400mm

This is a fairly tight crop of a shot from the E-M1 with a Canon lens adapter and a Sigma 70-200 f/2.8 lens zoomed in as far as it will go. So on a micro four thirds camera that brings us to 400mm. I shot on a tripod with pretty reasonable settings - ISO100, 1/500th second. Wide open f/2.8 aperture  of course because I can't control aperture on Canon lenses with this adapter. But the Metabones adapter is *soooo* expensive. That will have to wait. 

I have in mind to buy a micro four thirds native 100-300mm lens so that will be the equivalent of 600mm AND I'll have aperture control and auto focus. 

This was edited for detail using Macphun Intensify Pro. No other tool is so well suited to this particular job.