When I got my first 50mm f/1.8 nifty fifty lens, it didn't occur to me that I shouldn't use it wide open at f/1.8 all the ...

The RF 10-20mm F4L IS STM is a 5-star rated ultra-wide-angle zoom, but its maximum aperture is f/4 ... like the ultimate money-no-object astro photography lens. What do you think of Canon's ...