The "Lens Turbo" speed booster is a 0.72x focal reducer similar to the more expensive Metabones "Speed Booster," which was first released in an electronically-coupled Canon EF-to-NEX form. My camera history is deeply rooted in Pentax glass, and I was thrilled when I first heard of the "Lens Turbo" alternatives going on sale in China via eBay. Though the cost is still arguably a bit high for a lens adapter, at $199 it's significantly cheaper than the Metabones alternative. And the mount selection is much higher with the Lens Turbo options, currently offering Pentax K, Canon FD, Nikon F, and Minolta MD. Though there were only a handful of sample photos on the net demonstrating the quality of the unit, I decided to take a risk and order one. Though I can't offer much in the way of a scientific review, I can give some real-world examples and comparisons.
From left to right: Pentax-M 50mm f1.7 w/ Lens Turbo - Sony E-Mount 35mm f1.8 - Pentax-M 50mm f1.7 w/ "normal" adapter
The build quality on the unit is fantastic, and it actually adds a marginal amount of unexpected weight to the camera. It feels solid and heavy, and it's apparent that quality workmanship went into crafting the adapter. The front mount is a bit tight, though I haven't had too much trouble mounting/unmounting the Pentax glass (I'd rather have a tight mount than a loose one). The field-of-view is actually a bit narrower than I expected, when compared to the Sony 35mm f1.8 which I'm planning on replace with the 50mm/Turbo combo. However, I haven't done any scientific tests to figure out the exact resulting field-of-view. And it's worth noting that my Pentax-M 50mm f1.7 does produce a slightly narrower crop than the Sony E-Mount 50mm f1.8 when used with a "normal" adapter.
Left: Pentax-M 50mm f1.7 w/ Lens Turbo - Right: Pentax-M 50mm f1.7 w/ "normal" adapter
As you can see in the above photos, the Lens Turbo does seem to add a bit of light to the image (both of these images were shot in manual mode, everything kept at the same settings). There is also a hint of vignetting wide open, and the adapter seems to add a slight green tint to the image, as well as a hint of distortion. Both of the above images were shot wide open.
Above: Pentax-M 50mm f1.7 w/ Lens Turbo, shot at f8. Click on the image to view at full-size.
In the above image, you can see the performance of the Lens Turbo when stopped down. The sharpness near the center if the frame is fairly good, though it does soften up considerably near the far left and right edges of the frame. It's also interesting to note that, at least in my copy of the Lens Turbo, the Pentax-M 50mm f1.7 focuses at infinity DIRECTLY at the far stop of the infinity mark. Many "normal" adapters have imperfections in mounting distance that allow you to sometimes focus slightly past infinity, and I might have just lucked out with this particular copy of the lens and adapter.
Now for some real-world examples. Most of the photos below were shot wide open, and had levels adjusted in post, including sharpness levels. These shots were taken to test out how I'd be using the lens turbo on a day to day basis.
Conclusions:
While the Lens Turbo isn't the perfect alternative to a full-frame mirrorless camera, it's a step in the right direction in terms of achieving that aesthetic look. The results, even wide open on older Pentax glass, are satisfactory enough for most circumstances, and I'd even feel comfortable using it on paid client projects under the right scenarios. It succeeds in widening the field of view to nearly the same as what you'd receive on a full-frame body, and it does seem to marginally increase the exposure as well. Yes, the edges do soften up a bit, and it does seem to smudge and swirl the bokeh a bit. But it creates a unique look, and opens up the option to use manual glass in either their original focal lengths (with the Lens Turbo), or at cropped focal lengths (with a "normal" adapter).
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