Ursa Broadcast G2 Effective Sensor Size when using B4 Lens

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Jbird_RL

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Ursa Broadcast G2 Effective Sensor Size when using B4 Lens

PostTue Nov 22, 2022 9:38 am

We have purchased this camera to use in live broadcast virtual production, but to create a 'virtual' camera that matches this one, we need to put in the sensor size so that the images match.

Usually when the sensor isn't being cropped, working out the sensor size to put in for the Virtual Camera is just a case of looking it up, but how do we work out the size and width when the image is being cropped for a broadcast lens? The Product page says:
2/3 inch sensor size when using 4K B4 mount
but what does that mean in terms of sensor height and width, is it simply 9.6x5.4mm?

We have a Blackmagic URSA Broadcast G2 with a Fujinon Broadcast Lens (2/3" B4 mount) on it, which I think crops the effective sensor size. When overlaying the real image over our virtual camera, its clear the output aspect ratio is 16:9 (1:1.77778), but there is conflicting info online about how to work out the effective sensor size. I read somewhere the sensor size is an inch, but it has a teleconverter on it that scales the image up? Is this true? if so, how des that affect the effective sensor size?

Has anyone used this camera with the B4 mount for virtual production? What sensor size did you put in to get a good alignment?
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Steve Fishwick

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Re: Ursa Broadcast G2 Effective Sensor Size when using B4 Le

PostTue Nov 22, 2022 10:30 am

The UBG2 uses an optical adaptor to achieve an 'effective' 9.6x5.4mm, for B4 lenses. It is not actually that being approx. 1.5x or 14.438mm (H) x8.122mm (V) x 16.566mm, according to CVP's tool. That is a 4K crop of the total Super 35 sensor, which is identical to the Pocket 6K's ones.

For all practical purposes though, when using the supplied B4 adaptor, with B4 lenses, it should be treated as a standard ENG B4 camera and all characteristics will conform to those, as if it had a conventional 9.6 x 4mm sensor.
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Jbird_RL

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Re: Ursa Broadcast G2 Effective Sensor Size when using B4 Le

PostTue Nov 22, 2022 10:41 am

Steve Fishwick wrote:The UBG2 uses an optical adaptor to achieve an 'effective' 9.6x5.4mm, for B4 lenses. It is not actually that being approx. 1.5x or 14.438mm (H) x8.122mm (V) x 16.566mm, according to CVP's tool


Thanks for the quick reply Steve! Are you able to provide a link to this tool? And why is it 1.5x, does the camera/b4 mount have a teleconverter?
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Steve Fishwick

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Re: Ursa Broadcast G2 Effective Sensor Size when using B4 Le

PostTue Nov 22, 2022 10:46 am

https://cvp.com/tools/cameralens

Jbird_RL wrote:And why is it 1.5x, does the camera/b4 mount have a teleconverter?


Because in order to have 4K actual effective pixels BMD must use an optical adaptor to magnify the smaller B4 image to the target 4K crop, of this particular sensor. It's not a teleconverter and has no effect on the image, such as DOF. It is merely magnifying it, correcting for 3 chip aberrations, etc. If they used a different sensor, such as was in the G1 then the magnification would have to be slightly different. The only alternative is an actual 3 chip ENG camera, with no need for magnification but at a considerably higher price point.
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Jbird_RL

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Re: Ursa Broadcast G2 Effective Sensor Size when using B4 Le

PostTue Nov 22, 2022 12:04 pm

Thanks for the info, that's really helpful. If its not changing the image (I thought it might have an effect on the Focal Length and appear more zoomed in) then I think I can just assume the sensor size is equivalent to standard B4 lenses, i.e 9.6 x 5.4mm.
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Steve Fishwick

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Re: Ursa Broadcast G2 Effective Sensor Size when using B4 Le

PostTue Nov 22, 2022 12:42 pm

Jbird_RL wrote:If its not changing the image (I thought it might have an effect on the Focal Length and appear more zoomed in) then I think I can just assume the sensor size is equivalent to standard B4 lenses, i.e 9.6 x 5.4mm.


Yes that's exactly right, Jamie, for all practical purposes, as I say.
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Howard Roll

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Re: Ursa Broadcast G2 Effective Sensor Size when using B4 Le

PostTue Nov 22, 2022 11:05 pm

Jbird_RL wrote:We have purchased this camera to use in live broadcast virtual production, but to create a 'virtual' camera that matches this one, we need to put in the sensor size so that the images match.


The multiplication (tele-conversion) factor of the mount is 1.484X so the final effective sensor ends up a little wider than standard 2/3" at 9.73x5.47mm.

Good Luck
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Jbird_RL

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Re: Ursa Broadcast G2 Effective Sensor Size when using B4 Le

PostWed Nov 23, 2022 10:22 am

Howard Roll wrote:The multiplication (tele-conversion) factor of the mount is 1.484X so the final effective sensor ends up a little wider than standard 2/3" at 9.73x5.47mm.

Good Luck

Thanks for the Info Howard! Where did you find the information that the tele-conversion factor is 1.484? And how did you get to those numbers? 9.6 * 1.484 isn't 9.73, just a bit confused how you got to that.

Thanks in advance!
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BennoZ

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Re: Ursa Broadcast G2 Effective Sensor Size when using B4 Le

PostThu Nov 24, 2022 10:03 am

I also own this camera and it's not a little wider, but it's a lot wider than a normal 2/3" camera. Steve said it has no effect on the image like DOF, but don't forget the B4 mount costs you some light.
Also keep in mind you can set the sensor crop at 4k but also at 2.7K; that differs a lot in image size, so you have to check that setting.

Tip: Don't forget to set the lens correction switch to on, that will reduce the CA a lot :)
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Steve Fishwick

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Re: Ursa Broadcast G2 Effective Sensor Size when using B4 Le

PostThu Nov 24, 2022 12:29 pm

BennoZ wrote:but don't forget the B4 mount costs you some light.


Actually Benno, what you lose through the adaptor is made up by the larger photosites, dual gain and no 3 chip prism. Sensitivity is actually on a par with other true 3 chip ENG cameras, such as the Sonys.
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BennoZ

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Re: Ursa Broadcast G2 Effective Sensor Size when using B4 Le

PostThu Nov 24, 2022 12:34 pm

Steve Fishwick wrote:
BennoZ wrote:but don't forget the B4 mount costs you some light.


Actually Benno, what you lose through the adaptor is made up by the larger photosites, dual gain and no 3 chip prism. Sensitivity is actually on a par with other true 3 chip ENG cameras, such as the Sonys.

I’m sorry but I disagree, the light performance is very poor of the Broadcast G2: there’s a huge difference in light compared with other camera’s like a Sony PMW500 and such camera’s. Even the Ursa Mini Pro has more light. (But that can be because of the higher native ISO of 800 instead of 400 from the Broadcast G2?)

If you connect a EF lens on the Broadcast G2 there’s much much more light.
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Steve Fishwick

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Re: Ursa Broadcast G2 Effective Sensor Size when using B4 Le

PostThu Nov 24, 2022 12:40 pm

BennoZ wrote:I’m sorry but I disagree, the light performance is very poor of the Broadcast G2: there’s a huge difference in light compared with other camera’s like a Sony PMW500 and such camera’s. Even the Ursa Mini Pro has more light, but than can be because of the higher native ISO of 800 instead of 400 from the Broadcast G2.


You're not comparing like with like. I am referring to current UHD ENG cameras, that have photosites of around 2.5microns. SD and HD cameras by comparison are always going to be more sensitive. And the UMP G2 is only more sensitive at full sensor without the adaptor, in comparison with the UBG2 in ENG mode - try putting a B4 adaptor on them and compare again. I think the 6K Pockets are more sensitive than the Mini Pro, which would be the same for the UBG2 at full sensor, but I might be wrong.
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BennoZ

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Re: Ursa Broadcast G2 Effective Sensor Size when using B4 Le

PostThu Nov 24, 2022 4:01 pm

Steve Fishwick wrote:
BennoZ wrote:I’m sorry but I disagree, the light performance is very poor of the Broadcast G2: there’s a huge difference in light compared with other camera’s like a Sony PMW500 and such camera’s. Even the Ursa Mini Pro has more light, but than can be because of the higher native ISO of 800 instead of 400 from the Broadcast G2.


You're not comparing like with like. I am referring to current UHD ENG cameras, that have photosites of around 2.5microns. SD and HD cameras by comparison are always going to be more sensitive. And the UMP G2 is only more sensitive at full sensor without the adaptor, in comparison with the UBG2 in ENG mode - try putting a B4 adaptor on them and compare again. I think the 6K Pockets are more sensitive than the Mini Pro, which would be the same for the UBG2 at full sensor, but I might be wrong.


Because it’s off-topic I leave it with one thing: It’s just physics, when you magnify an image by glass the result is less light in a larger area, if you reduce it you get more light in a smaller area. So that’s why a Speedbooster gives you more light.
Here’s the exact reduce of light of the B4 4K mount of the Broadcast G2 camera from Blackmagic specs:
“Additional 1.1 stop light loss using 4K B4 mount.” (And exactly it is 1.1389 stops)

By the way, this B4 mount is the same as for the Broadcast G1.
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Steve Fishwick

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Re: Ursa Broadcast G2 Effective Sensor Size when using B4 Le

PostThu Nov 24, 2022 4:57 pm

BennoZ wrote:Because it’s off-topic I leave it with one thing: It’s just physics, when you magnify an image by glass the result is less light in a larger area, if you reduce it you get more light in a smaller area. So that’s why a Speedbooster gives you more light.
Here’s the exact reduce of light of the B4 4K mount of the Broadcast G2 camera from Blackmagic specs:
“Additional 1.1 stop light loss using 4K B4 mount.” (And exactly it is 1.1389 stops)


You didn't read what I wrote. Against that again is the bigger UHD photosites (3.8 microns as opposed to 2.5 microns) on the UBG2, no prism and dual gain. Look at the specs of the Sony PXW-Z450/750s and you will see sensitivity on balance is very similar.
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Re: Ursa Broadcast G2 Effective Sensor Size when using B4 Le

PostFri Nov 25, 2022 1:35 am

Jbird_RL wrote:
Howard Roll wrote:The multiplication (tele-conversion) factor of the mount is 1.484X so the final effective sensor ends up a little wider than standard 2/3" at 9.73x5.47mm.

Good Luck

Thanks for the Info Howard! Where did you find the information that the tele-conversion factor is 1.484? And how did you get to those numbers? 9.6 * 1.484 isn't 9.73, just a bit confused how you got to that.

Thanks in advance!


Divide the actual sensor size by the magnification of the teleconverter. 14.438mmx8.122mm each divided by 1.484.

This information was provided by Tim Schumann in this thread. viewtopic.php?f=2&t=152356

Good Luck

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