Macro lenses are usually the sharpest in any camera system, and the Phase One Schneider Kreuznach 120mm LS f/4.0 Macro ($6,490) is no exception. When paired with the 100MP image sensor in the Phase One XF 100MP it captures images with more detail than we've ever seen in the test lab, and real-world results are just as shockingly crisp. It's an exceptional performer at wider apertures, but loses quality when stopped down, so you'll need to use a focus stacking technique to get deep depth of field at macro ranges. It still earns an Editors' Choice nod, as the results you can get are truly extraordinary.

Design
The 120mm LS is a hefty lens, even for medium format. It measures 4.4 by 3.3 inches (HD) and weighs 1.8 pounds. The barrel extends significantly as you focus close, nearly doubling in length when working at 1:1 magnification. The lens barrel is metal and a large. A reversible lens hood is included, but the front element is so far recessed that you can get away without using it. The exception would be if you're using a front filter (the lens has an 86mm thread), as the hood will reduce the chance of light hitting it at an odd angle.

Optics are similar to the Mamiya Leaf Sekor F4.0 AF Macro that we reviewed in 2014, but this version of the lens features Phase One branding and a leaf shutter. The in-camera shutter allows you to capture images without introducing the vibration that you get with a focal plane shutter, and to sync with flashes at speeds as short as 1/1,600-second. The 120mm is part of Phase One's recently refreshed Blue Ring lens lineup, which is built with tighter quality control than previous optics and stores additional information, including its serial number, in image EXIF data.

The barrel is all metal, with a large, knurled manual focus ring occupying the bulk of its real estate. You can push the ring forward or pull it back to switch between automatic and manual focus. The only other control is a focus limiter switch that you can use to ensure the autofocus system hunts only over its macro range or only over its standard focusing range to acquire a lock.

The set focus distance is displayed in a cutout window, shown in both feet and meters. The lens focuses as close as 1.3 feet (40cm), measured from the image sensor. When working that close objects are projected onto the camera's image sensor at life-size—which is no small feat when you consider the sensor in the Phase One XF 100MP has more than twice the surface area of a full-frame 35mm camera.

Image Quality
I tested the 120mm with the XF 100MP, the highest resolution camera on the market. It's no surprise that the combination delivers the highest numbers we've seen from the Imatest sharpness test. At f/4 it's an incredible performer, notching 6,162 lines per picture height on a center-weighted test. There is a drop in resolution at the periphery of the frame—the score drops to 4,548 lines there—so keep that in mind when framing out macro subjects.

At f/5.6 the overall score improves to 6,702 lines, and at f/8 the lens is at its best at 7,274 lines. But in both cases the edges significantly lag behind the average score, showing 5,138 and 6,612 lines respectively.

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There's a slight drop in overall resolution at f/11, but performance is even across the frame when you stop down that far, with edges just behind the 6,935-line average score. That's also the smallest aperture I'd recommend using, as diffraction cuts quality at f/16 (5,806 lines), f/22 (4,172 lines), and f/32 (2,394 lines).

Macro photographers often use narrow apertures to capture images with sufficient depth of field. Thankfully the XF camera has a focus stacking tool built-in, so you can shoot at f/8 or f/11 to maximize sharpness when photographing a static subject. It's not a solution for subjects that move, but even at f/22 the 120mm delivers crisper results than most 35mm full-frame macro lenses can manage.

There's very little distortion, less than 1 percent, which isn't an issue in field conditions. When shooting at f/4 Imatest's Uniformity tool shows that corners are a little dim, about 1.4 stops behind the center, but that's very easy to correct in Capture One. At narrower apertures illumination is even from edge to edge.

Conclusions
The Phase One Schneider Kreuznach 120mm LS f/4.0 Macro delivers the crispest, most detailed results we've seen to date. The fact that we tested it along with a 100MP camera certainly helps it deliver exceptional images, but its peak is noticeably higher than that of the standard lens for the system, the 80mm LS f/2.8. It's not a perfect lens—it's bulky, expensive, and macro photographers will need to employ focus stacking in order to get optimum quality for images they'd normally shoot at very narrow f-stops. But at its best, it's an extraordinary performer, and worthy of being named Editors' Choice.