I will be purchasing a GH3 to use for filming Wildlife and Motorsports. I don't plan on using the camera for photos. I am haivng a hard time choosing a telephoto lens for this body. So far I've been considering a Sigma 150-600 Contemporary with a Metabones Speedbooster which would give me a maximum focal length over 800mm and 1 stop of extra light. Or the Lumix G Vario 100-300 which would give me a 600mm reach.
This will be set up on a railsystem with follow focus, field monitor, mic, mixer recorder, etc. Obviously I'm not concenred with AF, and OIS is just an added plus, but I probably won't ever use it since I'm on a tripod 99% of the time.
With that said, I'm looking for some input on my two candidates and I'm also looking for other potential options.
Any zooms that have an internal zoom mechanism so the front of the lens won't extend? (I like to use a mattebox)
Any older manual lenses that might fit the bill?
Thanks 
