Since the introduction of the Micro Four Thirds systems, enthusiast shooters have been mounting legacy lenses to modern cameras for economic and artistic reasons alike. But, aside from the $6,950 Leica M there hasn't been a compact full-frame digital body that could take advantage of older lenses designed for use with 35mm film.

The Mitakon Lens Turbo ($198 direct) aims to change that. It features a wide-angle rear converter, which effectively widens the field of view of the camera's sensor by 0.726x. It almost completely negates the 1.5x crop factor that shooters who uses 35mm film lenses on APS-C bodies have become accustomed to. Because it concentrates the light from the lens into a smaller-than-normal image circle, it also effectively increases the amount of light hitting the sensor, giving the lens a larger effective aperture than it would normally have.

Design and Features
The Lens Turbo is actually the second of its type to hit the market—the first was the Metabones Speed Booster—but it's the first one that we've been able to test. The Metabones version gives you a slightly wider field of view, and is available for Sony NEX and Fujifilm X mirrorless camera systems. Its magnification is 0.71x, but it's priced between $400 and $450, depending on which adapter you require. The Metabones is available for Alpha, Contarex, Contax C/Y, and Leica R lenses.

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You'll need a Sony NEX camera like the excellent NEX-6 in order to take advantage of the Lens Turbo; Fujifilm and other mirrorless camera systems are not supported. We tested a version that is compatible with Nikon lenses, but you can opt for one in a Canon FD, Minolta MD/MC, or Pentax K mount if you choose. Mitakon has plans to release versions for M42, Leica R, Contax C/Y, and Sony/Minolta Alpha lenses in the future. 

The adapter's depth will vary a bit by mount, but each is machined to match the native camera system's flange distance (the distance between the lens mount and the image sensor) in order to allow proper focus to infinity. Expect it to add a couple inches to the front of your NEX. You mount it just as you would any other lens, and there is a tab on the front of the adapter itself to allow you to attach and detach the lens that you'd like to use. The adapter itself is metal and is quite sturdy—it locks securely onto the camera, and the wiggle that you sometimes get with cheap adapters is nowhere to be found.

Performance and Conclusions
We tested the Nikon mount version of the adapter along with the Nikkor AI-s 28mm f/2.8 lens. This is regarded as one of Nikon's sharpest manual focus wide-angle optics, and it has an amazing close focus ability; it can hone in on a subject that's only 7 inches away from the image sensor. To understand just how sharp it could be on a native Nikon body we used Imatest to measures its performance on the 36-megapixel D800SEE IT.

We use 1,800 lines per picture height as the cutoff for acceptable sharpness using a center-weighted metric; at f/2.8 the 28mm AI-s managed 2,341 lines. There was some barrel distortion, about 0.6 percent, and edge performance was a bit lacking—it drops to 1,569 lines in the outer edges of the frame, about 56 percent of what it was in the center area. Stopping down improved performance; at f/4 we got an average score of 2,724 lines with 1,734 at the edges. At f/5.6 it's impeccable, 3,012 average with edges managing 2,136 lines; the drop-off from center to edge improved to 61 percent here. Finally, at f/8, we saw an average score of 3,096 lines with edges at 2,472—the edges were 73 percent as sharp as the center.

Those are a lot of numbers. And since we tested the Lens Turbo adapter on a 16-megapixel NEX-5NSEE IT, a direct score comparison to the 36-megapixel D800 isn't exactly fair. At f/2.8 the center-weighted score is only 1,534 lines. The center is actually quite sharp at 1,975 lines, but the edge resolution is a dismal 557 lines, about 28 percent that of the center. Stopping down to f/4 doesn't move the needle much, but we do see improvement at f/5.6. There the lens scores 1,717 lines overall, with 2,170 lines in the center and 590 lines at the edges. The drop-off percentage is about the same. The lens did better at f/8, notching an 1,827-line average with 2,153 lines at the center and 826 at the edges—the edge score is about 38 percent of that of the center.

What does all that mean? Basically, you're sacrificing some image quality at the center and mid-center of your image in order to gain the wider field of view. And you can forget about getting sharp corners. If corner-to-corner sharpness is what you're after, this adapter is simply not going to work for your needs. It's also not a good one for architectural work—the adapter introduces a noticeable amount of barrel distortion to your images. Our Nikkor test showed only 0.6 percent on the D800, but exhibited a noticeable 1.8 percent when paired with the Lens Turbo and the NEX-5N.

In terms of exposure, the Lens Turbo increases the effective f-stop of your lenses by about a two-thirds of a stop. An exposure at ISO 100 under identical lighting conditions required a 1/30-second exposure with the Nikon D800, but only required 1/50-second using the Lens Turbo adapter on the NEX-5N. We tested the adapter with a relatively slow f/2.8 lens, but if you pair it with an f/1.4 or an ultra-fast f/1.2 lens you'll be able to snap photos at higher shutter speeds in very little light.

The Mitakon Lens Turbo has a limited target market, but if you're the right type of photographer it's worth your money. If you're a NEX shooter who likes using legacy lenses, you'll now be able to get more out of older, manual focus glass. We tested with a Nikon lens, one that is actually compatible with modern full-frame digital cameras. But if you've got a stash of Canon FD or Minolta MD/MC lenses, you won't find any digital body that offers native support. And Pentax K shooters, who are still without a full-frame body, will be able to adapt legacy lenses and enjoy a full-frame experience. While the adapter clearly harms image quality, it doesn't do so to the point where the images are unusable. I was shooting outdoors on a bright day and had the option to stop the lens down as needed. Even at wider apertures I was able to get images out of the adapter that I'd be happy to print, and print large.