Purely for curiosity and theoretical interest. - The first 4K Blackmagic Cinema Camera, although it is smaller in the optical throat than the "big" URSA, nevertheless has enough real-estate within to accommodate the focal reducer 0.71 optical cell from an "Ultra" series Metabones Speedbooster.
There are caveats. The only lens system which may be possible to use may be the Nikon F-Mount AI/AIS family of manual lenses. The original Canon EF-Mount lenses which have a flange to focal plane distance of 44mm, cannot be moved the few millimetres rearwards, without destructive surgery to the BM4K front body.
The Nikon lenses with their 46.5mm flange to focal plane distance may be in with a chance with a very thin custom mount made to replace the Canon EF-Mount, which on the BM4K is 5mm thick, front to rear.
PL-Mount lenses have a very deep rear shoulder and many have rear elements which penetrate in the camera throat deeper than stills lenses designed for mirrored SLR cameras. Many will come up against the optical cell. The Cooke Speed Panchro Series II lenses are definitely not contenders for use with a Speedbooster.
The Nikon lenses likely will require trimming of some of the rearward protrusions inside the mount ring to enable them to be set back without catching on the internal fitments within the BM4K body. This of course will spoil the lenses as genuine Nikons.
In manually offering up Nikon lenses to a jigged up arrangement, it was not possible to move the lens rearwards enough for inifinity focus but it was within a whisker of being there. Sharp focus upon an object two feet distant from the focal plane occurs when the witness mark on the lens barrel reads ten feet.
There you go. - Some useless information for you.
Edited by Robert Hart, 30 July 2017 - 03:39 AM.