What 400mm at f/2.8 actually gives you
400mm is genuine long-telephoto reach, with the strong compression and narrow angle of view that comes with it.
f/2.8 is the working standard for a fast lens — a stop or two of low-light headroom over a kit zoom, and noticeably more background separation.
Size and filters
At 3450g (7.61 lb) this is a lens you plan around — it wants a larger body to balance against, and a tripod collar or monopod for extended use. A tripod collar is included, which moves the mounting point to the lens's centre of mass rather than hanging the weight off the camera's tripod thread.
Filters are 52mm — worth checking against what you already own, since polarisers and ND filters at this size are reusable across any other lens sharing the thread.
Autofocus, stabilisation and weather sealing
Autofocus is driven by a Dual Nano USM motor.
Optical stabilisation is built in, which stacks with in-body stabilisation on bodies that have it and does the work alone on bodies that do not.
The barrel is weather-sealed, so rain and dust are a manageable risk rather than a reason to pack up.
