control the AF using a dedicated AFL button.  The optical image stabilizer (OS) offers a claimed gain of up to 4 f-stops. As usual, the real-world efficiency will vary on a number of external factors such as your coffee level, focus distance, or simply because it's windy. Besides the standard stabilization mode, there's also a dedicated mode for panning.