Up until a few years ago, the idea of an 800mm lens fixed at f/11 would never have been considered a possibility. This is because DSLRs simply won’t work with a lens at a minimum f/11 aperture, as it can’t let enough light onto the autofocus sensor for the AF to function properly.
In fact, only the top-end DSLRs like the Canon EOS-1D X Mark III will autofocus at f/8 – but if, for example, you’re using a Canon EF 100-400mm at f/5.6 with a 2x extender, so are shooting at f/11, the AF won’t work.
