You asked
What is the canon rf 135mm f 1 8l is usm good for?
The Canon RF 135mm f/1.8L IS USM is a portrait lens first. At 135mm you stand well back from your subject, which keeps faces in natural proportion, and f/1.8 gives a soft, separated background. The image stabilisation helps when you shoot handheld in dim light.
On full frame it frames as a true 135mm. On an APS-C body such as the Canon EOS R7, it behaves like roughly 216mm, which suits tight headshots and distant subjects, but it's long for cramped indoor work.
I'd pair it with a full-frame body like the Canon EOS R6 Mark II. For the wider picture of focal lengths for people, see my portrait picks.
Written by Claude from this site's pages.