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There has been a bit of discussion on the practicality or otherwise of using a focal reducer situated in the throat of the big URSA to enable a closer visual match to the identical lenses used on the 4.6K cameras.
For those who are tempted to dig and delve, I suggest you DON'T, unless you have seriously developed fine engineering skills. That said, here follows, a bit of information. Please do not ascribe accuracy or validity to the following comments.
During the course of designing a one-off Nikon F-Mount for a big URSA PL v1, I also examined the possibility of an integrated Speedbooster optic. For the Big URSA, for Nikon lenses, there is a possibility because there is ample real-estate in the camera design to permit this. For the URSA Mini cams, I don't think so. There appears to be insufficient workroom to relocate a Nikon mount the necessary distance rearwards which is about 8mm when the BMPCC Speedbooster optical cell is tested.
The filter/cover glass in the throat of the BM URSA is situated too close to the front of the camera body for a Speedbooster optical cell to be located within the throat of the camera without this filter having to be relocated rearwards. That may introduce other optical and colour issues. As with the RED camera experiments thus far, the filter glass would have to be remade to a smaller diameter and installed closer to the sensor on the rear of the redesigned Speedbooster optical cell.
Unlike the recent RED experiment in which the optical elements themselves had to be ground flat on the edges to permit fitment within the throat of the RED camera, there is generous workroom within the big URSA's throat to fit the optics and reposition the Nikon Mount. With a PL-Mount and a modified clamp ring to permit a working clearance, this might also be possible, except with lenses which have a deep rearwards penetration into a camera throat.
With the aggressive BMPCC Speedbooster optical cell installed within the throat of the big URSA and the filter glass removed, Nikon lenses can be offered up and achieve infinity focus.
There is as I expected, a significant corner vignette from the BMPCC Speedbooster optical cell along with significant edge distortion. The optical cell from the 0.71 "Ultra" Speedbooster series may yield a wider image circle and maybe less edge distortion. It may not need to protrude so far back in the throat of the camera as the BMPCC optical cell which is about 13.25mm rearwards of the M/43 flange, therefore about 4mm from the sensor itself.
If the 0.71 optical cell can perform to the 21mm V1 URSA 4K sensor without a vignette, then it would be a relatively simple matter to remake the anti-reflection cone to carry the optical cell or the elements themselves in a custom assembly, to screw straight into the throat of the big URSA camera and shim between the rear shoulder of the BM throat and the rear face of the cell barrel for collimation.
As for removing the filter glass, I do NOT recommend that people tackle this. This element is surface-coated and could be easily damaged. The neoprene "O" ring retainer is a challenge to re-install correctly.
I have enquired with the folk who make and sell the Speedboosters to see if they will sell me a 0.71 optical cell. As a kitchen-table engineer, I do not like my chances but who knows.
There is probably not a lot of point to this exercise except as an option for folk who retain the big URSA and intend to use it in controlled lighting environments alongside the URSA Mini 4.6K or Pro, for a 4K or HD finish. Then the images conferred by the lenses may be a better match.
For those who are tempted to dig and delve, I suggest you DON'T, unless you have seriously developed fine engineering skills. That said, here follows, a bit of information. Please do not ascribe accuracy or validity to the following comments.
During the course of designing a one-off Nikon F-Mount for a big URSA PL v1, I also examined the possibility of an integrated Speedbooster optic. For the Big URSA, for Nikon lenses, there is a possibility because there is ample real-estate in the camera design to permit this. For the URSA Mini cams, I don't think so. There appears to be insufficient workroom to relocate a Nikon mount the necessary distance rearwards which is about 8mm when the BMPCC Speedbooster optical cell is tested.
The filter/cover glass in the throat of the BM URSA is situated too close to the front of the camera body for a Speedbooster optical cell to be located within the throat of the camera without this filter having to be relocated rearwards. That may introduce other optical and colour issues. As with the RED camera experiments thus far, the filter glass would have to be remade to a smaller diameter and installed closer to the sensor on the rear of the redesigned Speedbooster optical cell.
Unlike the recent RED experiment in which the optical elements themselves had to be ground flat on the edges to permit fitment within the throat of the RED camera, there is generous workroom within the big URSA's throat to fit the optics and reposition the Nikon Mount. With a PL-Mount and a modified clamp ring to permit a working clearance, this might also be possible, except with lenses which have a deep rearwards penetration into a camera throat.
With the aggressive BMPCC Speedbooster optical cell installed within the throat of the big URSA and the filter glass removed, Nikon lenses can be offered up and achieve infinity focus.
There is as I expected, a significant corner vignette from the BMPCC Speedbooster optical cell along with significant edge distortion. The optical cell from the 0.71 "Ultra" Speedbooster series may yield a wider image circle and maybe less edge distortion. It may not need to protrude so far back in the throat of the camera as the BMPCC optical cell which is about 13.25mm rearwards of the M/43 flange, therefore about 4mm from the sensor itself.
If the 0.71 optical cell can perform to the 21mm V1 URSA 4K sensor without a vignette, then it would be a relatively simple matter to remake the anti-reflection cone to carry the optical cell or the elements themselves in a custom assembly, to screw straight into the throat of the big URSA camera and shim between the rear shoulder of the BM throat and the rear face of the cell barrel for collimation.
As for removing the filter glass, I do NOT recommend that people tackle this. This element is surface-coated and could be easily damaged. The neoprene "O" ring retainer is a challenge to re-install correctly.
I have enquired with the folk who make and sell the Speedboosters to see if they will sell me a 0.71 optical cell. As a kitchen-table engineer, I do not like my chances but who knows.
There is probably not a lot of point to this exercise except as an option for folk who retain the big URSA and intend to use it in controlled lighting environments alongside the URSA Mini 4.6K or Pro, for a 4K or HD finish. Then the images conferred by the lenses may be a better match.