Fujifilm's mirrorless camera system has a very strong lineup of lenses behind it, ranging from the ultra-wide zoom Fujinon XF 10-24mm F4 R OIS ($899.00 at Amazon) all the way up to the Fujinon XF 100-400mm f/4.5-5.6 R LM OIS WR ($1,899), a telephoto beast that is sure to make wildlife and sports photographers very happy. It's an excellent lens from an optical standpoint, but one that's surprisingly large when you consider that it's matched with a mirrorless system that utilizes the APS-C sensor size. But it will pair well with Fuji's pro X-T1 ($318.00 at Amazon) body, and it's overall performance makes it an Editors' Choice.

Design
When I first saw the 100-400mm I was shocked at its size and heft. It measures 8.3 by 3.7 inches (HD), weighs about 3 pounds, and supports 77mm front filters. It's bigger than the Canon EF 100-400mm f/4.5-5.6L IS II USM ($2,199.00 at Amazon) (7.6 by 3.7 inches, 3.6 pounds), a lens that covers a full-frame SLR sensor. When you pair the Canon zoom with a larger, heavier body like the APS-C EOS 7D Mark II ($1,199.00 at Amazon) or full-frame EOS-1D X it feels like a natural fit.

But mirrorless cameras are, for the most part, smaller than SLRs. I used the Fuji 100-400mm with an X-T10 ($269.99 at Amazon) , and it's not a combination I'd recommend. The balance just isn't right. The X-T1 and the X-T1 Graphite Silver ($379.99 at Amazon) are a bit larger all around, and use a handgrip that's a bit deeper than that of the X-T10. If you're buying this lens, you should be pairing it with an X-T1 in order to get the best shooting experience. The X-T1 body is also sealed against dust and moisture, matching the weather-resistant design of the 100-400mm.

In full-frame terms the lens covers a 150-600mm field of view—the same that you'd get by using the Sigma 150-600mm F5-6.3 DG OS HSM Contemporary ($824.00 at Amazon) on a full-frame body, or what you'd you get by putting the Canon 100-400mm on an APS-C body like the 7D Mark II. It's a very useful telephoto range, but if you desire a bit more reach for wildlife or sports work you can pair it with the Fujifilm Fujinon Teleconverter XF 1.4x TC WR ($449.00 at Amazon) . The small teleconverter is also sealed against dust and moisture, and turns the lens into a 140-560mm f/6.4-8 zoom—in full-frame terms that's a 210-840mm coverage range.

The lens is sturdy with a barrel that's a mix metal and hard composite plastic. It ships with a removable tripod foot that mounts on a rotating collar, so you can mount to a tripod in landscape or portrait orientation without adjusting the tripod head. It also includes a reversible lens hood that mounts in a bayonet fashion and includes a moving cut-out that allows you to adjust a circular polarizer filter without having to remove the hood.

The narrow aperture control ring is knurled metal. It's joined by a very wide zoom ring, covered in textured rubber, that occupies the bulk of the middle of the barrel. At the front, just behind the front element, is a manual focus ring, also finished in textured rubber.

A few toggle switches sit at the base. There's a focus limiter—you can set the focus system to search across the full range, or from only 5 meters to infinity, which will speed focus when photographing distant subjects. You also get switches to change between manual and automatic aperture control, and to turn the in-lens optical stabilization system on or off.

The 100-400mm does extend when zooming. It has markings at the 100mm, 135mm, 200mm, 300mm, and 400mm positions. There's a lock switch that can keep the lens set to 100mm, so you can ensure that it remains at its shortest position when it's hanging at your side. The lens is a variable aperture design, and I was pleased to see how linear the progression is. At 100mm you can shoot at f/4.5, and the aperture narrows to just f/4.7 at 135mm, f/5 at 200mm, and f/5.2 at 300mm before closing down to f/5.6 at its maximum 400mm focal length.

You can focus as close as 5.7 feet (1.8 meters), which will be fine for portrait work, but it doesn't quite match the 1:3 macro magnification of Canon's 100-400mm. At its closest focus distance and full zoom the Fuji 100-400mm captures images at about 1:5 (0.19x) life size. As for focus speed, with the X-T10, the lens can lock focus in about 0.2-second, a little slower than some other Fuji lenses we've tested. If it needs to drive the elements a long distance—say, from infinity to its close focus range, it can do so in just 0.4-second, which is a solid result when you consider how much glass is in the lens.

Image Quality
I used Imatest to evaluate the quality of photos that the 100-400mm captures. At 100mm f/4.5 sharpness is excellent, at 2,571 lines per picture height. That's better than the 1,800 lines we look for in a photo. Sharpness is consistent across the frame, with edges that are almost as crisp as the center. Stopping down to f/5.6 delivers slight improvement—2,602 lines—and there's another small bump at f/8 (2,623 lines). You start seeing the effects of diffraction at f/11, with the score dropping to 2,586 lines, so you shouldn't stop down any further than that.

Image quality remains strong as you zoom. At 200mm f/5 it scores 2,570 lines, with similar results when stopped down. At 250mm f/5 the lens records 2,450 lines, and maintains that quality at f/5.6 and f/8, before dropping slightly at f/11 (2,337 lines).

See How We Test Digital Cameras

Our testing area doesn't allow us to evaluate sharpness via Imatest beyond 250mm—it's just not possible to back the camera up far away enough from our High Precision SFRplus test chart and get useful data out of the software. I used a small ISO 12233 chart to visually evaluate the detail that the lens is able to resolve at 400mm. Distinct lines are visible in the 2,600-line block at 400mm f/5.6, even toward the edges of the frame, which is in line Imatest results. That, along with images shot in the field, indicate that the lens is just as good at 400mm as it is at 100mm.

There's no visible distortion, nor is fall-off an issue. I used an ExpoDisc and to shoot a flat gray images at various f-stops and focal lengths and ran those photos through Imatest's Uniformity tool. At its maximum aperture the corners of the image lag behind the center by about 0.7EV, which is barely noticeable in field conditions. The sides show a 0.4EV dip—which isn't something you're going to notice in the real world. Given the size of the lens and consistent image quality from corner to corner, it wouldn't surprise me if it's capable of projecting an image circle that can cover a larger sensor, but is optimized for the APS-C chips used in Fuji's mirrorless camera line.

Conclusions
There's little to complain about with the Fujifilm Fujinon XF 100-400mm f/4.5-5.6 R LM OIS WR. Image quality is superb, and it offers plenty of telephoto reach for wildlife and sports work, with a teleconverter available if you need a longer lens. The lens is on the large size for a mirrorless camera, but telezooms are large by their very nature. And at $1,900, the zoom is less expensive than Canon's 100-400mm lens for SLRs, but is a lot more than Sigma's excellent 150-600mm Contemporary zoom. An SLR lens won't do you any good if you're committed to Fuji's mirrorless system, but you should take the price of glass into account when shopping for a new system. That said, Fujifilm devotees have access to a heck of a lens in the 100-400mm, and one that should pair well with the flagship X-T1 body. The 100-400mm zoom is an outstanding performer, and earns our Editors' Choice award.

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