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XHA1 Color temp shift craziness

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Jon Gulbrandson
XHA1 Color temp shift craziness
on Aug 26, 2009 at 3:39:12 pm

Hey everyone,

I have canon XHA1, recording to FS-C firestore. I've noticed this weird phenomenon happen twice now under certain lighting conditions.

In post, I'll scrub through the footage, and my color temperature will change from slightly cool to slightly warm. This is with AWB OFF and a manual white balance performed near the subject. I can see the shift in the vector-scope and in the parade views (as well as composite obviously).

The shift happens over a 5 - 6 second swing in a sin-wave like motion.

Now, both times, the set was lit with incandescent and florescent (the camera position remained stationary)... which makes me think there was some sort of natural fluctuation, unsynchronization, or other strange effect happening with A) the AC power b) maybe the wavelength of the varying light sources...

I can't help of think of it like tuning an instrument. Lets say you have two instruments playing the same note, but one is slightly flat... if you listen you can hear a Wah-wah-wah-wah effect because the wavelengths are going in and out of sync... or like, when your at a stop light w/ your blinker on, and the car in front of you has it's blinker on, and they'll phase in and out of sync...

I dunno.. i'm just trying to wrap my head around this... I did some google searching and couldnt' find anything..

any lighting guru's out there to shed some 'light' on this matter?


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Todd Terry
Re: XHA1 Color temp shift craziness
on Aug 26, 2009 at 3:48:46 pm

I'm 99% sure that is "phasing" caused by the fluorescent instruments... they are out of sych with your shutter speed and since you are using a combination of flos and tungsten, the ratio of the two types of light is constantly changing with the phase... ergo the color shifts in "waves."

If possible, try a test with the flos off. I think you will see that the color balance stabilizes.

You can combat this by playing with the shutter speed... if you are shooting 24fps (Canon's "24f" mode) you are probably using the default "normal" shutter of 1/48th. Try cranking it up to 1/60th. And... I think that camera has a "clearscan" mode (I use its big brother the XLH1 which does, so I think yours does too). If so, change the shutter to CS and you can dial in an exact shutter speed that will eliminate the phasing... which is probably going to be 60.1Hz.


T2

__________________________________
Todd Terry
Creative Director
Fantastic Plastic Entertainment, Inc.
fantasticplastic.com






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Jon Gulbrandson
Re: XHA1 Color temp shift craziness
on Aug 26, 2009 at 4:25:06 pm

Okay. Ya, so i wasn't too far off. I had a feeling the majority of the problem was because of the florescent and/or mixing that with incandescent. Luckily I can still pull the shot i need out of my footage, and thankfully when played at regular speed it's very, very hard to tell because the shift is so gradual.. i can only notice it when I scrub through at like 3x speed.

Thanks for your help! Lesson learned! I'll have to look into that clearscan feature and see if I have it.


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Todd Terry
Re: XHA1 Color temp shift craziness
on Aug 26, 2009 at 4:57:17 pm

I think you probably have clearscan...

If so (and if it works the same as the XLH1, which it probably does) keep toggling the shutter speed higher and higher as far as it will go... the very last shutter speed setting will say "CS" in the viewfinder instead of a number. Select that, then go into the camera menu and (I think) if you open the "Camera Settings" one of the menu items will say "Clearscan." You select that and then dial in the exact speed you want. As I said in the earlier post, for most "flicker" issues from AC sources if you put the clearscan on 60.1Hz it will eliminate the problem. You can just leave the clearscan setting at that number, and scroll the shutter up to CS mode when you need it. When you don't need it scroll back down to your "normal" shutter speed (1/48th for 24p or 1/60th for 60i) and the camera will remember the clearscan setting for the next time you need to kick it back on.


T2

__________________________________
Todd Terry
Creative Director
Fantastic Plastic Entertainment, Inc.
fantasticplastic.com






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Jon Gulbrandson
Re: XHA1 Color temp shift craziness
on Aug 26, 2009 at 5:07:14 pm

i just checked. you are absolutely right! I dialed my CS setting for 60.1 for the next time I run into this situation. Thanks a million!


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