M.Zuiko Digital 2x Teleconverter MC-20
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M.Zuiko Digital 2x Teleconverter MC-20

Also on show was a non-built-in teleconverter: the MC-20, which will be available in Summer 2019. This doubles the focal length of a lens it's attached to, but at the cost of 2 stops of light. The MC-20 can be used with 150-400mm F4.5 TC1.25x IS PRO as well as the existing M.Zuiko Digital ED 300mm F4.0 IS PRO and M.Zuiko Digital ED 40-150mm F2.8 PRO.

Combined with the forthcoming zoom and its internal teleconverter, we believe you'd get a 375-1000mm F11 lens (equivalent to a 750-2000mm F22 lens on full frame, in terms of depth-of-field and per-image light capture). Though the combination seems rather specialist.