UM46 vs UMPro - focus distance difference?

The place for questions about shooting with Blackmagic Cameras.
  • Author
  • Message
Offline

Emilian Dechev

  • Posts: 247
  • Joined: Fri Jan 15, 2016 5:09 pm

UM46 vs UMPro - focus distance difference?

PostMon Nov 27, 2017 11:18 pm

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?
Offline

Emilian Dechev

  • Posts: 247
  • Joined: Fri Jan 15, 2016 5:09 pm

Re: UM46 vs UMPro - focus distance difference?

PostMon Nov 27, 2017 11:24 pm

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...
Offline

Kristian Lam

Blackmagic Design

  • Posts: 559
  • Joined: Tue Aug 21, 2012 1:11 pm

Re: UM46 vs UMPro - focus distance difference?

PostMon Nov 27, 2017 11:41 pm

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.
Offline
User avatar

Jamie LeJeune

  • Posts: 740
  • Joined: Tue Feb 26, 2013 4:33 am
  • Location: San Francisco

Re: UM46 vs UMPro - focus distance difference?

PostMon Nov 27, 2017 11:47 pm

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.
www.incafilms.com
http://www.imdb.com/name/nm4601572/
Offline

Denny Smith

  • Posts: 7092
  • Joined: Thu Aug 01, 2013 4:19 pm
  • Location: USA, Northern Calif.

Re: UM46 vs UMPro - focus distance difference?

PostTue Nov 28, 2017 3:30 am

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
Last edited by Denny Smith on Tue Nov 28, 2017 5:44 pm, edited 1 time in total.
Denny Smith
SHA Productions
Offline

Emilian Dechev

  • Posts: 247
  • Joined: Fri Jan 15, 2016 5:09 pm

Re: UM46 vs UMPro - focus distance difference?

PostTue Nov 28, 2017 7:18 am

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? :?
Offline

Uli Plank

  • Posts: 1918
  • Joined: Fri Feb 08, 2013 2:48 am

Re: UM46 vs UMPro - focus distance difference?

PostTue Nov 28, 2017 10:00 am

EF lenses themselves are often not correct in their FFD. PL is a far more precise mount.
Offline

Emilian Dechev

  • Posts: 247
  • Joined: Fri Jan 15, 2016 5:09 pm

Re: UM46 vs UMPro - focus distance difference?

PostTue Nov 28, 2017 5:17 pm

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...
Offline

Denny Smith

  • Posts: 7092
  • Joined: Thu Aug 01, 2013 4:19 pm
  • Location: USA, Northern Calif.

Re: UM46 vs UMPro - focus distance difference?

PostTue Nov 28, 2017 5:46 pm

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
Denny Smith
SHA Productions
Offline

Uli Plank

  • Posts: 1918
  • Joined: Fri Feb 08, 2013 2:48 am

Re: UM46 vs UMPro - focus distance difference?

PostWed Nov 29, 2017 8:16 pm

And ultra wides are more critical than longer lenses.
Offline

Denny Smith

  • Posts: 7092
  • Joined: Thu Aug 01, 2013 4:19 pm
  • Location: USA, Northern Calif.

Re: UM46 vs UMPro - focus distance difference?

PostWed Nov 29, 2017 9:50 pm

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
Last edited by Denny Smith on Tue Dec 05, 2017 8:55 pm, edited 1 time in total.
Denny Smith
SHA Productions
Offline

Emilian Dechev

  • Posts: 247
  • Joined: Fri Jan 15, 2016 5:09 pm

Re: UM46 vs UMPro - focus distance difference?

PostTue Dec 05, 2017 8:34 pm

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?
Offline

Denny Smith

  • Posts: 7092
  • Joined: Thu Aug 01, 2013 4:19 pm
  • Location: USA, Northern Calif.

Re: UM46 vs UMPro - focus distance difference?

PostTue Dec 05, 2017 9:03 pm

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
Denny Smith
SHA Productions
Offline

Emilian Dechev

  • Posts: 247
  • Joined: Fri Jan 15, 2016 5:09 pm

Re: UM46 vs UMPro - focus distance difference?

PostTue Dec 05, 2017 9:32 pm

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".
Offline

Denny Smith

  • Posts: 7092
  • Joined: Thu Aug 01, 2013 4:19 pm
  • Location: USA, Northern Calif.

Re: UM46 vs UMPro - focus distance difference?

PostWed Dec 06, 2017 1:32 am

Good bye
Denny Smith
SHA Productions
Offline

Alastair Traill

  • Posts: 89
  • Joined: Tue Feb 04, 2014 1:00 am

Re: UM46 vs UMPro - focus distance difference?

PostWed Dec 06, 2017 12:59 pm

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.
Offline
User avatar

rick.lang

  • Posts: 7572
  • Joined: Wed Aug 22, 2012 5:41 pm
  • Location: Victoria BC Canada

Re: UM46 vs UMPro - focus distance difference?

PostWed Dec 06, 2017 4:40 pm

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.


Sent from my iPad using Tapatalk
Rick Lang
Offline

Emilian Dechev

  • Posts: 247
  • Joined: Fri Jan 15, 2016 5:09 pm

Re: UM46 vs UMPro - focus distance difference?

PostWed Dec 06, 2017 6:27 pm

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?
Offline

Denny Smith

  • Posts: 7092
  • Joined: Thu Aug 01, 2013 4:19 pm
  • Location: USA, Northern Calif.

Re: UM46 vs UMPro - focus distance difference?

PostMon Dec 11, 2017 3:23 am

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.
Last edited by Denny Smith on Tue Dec 12, 2017 2:18 am, edited 1 time in total.
Denny Smith
SHA Productions
Offline

Emilian Dechev

  • Posts: 247
  • Joined: Fri Jan 15, 2016 5:09 pm

Re: UM46 vs UMPro - focus distance difference?

PostMon Dec 11, 2017 6:46 am

Thanks Denny! I will advise the technician of my local store, since they were also not familiar with the problem.

Return to Cinematography

Who is online

Users browsing this forum: No registered users and 11 guests