The Panasonic Lumix G X Vario 12-35mm F2.8 ASPH. ($1,299.99 direct) is the company's take on a pro-level zoom lens for its Micro Four Thirds cameras. The lens covers a 24-70mm field of view in terms of full-frame photography, offers a maximum f/2.8 aperture throughout its zoom range, and is optically stabilized. It's impressively compact, but it lags behind our Editors' Choice Olympus M.Zuiko Digital ED 12-40mm f2.8 PRO, a lens that is sharper from edge to edge and less expensive. But Panasonic shooters may still opt for this lens; unlike Olympus Micro Four Thirds cameras, most of Panasonic's lineup lacks in-body stabilization.

The 12-35mm is pretty compact; it measures just 2.9 by 2.8 inches (HD), weighs in at 10.8 ounces, and supports compact 58mm threaded filters. The Olympus 12-40mm is noticeably bigger; that lens is 3.3 by 2.8 inches, weighs 13.5 ounces, and its larger front element requires you to invest in 62mm filters. It can focus as close as 9.8 inches, while the Olympus 12-40mm gets you a bit closer at 7.9 inches. The zoom ring is a knurled design with a rubber covering; the focus ring has a similar texture, but is bare metal that is cool to the touch. The standard accessories—a hood, front and rear caps, and a storage bag—are included.

I used Imatest to check the performance of the lens when paired with the Lumix GH3. The lens betters the 1,800 lines per picture height that we use as a cutoff for acceptable sharpness at every tested aperture and focal length, but its performance at the corners and edges is a bit disappointing. At 12mm f/2.8 the average sharpness across the frame is 1,866 lines; that's based on a center-weighted score—the center of the image is tack sharp at 2,065 lines, but part-way distances drop to 1,700 lines, and the edges drop to 1,500 lines. Stopping down the lens to f/4 improves things a bit: the average sharpness is 2,114 lines and the part-way score is 2,012 lines, but the edges still hover around 1,500 lines. There's a nominal amount of barrel distortion, about 0.9 percent, that is just barely noticeable in field conditions.

Zooming to 25mm eliminates the distortion and improves the part-way performance, but the edges are still weak. At f/2.8 the lens manages 2,033 lines, with edges that hover around 1,600 lines. Stopping down doesn't do anything to improve the sharpness. At 35mm you get some pincushion distortion, 1.1 percent, and sharpness is similar to what the lens shows at 25mm; at 35mm f/2.8 it manages 2,095 lines using a center-weighted score, with edges that hit 1,632 lines. Stopping down to f/4 finally improves the edges—they jump to just under 1,900 lines at that aperture, which is the only time the lens delivers excellent sharpness from edge to edge.

The Olympus 12-40mm, which can be used on Panasonic Micro Four Thirds cameras just as the Panasonic 12-35mm can be used on Olympus cameras, is sharper throughout its range and shows roughly the same amount of distortion. At 12mm f/2.8 it manages 2,536 lines on average, with edges that top 2,100 lines. It approaches 2,600 lines at 25mm, and manages 2,288 lines at 40mm.

But the Olympus lens isn't optically stabilized; Olympus bodies have that feature built-in. To this point, only the Panasonic GX7 packs sensor-based stabilization. If you're a GX7 shooter, the Olympus lens is clearly a better buy. But if you have a non-stabilized Panasonic camera, the benefits that image stabilization provide outweigh the slight softness at the edges of the frame that the lens exhibits. It's unfortunate that, given its premium price, the Panasonic Lumix G X Vario 12-35mm F2.8 ASPH. doesn't display the same edge-to-edge performance as our Editors' Choice Olympus M.Zuiko Digital ED 12-40mm f2.8 PRO. That lens is $300 less expensive, and is the right choice for Micro Four Thirds shooters with a stabilized body.