C70 color issues in the shadows when using all codecs (other than RAW)

[jmd123]

Hi All, 

I'm a new C70 owner, and have noticed a strange color issue that is only present when using XF-AVC, HAVC, or MP4, and is absent when using RAW LT, or ST. 

The issue presents itself in the shadows, when transitioning from light to dark. As seen in the attached photos (RAW LT on top, XF-AVC on bottom), the RAW LT image shows a clean and natural looking transition in the shadows on the lower portion of the forearm. However, in the XF-AVC, the shadows have a greenish/grey color, almost as if the shadows are absent of color or saturation. 

Raw LTXF-AVC Long GOP

I've tried this across all codecs, and across C-Log 2 and C-Log 3, and the problem persists. I've even tried turning off Chromatic Adoration correction, assuming that this might have been an issue. The image is about one stop "underexposed" (from middle grey) for mood, but by means would this be considered dark enough to expect this color shift. 

The images were captured in C-Log 2 Cinema Gamut with "natural" @ ISO 800. All images have used the Canon C-Log 2 to Rec709 transform LUT. 

Any thoughts would be much appreciated! 

[rs-eos]

Did you observe what the false color would be for this scene?  Curious if this scene was too underexposed?  And would the problem go away if exposed properly?

Still, it would make for an additional case of capturing in RAW.

[jmd123]

Thanks for the quick reply, Ricky. Yes, these image is exposed about one stop below middle grey, with the lightest skin tones being equally underexposed for mood. The region in question is nearly three stops below middle grey. 

I did try to lift the image in Resolve 18 to what might be considered "proper exposure," but this was unsuccessful in removing the coloration issue, as the noise appears to be at the sensor level. As the first post mentioned, the RAW looks fine, but the other codecs all show this green/grey coloration in the dark regions of the forearm. If I expose this scene (in camera) brighter than my intention (brightest skin tones +1-1.5 stops above middle grey), the problem does go away. though, this does take away from the mood I am trying to create, and this scene isn't unreasonable to expect color consistency across the dynamic range, even when exposed the way in which I have in the example photos. 

As in, I would say that this lighting is by no means "too dark", so I can't imagine that this coloration is a reasonable consequence of the environment/lighting. 

Thanks! 

[HNP_PDX]

Hi JMD123, 

I would double-check your sensor exposure. I've seen the color loss in the shadows that you have pointed out, and from my experience I would say that it only occurs in areas that are at least five stops underexposed from 18% grey. It looks as though you are reaching the bottom of the dynamic range, and the XF-AVC codec just can't hold the color detail like the 12-bit raw can. 

Considering you refer to lighting for mood, you might consider capturing it a stop or two over your intended exposure, and then pulling it down to taste in the post-production phase. 

I hope this helps! 

[jmd123]

Appreciate the insight. I’ll run some additional tests and get the exact exposure levels so I can report back. Thanks!