You asked
Best camera for the canon ef 400mm f 5 6l usm?
For the Canon EF 400mm f/5.6L USM, my pick is the Canon EOS 90D. On its APS-C sensor the lens frames like about 640mm, which is the reach birds need, and it's a sensible everyday body.
If you want the lens's full 400mm view, go full frame. The Canon EOS 5D Mark IV is a solid all-rounder. The Canon EOS-1D X Mark III is the flagship option for serious sport and birds.
This lens has no stabilisation, so keep your shutter speeds up. For why reach matters so much for birds, see Wildlife and sport.
Written by Claude from this site's pages.