Thread: Optically corrected b4-mount to mft adaptor for bmpcc.

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  1. #1 Optically corrected b4-mount to mft adaptor for bmpcc. 
    Optically corrected B4-Mount to MFT-Mount adaptors can be had off eBay a fraction cheaper than BM's own genuine URSA Mini 4K and URSA Min 4.6K B4-Mounts which appear to come in two styles.

    For the sake of anyone who has the BMPCC "and" the 4K URSA Mini and the genuine BM B4-Mount for it, here are some numbers which might be of interest relating to fitting it to the BMPCC with a custom spacer.

    CAUTION. These numbers ASSUME that the front face of the intermediate adaptor in the BM URSA Mini 4K is the same distance from the focal plane as in the big URSA 4K PL. I have not yet had the opportunity to check an URSA Mini 4K PL to make sure of it. It would seem that they are the same from on-line images I have studied.

    The six-screw attachment pattern for the PL-Mount with its offset upper attach screw APPEARS to be the same. This leads one to assume that the distance of the front face of the intermediate adaptor from the focal plane is also the same. However this is an ASSUMPTION, not verified fact at this point.

    The front face of the intermediate adaptor onto which the PL-Mount attaches is 40.2mm from the focal plane.

    The flange face of the MFT-Mount is 19.2mm from the focal plane.

    The distance from the front face of the intermediate adaptor to the flange face of the MFT-Mount is 21mm.

    If you decide to rat the MFT rear-mount ring off your BMPCC Speedbooster for a quick mount rather than machine the profile in the rear of a custom adaptor, the thickness of the Speedbooster's MFT rear mount ring forward of the flange face is 1.9mm, that is. not including the tail extension.

    The adaptor thickness from the front face to rear face then becomes 19.1mm.

    These dimensions have been arrived at by measurement of the PL-Mount off the big URSA which is 11.8mm thick from flange face to rear face which butts against the intermediate adaptor front face. There is a tail which enters into the bore of the intermediate adaptor. This is not included in the 11.8mm dimension.

    I make no firm claim as to their accuracy however, the dimensions were fine for the Nikon F-Mount adaptor I made for the big URSA PL version.

    I have a BM B4-Mount adaptor for the URSA Mini 4K on its way with the intention of mounting it to the big URSA for event work at HD windowed sensor resolution. If it does not fit straight to big URSA, I will make an intermediate adaptor to suit.

    Once I have it in hand, I will be able to verify the ASSUMPTIONS I have made here.

    CAUTION. The B4-Mount for the URSA Mini Pro appears to be of a different design and the description above is not correct for the B4_Mount which suits the URSA Mini Pro camera.

    Blackmagic Design have neither been advised of nor endorse this notion.

    Furthurmore any fitment of this arrangement to the BMPCC camera will almost certainly void any remaining warranty.
    Last edited by Robert Hart; 07-26-2017 at 11:48 AM. Reason: errors
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  2. #2  
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    Sep 2012
    I believe the BM converter is a mild 1.1x tele which would be insufficient to cover the S16 sized broccoli(anyone else notice how aggressive spell check is becoming)/micro sensor. It works for an HD crop on the 4K and 4.6 because it's only a 10-11mm window. Why BM doesn't release an mft-b4 adapter for their studio cameras remains a mystery to me.
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  3. #3  

    I suspect it should cover. The third party adaptors are about 1.1 but one affordable brand suggests that around f5.6 is about where one should shoot with or the image may go soft. That is not much comfort given that long-reach lenses tend to be used in difficult and uncontrollable situations.

    P+S Technik's own IMS/B4-Mount correcting adaptor does not grow the image circle and it is fine with some Fujinon ENG lenses for sensor coverage on the SI2K which is a smaller sensor than the BMPCC. If there is an issue with vignetting at all, I suspect it might be at the wide end with a momentary opening then a corner vignette again in the zone approximately 20mm to 35mm or thereabouts that I have seen with an older TV-Nikon ENG lens before I got some rescue Fujinons to fix and then try out.

    Because there are already B4-Mount to MFT adaptors already out there, my guess is that BM will not want to get into a competition game with other vendors. A dumb metal adaptor to take up the space between the rear of an existing URSA Mini/Mini Pro B4-Mount with a MFT tail on its rear would not be too expensive. It does represent one more product which at best would be niche and possibly unprofitable and to survive in business, that counts.

    When P+S Technik made their B4-Mount, they also decided that as they already had a thick planar element in there, why not marry up their Evolution optical viewfinder by adding a splitter within that planar element. It did not sell all that well though. It was a beautiful piece of work but very costly. I hope they did not get burned but they may well have.

    This bit of event coverage vision was shot with a Fujinon on the SI2K, mostly in low light. Sharp focus is not a strong point with these lenses wide-open but there is nothing quite as useful as having that long reach.
    Last edited by Robert Hart; 07-27-2017 at 05:35 AM.
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  4. #4  
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    Sure, the SI 2K is a 2/3" sensor, 9.59mm horizontal at DCI 2K no reason a B4 lens shouldn't cover especially at 1920 which is closer to the 8.8mm standard of 2/3".

    The BM adapter 1.1x seems to use high quality corrective optics and the people who use it seem to like it but it's not going to cover the pocket sensor without compromises at the wide end. The Fotodiox version is a piece of junk, that I believe is 1.4x given the 1 stop loss. Any adapter that has to be stopped down to f5.6+1 stop adapter loss for an f8 isn't doing any "correction". With that performance you can really just leave the optics out and shoot with a straight mechanical adapter and piss off the wide end.

    Here's a semi useful comparison of image circles using a variety of standard def B4 lenses, mostly vintage. You can see how a 1.1x might get you close with some of them. I believe the older lenses actually cover better because their optical formulae are probably based on 1" tubes rather than 2/3"ccd.

    Good Luck
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