Horrible lenes. You're not going to be 'zooming' them; their fix length, 500mm, 800mm even 1000mm. Be aware that they are stupidly hard to use well. The depth of field is tiny so (manual only) focusing is super critical even at the really slow speed they are (f/8, f/11 or even f/16). Get a 1.4x or 2x teleconverter for the lonest lens you already have IMHO.
The shot above should of been great, but in reality it's unuseable due to the terrible bookeh as andy says above.