Photography is all about compromises. If you want long, it's not going to be light. If you want fast, it's probably not going to be cheap. If you want a smaller crop-frame camera system, then you're going to give up depth of field control....unless, of course, you have a Speed Booster.
Basically, the Metabones Speed Booster is the opposite of a teleconverter. Instead of extending the focal length and dimming a lens, the Speed Booster shortens the focal length and increases it's brightness. For an APS-C sensor camera, it basically gives back the effective field of view and depth of field of a full frame lens on a full frame body.
Even though it's designated for G-type lenses, the Nikon version of the Speed Booster is compatible with D-lenses as well. In either case, the aperture control is limited, however, and only supports use from wide-open to f/8. The aperture markings are in half-stops. Newer versions of the adapter have a "click-less" aperture ring for smooth video operation. Just a reminder, the E-Mount includes the A7 and A7r cameras, so this adapter can be used to mate FX Nikon lenses onto the full frame Sony bodies.
The bottom of the adapter also surfaces as a tripod collar (Arca-Swiss style), making tripod mounting more feasible than using the screw attachment on the camera body.
In use, the Speed Booster optically reduces the focal length of a full frame lens by 0.71x, and decreases the working depth of field by one stop. However, when factored in with the 1.5x or 1.6x crop factor on a Nikon or Canon body, the effective field of view is almost the same as if the lens were used on a full frame body. The depth of field is also reduced, so that too would be as if the lens were used on a full frame body, meaning that there is one addition stop to increase the amount of bokeh that a lens can produce. (The Micro Four Thirds version of the Speed Booster is also constrained to a 0.71x reduction factor. Unfortunately, while the m4/3 Speed Booster can reduce the field of view, it can't replicate the field of view of a full frame camera on that system.)
Because the primary purpose of the Speed Booster is to regain the "bokeh" that is lost in a crop sensor camera, it seems reasonable to assume that the majority of the users of this device will be pairing it with fast primes. To that end, the Nikon AF-S 85mm f/1.8G, mated to a Sony NEX-6. Just as a bit of warning: the Speed Booster mounts very tightly with the lens, so there's a fair bit more resistance to mounting a lens on the Speed Booster than on its native mount.
|Nikon AF-S 85mm f/1.8 G speed-boosted on Sony NEX-6|
An added benefit to this device is that it condenses the light from the larger full frame image circle into the area of the crop frame image circle... this has the benefit of improving the image quality and perceived sharpness of the lens. In practice, this is true for the center of the image, but the benefit isn't quite there in the edges. To that end:
|Nikon AF-S 85mm f/1.8G Speed Boosted on Sony NEX-6|
This photo was taken at f/1.8, albeit at close range in order to maximize the background blur. With the Speed Booster, the 85mm lens becomes effectively a 60mm f/1.2 full frame lens mounted on a crop body.... or in other words, some that gives more or less the same depth of field and angle of view of an 85mm lens on a full frame body. There's a tad bit of barrel distortion, but otherwise, the Speed Booster preserves the bokeh characteristics of the lens being used. If you are using a creamy and wonderful lens like the Nikon 85mm f/1.8 (or the f/1.4 if you are so lucky), then that won't change. If you are using a lens with busy and harsh backgrounds, that won't change either.
Ultimately, this is a useful, if expensive, piece of equipment to use if you have equipment from two different manufacturers. At a cost of $430 USD, it's not the casual throw-it-in-your bag piece of kit that a normal lens adapter is. However, it is an interesting tool for people who are firmly dedicated to crop sensor cameras; since ultra-shallow depth of field is not (usually) a frequently performed task, it can be cost effective to keep one fast lens for boosting rather than having one crop and one full frame system.
With thanks to Broadway Camera