Verdict. The Canon RF 16mm f2.8 STM is a compact ultra-wide prime lens for the full-frame EOS R mirrorless system, aimed at landscape, architecture, astro-photography and vlogging. Announced in September 2021 alongside the RF 100-400mm, the RF 16mm f2.8 costs around $299 or 319 pounds, making it not just the widest prime lens in the RF system 2cbn4.