Color difference between A and B cam

Tue Sep 22, 2020 11:35 am

Hello,

we have two Alexa Minis on our current show, same firmware etc. same LUT,

but on the same shot it shows quite a difference in the green/blue channels.

Does this have to do something with the calibration of the chip? Can this be fixed without exchanging the camera bodies?

thanks and all the best,
Xiaosu
 
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Re: Color difference between A and B cam

Wed Sep 30, 2020 6:25 pm

Dear Xiaosu,

without knowing the extent of the color mismatch - did you do a white-balance on the same target?

Kind regards,
Jan Heugel
Jan Heugel
Application Engineer
Jan Heugel
 
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Location: Munich, Germany

Re: Color difference between A and B cam

Wed Sep 30, 2020 10:19 pm

Unfortunately the difference is quite big, here is a picture:

Top Left: A CAM
Bottom Left: B CAM

Top Right: A CAM
Bottom Right: B CAM using a LUT that I generated in order to try to match the cameras - unfortunately it does not work for all the lighting situations...

As you can see it's not just the colors or a slight green/magenta or WB shift. It's the contrast and lighting levels as well.

Yes the lighting has been the same in both cameras, it's even the exact same lens. Same settings, same LUT, same everything.

We are quite puzzled and getting another camera body in order to remedy the problem. What could be the problem?
Xiaosu
 
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Joined: Thu Feb 01, 2018 1:00 pm

Re: Color difference between A and B cam

Fri Oct 30, 2020 7:06 pm

We had the same issue on a shoot with two MINI LF's.

The color diff was pretty significant

See below
Aaron Bernakevitch
 
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Re: Color difference between A and B cam

Wed Nov 04, 2020 12:35 pm

Thanks so much for sending us the original frames.

Some questions:
Which lenses were used for the shots? On one of the shots with the EasyRig in the Frame, the Metadata says Vantage A 35. Was the same lens used for both cameras?
Which lenses were used on the shot with the tree in the background?
The Metadata shows that no internal NDs were used, but did you use external ND filters for any of these takes?
Oliver Temmler
Product Manager Storage Media
ARRI Munich
Oliver Temmler
 
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Re: Color difference between A and B cam

Mon Nov 09, 2020 4:20 am

Hi Oliver

We were using the rehoused Nikons from Zero Optik

Easy rig shot both were shot on a 35mm

For the tree shot's I think it was the 85mm and the 35mm. No Pola. But we had the same schneider ND's.
Aaron Bernakevitch
 
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Joined: Wed Feb 07, 2018 7:37 pm

Re: Color difference between A and B cam

Mon Nov 23, 2020 2:59 pm

Sorry for taking so long to reply. I wanted to discuss this with a few colleagues first.

The sensor calibration performed by the ARRI factory and ARRI service centers maps the sensor’s response to light for proper sensitivity to brightness across the dynamic range for all color channels. The color rendition of the camera, however, can vary depending on all elements of the optical system: filters, lenses, and internal optical elements in front of the sensor.
Even with the same lens in front of two cameras, you may still get slight differences, as in those image from Xiaosu or your pictures with the Easyrig in the frame. In your example with the young gentleman in front of the tree, my guess is that the differences are largely caused by everything that happens in front of the lens mount. This should be compensated with an individual white balance and CC correction.

The factory pre-sets for tungsten, daylight and so on are alright if you just use one camera in a scene. With two cameras, however, the same RGB gain may actually pronounce said differences in the optical path and the resulting color rendition.
To match two cameras for use in the same scene, we recommend to create individual white balance presets for each camera. This means, you should auto-white both cameras with a neutral grey target in the lighting for that scene and store that e.g. as your "ext day" white balance.

That said, I know that some cinematographers like using the Kelvin-dial to make shots a bit warmer or cooler, which is totally relatable. Unless you shoot ARRIRAW, this can permanently impair the image quality, as it will increase the noise in one of the color channels and the noise will stay, even if the adjustment were taken back a bit in grading. We therefore recommend to look at white balance more the technical baseline, and to use the CDL correction for quick and easy adjustments towards a warmer/cooler image.

I hope this helps.
Best, Oliver
Oliver Temmler
Product Manager Storage Media
ARRI Munich
Oliver Temmler
 
Posts: 48
Joined: Wed Aug 13, 2014 3:16 pm
Location: Munich, Germany

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