Sigma 300-800mm f/5.6

Focusing distance: 600cm / 236.2in
Magnification: 0.14x
Filter size: 46mm
Lens size: ø156.5mm x 549.4mm / ø61.6in x 216.3in
Weight: 5915g / 208.6oz
Construction: 18 elements in 16 groups
Accessories: Shoulder strap, circular polarizing filter, Hood, Case, Tripod adapter (built-in)

Eddie's take on it

The mythical Sigmonster is a full frame lens that, with the 4/3 crop factor, delivers a 600-1600mm equiv. field of view, which is absolutely ridiculous.

Over the years, I have never seen it for sale used in 4/3 mount. It would be intersting to test it out, but given the extreme focal lengths, modest aperture and massive weight/size, I'm not sure how usable it would actually be on a 4/3 or m4/3 camera, if you wanted to do serious work with it.

One clear alternative to the Sigmonster is the Olympus 300mm f/2.8 with the EC-20 2x teleconverter. This give you 600mm reach (1200mm full frame equiv.) at f/5.6 and is actually very high quality.