The Olympus M.Zuiko Digital ED 12-40mm f2.8 PRO ($999.99 direct) was launched along with the company's top-end OM-D E-M1 camera, adding a true professional-level zoom to its Micro Four Thirds lens library. It's a pricey lens, but the constant f/2.8 aperture, sharp optics, and all-weather design justify its cost. It's an easy choice as an Editors' Choice for standard zoom lenses for the Micro Four Thirds system.

The lens, which is on the large size for the Micro Four Thirds system, measures 3.3 by 2.8 inches (HD) and weighs in at 13.5 ounces. It's compatible with 62mm threaded filters and pairs well with the E-M1 and E-M5, but PEN shooters may find that it's a bit hefty for those slim camera bodies. There's no in-lens image stabilization; Olympus puts that feature into the body of the camera. You can use the lens with a Panasonic Micro Four Thirds camera, but unless you're shooting with the GX7 you'll be without stabilization.

The 12-40mm zoom range is the equivalent of a 24-80mm design in full-frame photography. It's a standard range for top-end zoom lenses, and the maximum f/2.8 aperture is maintained throughout. The lens is protected against splashes, dust, and cold weather, just like the OM-D E-M1. Its construction is sturdy and metal, with a wide, textured zoom ring and a narrower manual focus ring. There's a clutch system to toggle between manual and autofocus; just pull back on the focus ring and the lens switches to manual operation, or push it forward to go back to autofocus. The only other control on the lens is the L-Fn button; you can program its function via your camera body.

I used Imatest to check performance when paired with the 16-megapixel OM-D EM-1. It's quite sharp at every tested focal length, even at the edges of the frame. At 12mm f/2.8 it scores 2,536 lines per picture height using a center-weighted testing method, better than the 1,800 lines that we require for an image to be called sharp. The edges are impressive at 2,100 lines. Performance is just as good through f/8, and barrel distortion is a modest 1.3 percent; that's easily corrected in Lightroom.

There's no loss of sharpness as you zoom. At 25mm f/2.8 the lens manages 2,595 lines, with sharp edges, and only 0.5 percent distortion. Again, sharpness stays steady through f/8. Zooming to the 40mm and you'll see a slight drop off to 2,288 lines with negligible distortion. It's here where you can narrow the aperture just a bit to improve performance; at f/4 the lens crosses the 2,400 mark, and it peaks at just shy of 2,500 lines at f/8.

If you're looking for a professional zoom lens to complement your Olympus Micro Four Thirds camera, this is the one to get. It's a bit large for PEN bodies, but if pairs perfectly with the OM-D E-M1, largely in part to that body's weighty design and deep handgrip. Like the OM-D series, the lens is protected from dust and splashes, and it is rated to operate in temperatures as low as 14°F. The only real knock on the design is the lack of image stabilization. This isn't an issue for Olympus owners, but Panasonic shooters who are looking to pair the lens with a Micro Four Thirds body may be better off with a stabilized Panasonic lens. The Olympus M.Zuiko Digital ED 12-40mm f2.8 PRO earns our Editors' Choice award with ease.