This Canon, I mean cannon, I mean “Prinxgalaxy”, is just your average 400mm, except you can use it as a baseball bat (tried&tested). It has no stabilisation and is an F6.3 so good luck photographing kingfishers in the rain, however its price is just 5% of the Canon 400mm, and you can pick up a 2x teleconverter for M42 mount lenses for around £10 too, though then you end up with an F8.0.
It’s fair to say I don’t use this lens very much, you can only really use it in the UK (weather joke incoming:) between the 15th and 16th of June, at around 3pm, for 4 minutes, pointing at the sun – then you’ll get a good photo of your dog.
Jokes aside, the lens does perform reasonably well when filming, on a steady tripod with an extra lens mount. The best feature of it in fact, is the smooth aperture ring. I use this to make my own fades when filming and even to change perspective – a shallow depth of field portrait suddenly becomes a deep cityscape – something you can’t really do in post.