Experiment

Tried out a Rokinon 800 mm lens today. From Amazon it cost about $200. This dove was maybe 200 feet away and the photo has been cropped, Focus was with live view magnified and mirror lock-up was used. Also exposure and contrast were increased in Lightroom.

Image is soft and compares unfavorably with the Canon 70-200, f2.8L either alone and with a 2X extender.

Additionally the 800 mm is hard to focus and the Canon adapter is wobbly.

In summary it is not worth fooling with so I sent it back for a refund.