Weird colours from Ursa Mini Pro. Broken Sensor?

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MattNorth

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Weird colours from Ursa Mini Pro. Broken Sensor?

PostFri Feb 16, 2018 7:49 pm

I've been testing my new Ursa Mini Pro today and I noticed some strange patches of colour. Here's a still taken from the footage. I've boosted the saturation to show the colour pattern. Can anyone tell me what's causing this? Do I have a defective sensor?

Ursa mini pro still_1.2.1-min.jpg
Ursa mini pro still_1.2.1-min.jpg (118.71 KiB) Viewed 468 times
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Denny Smith

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Re: Weird colours from Ursa Mini Pro. Broken Sensor?

PostFri Feb 16, 2018 9:17 pm

Yiu need to contact BM Support directly, as this is not normal.
Cheers
Denny Smith
SHA Productions
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robert Hart

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Re: Weird colours from Ursa Mini Pro. Broken Sensor?

PostSat Feb 17, 2018 6:42 am

To be at first fair before going after the camera sensor :-

Could you provide information about all the light sources which were in that room? A behind the scenes still image would be even better. Those green zones might be spill from discharge sources or LEDs which may have a green spike in their colour spectrum.

John Brawley would be your best advisor on this subject. I only know just enough to be a danger unto myself.

Could you test the camera on a clean white panel, evenly lit by outdoors sunlight at various iris settings and exposures with ND filters? You would need to add extra ND in front of your lens to bring the light level down as close to indoors conditions as you can.

If an identical pattern is reproduced, then the sensor or filtration in front if it may be at fault. There are several opportunities for light to bounce back and forth between planar surfaces in the ND filter stack and from within the back of the lens itself.

Uneven surface coating, slight smudges on the rear element of the lens could be enough to vary reflective contamination of the image.

Hopefully these will be notions that folk better skilled than I can comment upon or dismiss as wild imagination.
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MattNorth

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Re: Weird colours from Ursa Mini Pro. Broken Sensor?

PostSat Feb 17, 2018 11:56 am

Hi Robert,

Thanks for your input. I've been doing some more testing today and I think this has more to do with the lenses. I tested the lenses (TLS rehoused Cooke Speed Pancros) on a different Ursa Mini Pro and the effect is the same. I then tested the camera without a lens but with an ND in place to control the exposure. There was no dramatic colour variation. Saying that I have an Alexa Mini kit and this doesn't happen on that camera with these lenses. Could the sensor be more sensitive to these colour variations/light refractions?
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robert Hart

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Re: Weird colours from Ursa Mini Pro. Broken Sensor?

PostSat Feb 17, 2018 6:40 pm

QUOTE: "Thanks for your input."

You are welcome. I hope I have not misled you in any way.

QUOTE: "I've been doing some more testing today and I think this has more to do with the lenses. I tested the lenses (TLS rehoused Cooke Speed Pancros) on a different Ursa Mini Pro and the effect is the same."

Those old Cooke ( Taylor Taylor and Hobson ) Speed Panchro Series 2 lenses by today's standards are more flarey and the coatings may be by now uneven in reflection suppression. The URSA Mini Pro as far as I know has two ND filter disks behind an IR filter. Even with no ND selected, to ensure focal consistency there will still be clear planar elements in the disks.

Therefore maybe a total of five planar surfaces exists, capable of creating several generations of internal reflections versus one for the Alexa Mini which as far as I know, may have only a IR/anti-alias/contrast filter in the optical path between lens and sensor.

QUOTE: "I then tested the camera without a lens but with an ND in place to control the exposure. There was no dramatic colour variation. Saying that I have an Alexa Mini kit and this doesn't happen on that camera with these lenses. Could the sensor be more sensitive to these colour variations/light refractions?"

I cannot answer this question except by wild guesswork. The colour defects in your sample images appear to be across the span of the URSA MP sensor. There can be colour defects towards the edges or corners of some sensors with some wide-angle lenses and IR filter combinations but these generally appear as a symmetrical pattern.

I understand that the green channel in the URSA MP sometimes may need boosting in colour grading.If the green channel needs bringing up, then a green colour defect in the image will itself become aggravated.

Another aspect to the old Speed Panchros is that their sweet spot seems to be in the ballpark of f5.6-f11. Indoors you may well have had to open them up to fully wide. Compared to modern lenses, small optical aberrations ( maybe astigmatism ) I might expect from older lenses.

They may even have manually-figured or manually-finished elements in them, They may be provoked by wide-open or near wide-open aperture into yielding sharpness variations and related chroma variations in areas of the image.

I am sorry I cannot be any more help but hopefully my comments may prompt the right answers by people more knowing than I.

Hopefully a lens tech like Matthew Duclos might chip in and put right any wrong assumptions I have made here.

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