What 12mm at f/2 actually gives you
At 12mm this sits in ultra-wide territory, where straight lines near the frame edge stretch noticeably and a small step forward changes the composition a lot. On the APS-C bodies it covers, it frames like 18mm would on full-frame.
f/2 is fast enough to keep shutter speeds up in poor light and to separate a subject from its background clearly.
Size, close focus and filters
At 236g (8.3 oz) it is light enough that it stops being a decision — the kind of lens that stays on the body or in the bag without thought.
It focuses to 13cm, which is close enough to fill the frame with small subjects.
Filters are 67mm — 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 and stabilisation
Autofocus is driven by an STM motor.
There is no optical stabilisation in the lens, so handheld steadiness depends on the body having IBIS, or on keeping the shutter speed up.
