Metabones Ultra 0.71x EF to MFT is for APS-C
This is sort of incorrect.
Both the Metabones XL 0.64x EF to MFT and the Metabones Ultra 0.71x EF to MFT are primarily intended to be used with full frame EF lenses. If one reads the official Metabones product page for the Ultra 0.71x EF to MFT, one sees that there are several caveats to using an APS-C lens with it, and that it is marginally usable for recording 16:9 video, but not necessarily 4:3 stills, with APS-C lenses.
Unmodified EF-S mount lenses will not physically attach to this adapter. Other "APS-C only" lenses in the EF mount will attach, but the other caveats regarding using lenses with an APS-C size image circle with this adapter remain valid.
The full name listed at B&H is the
T Speed Booster Ultra 0.71x Adapter for Canon Full-Frame EF-Mount Lens to Micro Four Thirds-Mount Camera.
Only FF Canon EF lenses or equivalent third party lenses will project an image circle large enough to cover the µ4/3 sensor when used with the Metabones Ultra 0.71x EF to MFT. It does not provide full sensor coverage with third party APS-C lenses in the Canon EF mount that only cast an APS-C size image circle.¹ The apparent reason Metabones "recommends" the Metabones Ultra 0.71x EF to MFT, rather than the Metabones XL 0.64x EF to MFT, for APS-C lenses is because the vignetting issues will be much less severe with the 0.71X focal reduction than with the 0.64X reduction. They still point out that there may be issues with image circle coverage using APS-C lenses with the EF mount.
Below are two notes of particular interest from the Metabones product page:
EF-S lenses require modification to fit or may remain incompatible even after modification (but third-party DX lenses can be used without modification)
Even if the EF-S lens is modified to fit the EF mount of the adapter, there may be mechanical clearance issues with some EF-S lenses that have rear elements that protrude further out the back of the lens flange than EF lenses do.
Some EF-S and DX lenses² may not have enough coverage of the entire image circle for stills after widening by 0.71x (but video will have enough coverage).
The video comment seems to assume one is shooting in 16:9 format and thus not using the extreme corners of the 4:3 aspect ratio sensor. With a 17.3 mm width, only the center 9.73 mm of the sensor's 13 mm height would be used for 16:9 video. The resulting 19.84 mm diagonal is very close to 26.9 x 0.71 = 19.1 mm.
You might also find the press release from Metabones announcing the Speed Booster ULTRA 0.71x M43 with Autofocus to be helpful. It uses the phrase "DX and APS-C format lenses" several times. It also says (bold and italics added):
However, the new ULTRA design makes very effective use of exotic materials at the furthest limit of glassmaking technology, and as a result is almost perfectly corrected for use with all fullframe SLR lenses regardless of aperture. The Speed Booster ULTRA m43 will also work extremely well with many DX and APS-C format lenses provided the image circle of the lens is large enough.
Here's the full lens compatibility statement further down in the press release:
Lens Compatibility: Any full-frame (36 mm x 24 mm format) SLR lens with the appropriate mount can be used. DX format lenses can be mounted but may result in vignetting when the full Micro Four Thirds format is used, depending on the lens. DX lenses are fully supported when used on the Panasonic GH4 in Cinema 4k mode, and will function without abnormal vignetting. Canon EF-S lenses are not supported, but third-party EF-mount APS-C lenses can be mounted and will have full electronic functionality.
Thus, it seems the Metabones Ultra 0.71x EF to MFT was never intended to be 100% compatible with APS-C only lenses.
26.9mm is the 'nominal' diagonal of a Canon APS-C image sensor, which can range from 26.63mm to 27.26mm depending on the exact APS-C sensor made by Canon. One would assume all of Canon's APS-C only EF-S lenses can cover their largest APS-C sensor with a 27.26mm diagonal.³ One might also assume, with a fairly high degree of certainty, that most third party 'APS-C only' lenses would as well, since the same designs are also offered in other mounts, such as Nikon F and Minolta/Sony A, for cameras with slightly larger APS-C sensors with diagonals closer to about 28.2mm.
It is also the case that most zoom lenses expand the image circle as they are zoomed to longer focal lengths. EF-S lenses must have an APS-C sized image circle at their widest focal length setting, but that image circle will expand when zoomed to longer focal lengths (if there's nothing physically blocking the light at the rear of the lens). This same concept is illustrated with the EF 8-15mm f/4 L Fisheye lens. At the widest setting, the image circle does not completely cover a FF sensor. In fact, the full image circle is contained within the rectangular sensor. Only at 14-15mm does the full sensor receive light projected by the lens.
The only Metabones adapter for EF-S lenses to MFT that will provide a large enough image circle to cover the complete µ4/3 sensor at all lens focal lengths and apertures is the T Smart Adapter for Canon EF or Canon EF-S Mount Lens to Micro Four Thirds-Mount, which does no focal reduction a/k/a Speedboosting.
¹ Third party APS-C lenses for Canon EOS cameras use the EF mount, rather than the EF-S mount, which is still protected by patent. The patent has expired on the EF mount introduced in 1987. Even though they don't cast an image circle large enough for FF sensors, they can be attached to FF EOS cameras.
² "DX lenses" is used here as a broad term to refer to any lens intended for use on APS-C cameras only. This should be self apparent, since the adapter's product page on which the statement appears is not a Nikon F-mount adapter.
³ The 22.7 x 15.1 mm sensor with a calculated diagonal of 27.26 mm was used both in the original 10D, released before EF-S lenses were introduced, and in the EOS Digital Rebel/300D that was the first EOS camera with an EF-S mount flange with the extra slot needed to accommodate the extra tab on the lens bayonet. Unmodified EF-S lenses will not mate to the 10D in the same way they will not mate to any full frame Canon EOS camera. The Digital Rebel/300D was the first EOS camera with an EF-S flange on the front of the camera. The 10D was introduced in early 2003. The Digital Rebel/300D came out about six months later in the third quarter of 2003. The first EF-S lens was introduced simultaneously with the Digital Rebel/300D.