SPEEDBOOSTER FOR BM URSA V1 PL.

[Robert Hart]

I am in the process of making a special insert for the BM URSA 4KV1 PL camera to accommodate a 0.71 "Ultra" optical cell from a Metabones Speedbooster.

Does anyone by chance know what the pitch of the threaded section of the optical cell is. It is very fine and likely to be a nightmare for a backyard engineer like me to reproduce. I could machine a piece of Delrin and use its inherent compliance to screw the optical cell into without having to cut an actual thread. I would prefer to do the job properly.

Metabones simply does not reply to messages so there is not much point to trying for info from them forever.

I would like also to get hold of a spare anti-reflection cone for the URSA. I may go the nolathane casting route for mounting the speedbooster optical cell within a spare cone. With care and use of a suitable separation material, I should be able to preserve the functionailty of the thread for fine backfocus adjustments.

Does anyone know how best to go about getting hold of BM spares. Navigating to someone who can actually help, assuming the company would want to, is proving an enigma.

[Phil Rhodes]

Much as I appreciate what you're doing, I suspect you may be in quite some trouble with both those requests. There are other manufacturers of very similar optical components, although they're in China and I'm not sure how good the communications are likely to be.

 

Blackmagic spares, not a clue.

[Robert Hart]

Like enemies, they may well be keeping parts of their registered designs even closer.

By trial and error, I may be able to arrive at the correct thread pitch with the lathe gears but it is really fine and likely beyond my meagre engineering abilities. It is an internal cut and my machine does not turn true off the main bed which is what is used when cutting threads.

There's a guy down the hill from here who is a German-trained wizard at this stuff but prefers to CAD everything these days which means big money relatively speaking for a one-off job.

Edited July 19, 2017 by Robert Hart

[Robert Hart]

How's this for a bit of fortuitous coincidence. It seems from calculations and from offer-up tests that the front face of the big URSA PL's intermediate adaptor is exactly spot-on for the rear mounting face of the Nikon 5-screw mount with a Speedbooster optical cell in the throat of the camera. The internal diameter is too wide for the Nikon F-Mount to be directly attached by screws. However for test purposes the mount can be clamped with a thin hard spring-steel grade washer arrangement with screwholes in the PL-Mount pattern.

The washer has to be thin enough that the 1.2mm rear overhangs on the Nikon iris ring clear the washer. The Nikon mount ring also need so have the 2mm deep AI clearance of the FM2n style mount. The later mounts have a shallower clearance. For a substitute intermediate adaptor with correct internal diameter for a Nikon mount, there is a space required behind for the small 1.5mm deep rear shoulder of the Nikon mount ring and the corrugated spring, which requires some clearance channels to work properly.

I also offered up an old Angenieux T3.9 25-250mm zoom lens. It is not viable for a speedbooster because its image circle is too small and vignettes to the big URSA's 4K sensor. It would be sort-of okay with a faux-cinemascope top and bottom of frame crop but it appears there would remain a subtle side edge darkening of an apparent half-stop.

Interestingly, the Sony FS7 sensor is quoted as 25.5mm wide and the BM4K sensor as 25.34mm wide. I have been advised of anecdotal accounts of the Speedbooster "Ultra" EF-Mount to Sony E-Mount version, conferring a full image to the sensor of the Sony FS7 camera. I think the lenses used were Nikons via adaptors and Zeiss CP2. I have not seen it working myself.

Depending upon the intermediate adaptor arrangement of the BM URSA Mini 4.6K, it may be possible for this camera to also provide a useful image with a 0.71 Speedbooster but the chances of corner and side edge image softness may be greater with some lenses.

I have not yet been able to get my greasy hands on a URSA Mini 4.6K.

NOTE: The URSA Mini Pro 4.6K is not viable for a speedbooster optic being installed in the camera throat because the workspace is occupied by the inbuilt ND filter system.

Edited July 30, 2017 by Robert Hart

[Phil Rhodes]

This is all very interesting, though I suspect you'll spend a lot of money on machining to make it all work.

[Robert Hart]

The most expensive part for me will be the raw metal stock as it is a DIY project and I already have the tooling. Hollow-cast gunmetal bronze stock is not the cheapest stuff to buy but is friendly stuff to work with and has good strength.

The modified intermediate adaptor should be easier than the custom Nikon F-Mount adaptor to fit the PL-Mount fastener pattern. It is basically a modified copy of the existing intermediate adaptor.

It will have a narrower throat, a shallow wider clearance bore at the rear so no pressure goes on the front of the anti-reflection cone, fewer holes drilled in it and a small relief in the front to accommodate the small rear locating shoulder of the F-Mount. There is a lot less detail involved although the dimensions of course have to spot-on.

To accommodate the speedbooster optical cell, the arrangement can be as simple as fixing it inside the existing anti-reflection cone with custom washer-shaped pieces and some tacks of adhesive or remaking the cone as a complete barrel to support the optical cell, with internal thread for very fine focal adjustments.

A PL-Mount version looks like it may be doable but the lens choice will be limited to the Zeiss CP2 and possibly the Rokinon Xeen range. These are already available in EF-Mount and F-Mount versions which makes the PL version somewhat pointless except for more robust construction and solid fixture which really matters when using wireless follow-focus.

Edited July 31, 2017 by Robert Hart

[AJ Young]

Please excuse me if I'm incorrect with my question and response, but are you essentially trying to install a built in focal reducer for the PL mount on an Ursa?

 

I don't think that'll work for most PL lenses because their image circles are designed for a S35 film or sensor size. The Metabones focal reducer (speedbooster) is designed for full frame image circles.

 

However, I believe more and more lens manufacturers are making FF PL lenses, like those new Cookes or the cine line of the Sigma Art lenses. But, you'll be SOL if you want to use standard/super speeds, Ultras, Cooke S4's, etc.

[Robert Hart]

AJ Young.


You are correct. My URSA is the PL-Mount version. Many PL-Mount lenses are not suitable for the image circle being small. Some PL-Mount lenses are not suitable because lens structure physically will overlap the front element of the speedbooster optical cell.

Because I am using my old Nikon F-Mount stills lens set, the image circle is not a problem for me. My mount is the four-screw Nikon F-Mount as fitted to the FM2n stills camera. The later five-screw mounts require approx 1mm to be machined off to extend the small shoulder around the flange face rearwards to allow overhangs on the aperture control ring of older AI-AIS lenses to have clearance.

Metabones specifies that in their own PL-Mount flavour of their 0.71 "Ultra" Speedbooster, the only PL-Mount lens system which is approved to work with the PL-model for Sony E-Mount is the Zeiss CP2 family defined below. The limitations are defined "opto-mechanical". On other flavours of the 0.71 "Ultra" family of adaptors, a wider range of stills lenses can be attached.

Except for achieving a closer match perspectively between the BM4K and BM4.6K sensor images and about a stop of light gain, there is probably not a lot of point to using a Speedbooster in the URSA 4K cameras. The quality of the wider lens images appears to be compromised though that may be remediable with better trimming of the position of the speedbooster optic.

http://www.metabones.com/products/details/MB_SPPL-E-BT1

Due to the rearwards set-back of the PL-Flange, there are mechanical reasons why a PL-Mount is not practical although it could probably be made to work with a unique thinner clamp ring arrangement.

I offered up to my arrangement an old 25-250mm Angeniuex zoom. It does vignette and exhibit darker areas in the edge of the image.

The Peleng 8mm fisheye lens is intended to confer a circular image within the standard stills image full frame. I understand these were modified for PL-Mount for motion picture application and the image circle covered the standard 35mm motion picture frame.

In this linked vision clip, the last lens image from an 8mm Peleng fisheye is signficantly cropped and thus illustrates your point.

Edited August 6, 2017 by Robert Hart

[Robert Hart]

This is my arrangement. It is a custom intermediate adaptor with Nikon F-Mount attached on front. It replaces both the PL-Mount and the silver-coloured metal do-nut within the URSA PL cam's body to which the PL-Mount attaches.

You will observe that the front of the intermediate piece made of bronze is recessed significantly within the camera body compared with the normal intermedate adaptor because the speedbooster optic shortens the flange to focal plane distance.

You will observe a few extra holes in the bronze piece. this is because it is multipurposed to function as a Nikon mount adaptor to be attached to the front of the normal silver-coloured intermediate adaptor "do-nut" by using the PL-Mount screws which secure through another set of differently arranged holes.

Edited August 6, 2017 by Robert Hart

[Phil Rhodes]

My understanding is that the position of an optical reducer like that is very critical (in the way that the expander for a B4 mount lens is not, because you can adjust it out with the back focus on the lens.)

 

P

[Robert Hart]

I suspect as you describe that the optical cell of the speedbooster is not correct. The image from the 14mm rectilinear lens shows a ring of sharp focus which travels to the centre with focus trims. This lens is a little soft in its corners but not as much as is seen in the image. I have yet to make an adjustable support for it as soon as I can find out what the thread pitch for the optical cell is. I also need to find out what the pitch of the thread on the back of the BM cam's anti-reflection cone is. Then I can make the internal adjustable fitting.

[Robert Hart]

For curiosity only. The accompanying diagrams are not engineering drawings and measurements have not been meticulously checked. The diagram on the right is an estimation of the section of a replacement mount for the speedbooster which would replace the anti-reflection cone in the URSA 4K PL camera turret.

A reminder. This is most definitely not a BM approved adventure into the working of the camera. Any such adventure by you into your camera will void your warranty.

[Phil Rhodes]

Did we discuss whether this will work on an Ursa Mini 4.6K?

 

P

[Robert Hart]

It might possible to make it work on the URSA Mini4.6K PL if the throat of the camera body and the anti-reflection cone are identical to the big URSA 4K PL. The intermediate adaptor in the front of the URSA Mini4.6K may be different and may have a different pattern of the mount attach screws.

There may not even be an intermediate adaptor like there is on the big URSA. I have had no opportunity to study an URSA Mini 4K PL closely. I had hoped I might find one which was totalled but substantially intact around the region of the mount but no luck thus far.

I did get it to within a whisker of working in the original BM4K EF-Mount camera body. I think this may have a shorter "snout" on the front of the camera body compared to the PL version but I am by no means sure. The rear of several Nikon lenses interfered with fittings just inside the throat of the BM4K EF, which is narrower but still accommodates the Speedbooster optics with about a half-millimetre clearance radially.

Fitment within this camera would probably have to be via press fit into a soft cylindrical cuff as there is insufficient workspace to cut a 0.05mm pitch internal thread and an external 0.10mm external thread on a metal cylinder of only 0.5mm wall thickness.

Given that the Speedbooster optic seems to pull in about 32mm of the 36mm stills image frame width to the 4K sensor, there may be a vignette of the corners on the 4.6K sensor with the wide lenses.

I guess I could draw the 4.6K sensor area onto a piece of groundglass to do some image-circle tests but my old experimental pieces of surface-ground microscope slides are buried deep in the shed somewhere.

A Speedbooster can never work on the latest URSA Mini Pro because the ND filter system occupies conflicting space within the throat of the Pro camera.

Edited August 6, 2017 by Robert Hart

[Robert Hart]

Here is an image of the Speedbooster assembly fitted. Yeah I know, I am slack because I have not yet matte-blacked the shiny bits.

Edited August 7, 2017 by Robert Hart

[Phil Rhodes]

If you can figure out whether that's possible on the Mini, you might even sell a few conversions - if that's something you're interested in.

 

That said I would proceed with great caution when considering anything like that.

[Robert Hart]

I had no plans for making any sort of a business out of it.

I put in hand-building, seven hours just to set up the thread-cutting gears and machining the internal assembly, plus about the same for the combined Nikon F-Mount adaptor which doubles as the Speedbooster's Nikon mount as well.

Add the cost of buying in a whole Speedbooster just to use the internal optical cell, having to cover product liability, packing, shipping and Australia's overvalued dollar, I could not put them out there for a cost that folk would be prepared to pay.

I think the original 4K URSA Mini PL may have the same screw pattern because the URSA Mini B4-Mount fits the big URSA PL just fine.

If I can find a knackered 4K URSA Mini PL cam and 4.6K URSA Mini PL cam to study and take measurements from, likely I could find some way to make it work.

BM may have a sea container of rejected parts and non-repairable camera returns by now. I might do the begging bowl on them but I don't rate my chances. Whilst BM does not discourage folk from designing and making accessories to fit BM products, something which invades the camera like the inbuilt speedbooster is unlikely to be assisted as the chances of owner-warranty being voided are too great.

[Robert Hart]

For interest of anyone who would like to have a speedbooster in their URSA, I have obtained some numbers from a local machinist.


Costs thus far :-

For both the internal support for the speedbooster optical cell and Nikon F-Mount base only = AU$460-00, plus material.

GST if in Australia = AU$46-00.

Nikon F-Mount supplied separately by camera owner. = Cost unknown. Cost $AU Zero if the Speedbooster is Nikon Mount type.

"Ultra" 0.71 Speedbooster optical cell supplied by camera owner. = AU$610-89.


With used big URSA cams going inexpensively on eBay, a cost of about AU$1010-89, plus AU$102-00 GST, probably represents over-capitalisation for the benefit of near-to full stills frame fields-of-view and being wedlocked to Nikon F-Mount lenses.

I have not been able to obtain any information regarding permission related to fitments designed to match registered design and separate supply of the 0.71 "Ultra" optical cell from either Metabones or Caldwell. Each entity seems unwilling or incapable of replying to email queries.