At a 35mm-equivalent focal length of 600mm, Olympus delivers the first real pro-level supertelephoto lens for the Micro Four Thirds (MFT) mount with its M.Zuiko 300mm f4 IS Pro. Mirrorless MFT cameras have a smaller lens mount and tend to be substantially smaller than APS-C-based mirrorless and dSLR models, so this comparatively compact lens can significantly decrease the schlep factor for serious wildlife and sports photographers. Provided, that is, you're willing to sacrifice a little of the photo quality and resolution those larger-sensor cameras afford.
At $2,500, £2,200 or AU$3,500, the lens unsurprisingly bears a pro price tag as well. But if you need the distance or the its comparatively close focus and shoot with an MFT camera, especially one of Olympus' newer Sync IS-supporting cameras, it's worth it.
In those cameras, such as theI tested it with, Sync IS combines two-axis optical and three-axis sensor-shift stabilization for a total of five axes, using a new IS control unit and gyros in the lens and body. Olympus claims 6 stops of stabilization in this configuration, and my testing bears that out. I was able to handhold as slow as 1/10 second, though I could only get consistently stable results at 1/13 second -- if you're steady-handed, you'll probably do even better. If you want to slip it onto your Panasonic, you should still get about 4 stops, though I didn't test that.
Olympus claims that this is the company's "highest resolving lens ever" with "very little edge distortion." I can't validate the former statement, but my photos bear out that it resolves extremely well through f16, with the expected slight sharpness drop-off by f22, and I saw practically no distortion, vignetting or aberration/fringing. It incorporates a new coating, Z Coating Nano, a layer of nanoparticles and air on the front of selected lens elements to reduce the amount of light that gets reflected off the elements' surface (coatings reduce flare and fringing).