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UM46 vs UMPro - focus distance difference?

PostPosted: Mon Nov 27, 2017 11:18 pm
by Emilian Dechev
I just got the Ursa Mini Pro and I am doing some random comparisons with my older Ursa Mini 4.6K

Aside from the UM Pro being a bit more magenta, rather than the UM 46, which is a bit more warmer,

there is a very odd difference, that came up.

Using the same 50mm Lens at the EXACT same camera distance and focus distance, at about 1.2 meters to the test object, the 2 cameras had different focus points!

I believe, that both cameras have the same sensors, same bodies and all of their physical distances are the same.

The UM 46 was giving me a shorter focal distance with about 3-4cm. That is not much, but I wonder, how is that physically possible?

Re: UM46 vs UMPro - focus distance difference?

PostPosted: Mon Nov 27, 2017 11:24 pm
by Emilian Dechev
WOW just repeated the test with a 14mm lens - the focus difference was big like 40 cm ??

So the UM46 focused properly at about 100cm, while the UM Pro focused at about 60cm...

Re: UM46 vs UMPro - focus distance difference?

PostPosted: Mon Nov 27, 2017 11:41 pm
by Kristian Lam
The shimming distance could be different and the URSA Mini Pro can be shimmed via the removable lens mount. That will cause the difference in the focus distance.

Re: UM46 vs UMPro - focus distance difference?

PostPosted: Mon Nov 27, 2017 11:47 pm
by Jamie LeJeune
Even different units of the same model camera will have slightly different default sensor tints. I've seen this not only when shooting with multiple 4.6K Ursa Mini's, but also with every other make and model of camera I've ever used for multicam setups. On the Ursa Mini the temp and tint controls to make a match in camera are easy to set. The pro broadcast cameras I used to rent had extremely complex color matrix setups that took a lot of time and skill to adjust for a multi-camera match. Luckily the rental house would always take care of it.

I'm no expert on this, but I'd bet that the focus distance difference you're seeing has to do with the swappable mount on the Ursa Mini Pro. The exact flange focal distance on the Pro can be adjusted with the shim kit and a torque wrench with 2mm hex head. See page 200 in the most recent manual for details.

Re: UM46 vs UMPro - focus distance difference?

PostPosted: Tue Nov 28, 2017 3:30 am
by Denny Smith
Yep, i discovered this with the. I have the Cinema and the Micro Studio using a PL adapter, the two cameras had a slightly different FFD, so I had to re-shim the PL adapter to offset the difference, wasn’t much, but more obvious with wider angle lenses.
Cheers

Re: UM46 vs UMPro - focus distance difference?

PostPosted: Tue Nov 28, 2017 7:18 am
by Emilian Dechev
Thanks for the info!

I knew, I should do some shimming when replacing the EF to PL mount, but I never thought the default EF is also out of alignment.

Now I have to assume that the camera by default from the shelf is not focus accurate with EF lens? :?

Re: UM46 vs UMPro - focus distance difference?

PostPosted: Tue Nov 28, 2017 10:00 am
by Uli Plank
EF lenses themselves are often not correct in their FFD. PL is a far more precise mount.

Re: UM46 vs UMPro - focus distance difference?

PostPosted: Tue Nov 28, 2017 5:17 pm
by Emilian Dechev
Uli Plank wrote:EF lenses themselves are often not correct in their FFD. PL is a far more precise mount.


Sure, but 40cm is too much difference...

Re: UM46 vs UMPro - focus distance difference?

PostPosted: Tue Nov 28, 2017 5:46 pm
by Denny Smith
Also, the tolerances in the EF Mount (as well as the MFT mount) are not very consistent from one mount to another, especially from different manufacturer runs, this combined with the lens mount, can create quite a big difference. Also, when judging focus in a test target, you need a monitor with pixel zoom in to be accurate.
Cheers

Re: UM46 vs UMPro - focus distance difference?

PostPosted: Wed Nov 29, 2017 8:16 pm
by Uli Plank
And ultra wides are more critical than longer lenses.

Re: UM46 vs UMPro - focus distance difference?

PostPosted: Wed Nov 29, 2017 9:50 pm
by Denny Smith
Yes, yes they are! I discovered this with my 9.5mm lens in the Micro Camera, the VA showed focus point was different than the SmallHD 501,with its 2x Zoom.
Cheers

Re: UM46 vs UMPro - focus distance difference?

PostPosted: Tue Dec 05, 2017 8:34 pm
by Emilian Dechev
And the official answer from BM rep:

"I believe this may be within expected tolerances for the EF mount on the URSA Mini Pro as the mount has such a wide variety of lenses available there needs to be a certain tolerance in place to allow back focus and infinity focus on all lenses.

If the customer would like to be able to achieve the focussing distance on a specific lens then you can use shims on the URSA Mini Pro to adjust for this.

The URSA Mini Pro shim kit can be purchased as a spare part in which they are fitted behind the lens mount on the camera."


Alright I will try that :)

But from a customer point of view - why should I get a several thousand dollar camera, that comes as "EF mount" by default, but that "default state" is not usable, because it cannot focus. So now I need to pay for additional accessory in order to use it?

Re: UM46 vs UMPro - focus distance difference?

PostPosted: Tue Dec 05, 2017 9:03 pm
by Denny Smith
It is the EF Mount, which is a still camera mount with very slopy focus setting tolerances, and expecting Cinema Camera Lens Mount Tolerances from a still camera designed lens mount is unrealistic.

Also, not all EF Mount lenses are created equal, unlike the PL mount which is very precise and was designed for video cameras. Using a still camera lens on a video camera is a “crap shoot” in regard to actual focusing tolerances and the distance scale will be off most of the time, as a still camera uses it’s auto-focus system to focus these lenses. Even Sigma recognized this issue with their Art series lenses, and supplied a programming pod/module to reset the lens focusing performance and back distance to achieve accurate focus with the art series lens and EF and Nikon F Mount still camers.

Normally the EF Mount is not shimable, but BM came up with a shining system for their EF Mount on the Ursa Mini Pro, to correct for the poor manufacturing tolerances on most EF lenses. Even Canon came up with a “locking” EF Mount for their C200/300 camera to help reduce the mount movement present in the EF Mount.
Cheers

Re: UM46 vs UMPro - focus distance difference?

PostPosted: Tue Dec 05, 2017 9:32 pm
by Emilian Dechev
Thanks for the answer Denny, but I need to be able to work with cinema EF lens, not still lens.

And even considering all of your points, my older Ursa Mini 4.6K EF mount is having 100% accurate focus on all cinema EF lens I have been working with.

Why these Ursa models are different in that regard?

BM rep answered that: "this may be within expected tolerances for the EF mount, and if the customer would like to be able to achieve the focussing distance on a specific lens then you can use shims on the URSA Mini Pro to adjust for this"

OK so he is telling us, that it is technically possible to get the UMP to 100% accuracy. Will see about that :)

Nevertheless, it would have been great, if a camera that is sold as "EF mount" already has the needed shims pre-installed, so the customer is not buying a camera, that is not "EF mount ready".

Re: UM46 vs UMPro - focus distance difference?

PostPosted: Wed Dec 06, 2017 1:32 am
by Denny Smith
Good bye

Re: UM46 vs UMPro - focus distance difference?

PostPosted: Wed Dec 06, 2017 12:59 pm
by Alastair Traill
In film camera days you could replace the film with a flat steel plate and measure the back focal distance with a dial indicator. If the distance was incorrect it could be adjusted. Lens back focal distances could be checked with a collimator and if necessary be adjusted. Obviously with a video camera you cannot use a dial indicator and even if you could there is a filter in the way. You really need to know whether the problem is lens or camera based. If you start changing the back focal distance of the camera without knowing the cause of the problem you risk upsetting other lenses. It is also important to realize that the shorter the focal length of the lens the more critical is the distance. An error of 0.0005” with a zoom of 10 mm minimum focal length can cause problems. With a shorter focal length the tolerances are much less again – in fact the effects are related to the square of the focal length. I do not know how video camera back focal is measured but I guess a reference lens and a collimator would be used.

Re: UM46 vs UMPro - focus distance difference?

PostPosted: Wed Dec 06, 2017 4:40 pm
by rick.lang
Remember back in the day (2012 seems like just yesterday) the BMD EF FFD was precisely 44.0mm. BMD re dived a lot of complaints until they relaxed the FFD to accommodate some very popular lenses from third parties. So give BMD some credit for trying to do it right in the first place. The upshot has been to go to PL mount lenses if you want consistent behaviour of your lenses (with shims and back focus possibly). I understand the frustration, but keep this in mind too.


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Re: UM46 vs UMPro - focus distance difference?

PostPosted: Wed Dec 06, 2017 6:27 pm
by Emilian Dechev
Sure I understand.

Its just that, this "EF needed shim" could have been pre-installed, when you buy a camera listed as "EF mount".

After I get the shims, I will post the results.

Does anyone here have a Ursa Mini Pro with shim installed for EF use, and can confirm that wide cinema EF lens (Zeiss CP, Canon, Xeen 14mm, 21mm, 24mm) are hitting the focus marks?

Re: UM46 vs UMPro - focus distance difference?

PostPosted: Mon Dec 11, 2017 3:23 am
by Denny Smith
As Rick pointed out, BM changed the FFD setting on their EF Mount lens from Canon 44.0mm standard (which your Canon Cine lenses are probably set for) to 43.xx to allow proper focusing with lenses like the Sigma 18-35 which would not focus properly at 18mm wide open (f1.7). The UMPro shim kit should allow yiu to reset the FFD back to 44.0mm. Good luck.

Re: UM46 vs UMPro - focus distance difference?

PostPosted: Mon Dec 11, 2017 6:46 am
by Emilian Dechev
Thanks Denny! I will advise the technician of my local store, since they were also not familiar with the problem.