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arri alexa magenta

arri alexa prob

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#1 bledarcili

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Posted 06 July 2016 - 12:44 PM

Hi , i have shooting too days for music video with arri alexa clasic pro ress 4444 and i see magenta and cyan in image, please can you help me???

I use tiffen ND filster 0.6 ,0.9 with arri MK2 lens.

See in link please:

 

https://www.flickr.c...s/23280449@N06/

 

Thanks

Bledi


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#2 David Mullen ASC

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Posted 06 July 2016 - 01:11 PM

The green cast is just the basic Alexa Rec.709 LUT, it always leans a little yellow-green and it gets worse when using ND filters, often you need some minus green correction added in the camera, but it is also easily shifted in post.

 

What's weird is the magenta diagonal band across the middle of the frame no matter which direction the camera is pointed. I've never seen that before.  If you were using a pola or variable ND, I suppose it could be some sort of odd polarization artifact but you said you were just using straight ND filters.  So I have no idea but it looks to be some sort of optical artifact somewhere.


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#3 Bruce Greene

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Posted 06 July 2016 - 03:25 PM

What ISO was the camera set to? Kind of looks like clipping of highlights. Did you shoot at ISO 200?
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#4 Shawn Sagady

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Posted 06 July 2016 - 04:46 PM

The good news is the magenta band looks static so you should be able to fix it in the grade with a power window, but not sure what would cause such a specific artifact, can only think a defect in the filters somehow?  Or some kind of minor light leak on the sensor hitting from a weird angle.


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#5 Stuart Allman

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Posted 06 July 2016 - 05:20 PM

Bledi,

 

My guess is that this is some type of internal reflection.  Try moving the matte box such that the ND filter is right up next to your lens, without them touching of course.  My first inclination is that you have an internal reflection going on and moving the filter close to the lens should at least change the characteristic, if that's the root cause.  I know I have to watch out for things like this on my matte box.

 

As a second suggestion, try a different lens if you have one available and see if there is some type of reflection going on because of the lens geometry and your matte box.

 

Stuart Allman

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