Over a decade ago I wrote about building your own DX focal reducer from cobbled parts (technically it was an afocal wide converter). The goal was to get back the 1.5x crop factor and make a lens work as you'd expect from it's marked focal length. The result wasn't very pretty, but it gave us true wide angle for our D1 cameras long before Nikon got around to making wide angle DX lenses. Unfortunately, those focal length reducers couldn't be done simply, mostly because the mirror box pushes the mount forward, so you have to do the corrections far forward of where they're optimally done. The nice thing about mirrorless cameras, though, is that the sensor to mount distances are far shorter; short enough to allow for a far simpler focal reducer approach.
Today, Metabones has announced a commercial focal length reducer and mount converter for NEX systems called the SpeedBooster. The initial versions work with Canon EF lenses and provide a 0.71x focal length reduction. In reducing the focal length, you also get an aperture change (as with teleconverters, which do the opposite of a focal reducer): you gain approximately one stop of aperture. The EF versions of these SpeedBooster adapters feature auto-aperture, IS support, EXIF data transfer, and even partial autofocus support on many recent Canon lenses (post 2006). The adapter also has a detachable tripod foot that's also an Arca Swiss plate. The first version to be made available will be Canon EF to Sony NEX (E-mount), available later in late January (25th) for US$600. Other versions will be at different prices (Leica R to Fujifilm XF or Sony NEX is listed at US$400 on their site).
Since there's a lot going on here, let me reiterate what the SpeedBooster does:
- Mount conversion — initial version for Canon EF lenses to Sony NEX, but conversions to m4/3 and Fujifilm XF mounts are also coming. Also, Metabones claims they will eventually have Leica R, Alpa, Contarex, Contax C/Y, and Nikon F versions (if they did everything they currently write about, that would be 18 different versions of the SpeedBooster.
- Focal length conversion — the focal length is reduced 0.71x. Thus, a 50mm Canon EF lens becomes a 35.5mm lens. That's not quite a perfect reduction between full frame and APS, but close enough for most of us (the 50mm should become 33.3mm to be a "perfect" 50mm equivalent on NEX).
- Aperture adjustment — the effective aperture is increased by one stop. So an f/1.4 lens becomes an f/1 lens. This is again just about the right change for going from full frame to APS: you'd get about the same DOF on the Sony NEX with a lens mounted on this adapter as you would from a full frame camera if you kept all the other parameters equal. Some may wonder how the aperture gain is achieved. Simple: the image circle is reduced (concentrating the collected light into a smaller area).
- MTF gain — the "compression" effect of the focal length reducer also tends to reduce the size of aberrations, which are a primary driver of MTF. Metabones uses a Nikkor 50mm f/1.2 as their example for plotting the lens' normal MTF versus with use of the SpeedBooster, and there's indeed a clear MTF boost in the central area on an m4/3 version of the adapter. The NEX version, however, shows more MTF loss as you move to the corners (the center is still higher than the original lens). The MTF gain claim is a little less reliable than the other claims: there's going to be high variability in the size and position of the gains depending upon the lens used and the format you're adapting to.
- Telecentricity gain — digital sensors like light to hit at less than 15° to perpendicular. In some wide angle lens designs that's difficult to achieve, so you get impacts from the slanted light. One simple to see impact is vignetting. One by-product of the focal reducer is that light is slightly more tele centric. The difference isn't dramatic, but I'll bet we see visible differences in some adapted lenses' vignetting performance.
If you want to read more about the technical details of the SpeedBooster focal reducer, Metabones has a White Paper on their Web site that describes the details at length.
The Metabones adapter was designed by Brian Caldwell, the man who created some of the best corrected lens designs for Coastal Optics (the 60mm f/4 Macro, for instance, is one of the best performing lenses I know of for Nikon mounts, and it can pass UV and IR light as well as visible).