This diagram from the patent shows the optical construction of the Converter Adapter (labelled CL) with a Master Lens (labelled ML) in front of it.

A new patent application filed by Canon, and first detailed on Canon News, lays out the schematics for its own version of a speedbooster adapter that would enable Canon EOS M users to adapt EF lenses onto the EF-M mount.

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Japanese patent application 2018-189864 details an adapter that includes both a 0.8x focal length reducer, as well as a 'variable flare cutter.'

As with the speedbooster adapters, Canon's adapter would use a series of lenses to reduce much of the full-frame field of view onto an APS-C sensor, such as those used inside Canon's EOS M cameras.

Where things get interesting is that Canon isn't stopping there. Similar to how Canon has introduced a line of EF to RF adapters with added features, including an integrated control dial and drop-in ND/CPL filters, the adapter detailed in this patent adds yet another component: an adjustable aperture or set of apertures that effectively mask off sections of the adapter to reduce the potentially negative impact of stray, non-image forming, light rays.

This diagram from the patent highlights two separate locations where the variable aperture could be located within the converter (the front of the converter being the left side and the rear of the converter being the right side).

The patent explains this is done by calculating, on the fly via communication through integrated contacts, the ideal pupil sizes and locations of the in-adapter apertures, based on the attached lens' current aperture and focus distance. With this information, the the adapter could ideally adjust its multiple variable flare cutters.

Within the patent, an example scenario is detailed showing how a full frame 50mm F1.4 lens would effectively become a 40mm F1.2 lens with an image height of 13.66mm and 18mm back focus — precisely the size needed for EOS M cameras.

The resulting combination would act as a 64mm F1.9 equivalent. Not quite as wide or with such a bright equivalent aperture as the full frame lens used on full frame, but still better than using a pass-through adapter.