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Old February 16th, 2005, 08:16 PM   #1
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strange Color Shifts.....manual WB

I recently shot with the Xl2 in a park setting. I was filming some kids on a wooden jungle gym. As I was filming I adjusted the iris to properly expose as they moved from shadow to sunlight. The kids were wearing red, white and blue sweat shirts and had on light denim blue jeans. However the majority of the video has the blues looking purple and thje blue jeans looking like light purple jeans! The other colors seem to be OK. I had both neutral Density filters on and on occasion when I'd stop down for a few seconds the colors come through accurately. So there is some relationship with the iris that caused this shift to occur.

I shot with Cinnegamma On and Cine Colors too. 24p and 16x9. All other settings default. Never had this problem before and I am wondering if someone can explain why this might happen. The only new element that is in the equation is I was using the 16x manual lens for the first time. But my brain says the lens shouldn't cause this type of color shift. But that same brain has been known to be wrong!!!

Anyone ever see this before?
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Old February 16th, 2005, 09:47 PM   #2
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If you used fixed color balance and didn't change when moving from shade to sun you should expect a noticeable change in color because in shadow the subjects are illuminated by the blue sky and in sunlight by the yellow sun. Sunlight is at a much lower color temperature than sky light. If balanced for shadow light images illuminated with sunlight will have lots of red (and blue things will become purple). If balanced for sunlight images exposed in the shade will look very blue. Moving from sun to shadow and back is probably the only situation in which you might want to use auto color balance but be sure to have something neutral in the picture so you can check on what it did in post and correct if it didn't do a perfect job. If you use manual set for one you can then try to correct the other in post.
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Old February 17th, 2005, 07:22 AM   #3
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I was color balanced using the outside preset. I have filmed outside before and never seen this kind of anomally. I intend to pop the tape in and watch the data code and see if I can find a pattern to this issue. My only other theory is that maybe I could have accidentally put the color balance in auto mode and that caused the blues to shift.

The funny thing was I was testing the manual lens for clarity and focus and was paying little attention to the colors at all. My wife came in and saw the footage and said " those colors look terrible!" I said "no they don't...look how clean they are." she said yes but the blues all look purple! I started to look and was like...damn! So I paid closer attention and sure enough they were purple 90% of the time and blue 10%. The weird thing is that the skin tones seem consistent. Sometimes it takes a women to see something that is a fashion mistake. I mean.....red, white and purple? How did I miss that? I was so focused on the clarity I looked right past it.

I'll try and figure it out and let you know if I come up with an answer.

Thanks!
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Old February 17th, 2005, 07:41 AM   #4
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There are lots of things which might have caused what you got including a failure of some sort. My comments were based on what might be expected if everything was working OK. Whatever the case, mixed sunlight and shadow (or mixed source color temperatures under any conditions) are a problem. You can't be simultaneously correctly balanced for both. Good luck!
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Old February 20th, 2005, 01:04 AM   #5
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Me too...

The first real test of my shiny new XL2 was at an indoor ice rink. I had manually set the white balance against the ice. I was playing with the AE shift, trying to bring out the colors of darker clothing and intentionally overexposing the ice. I was only later after I got home and hooked the camera up to a monitor for playback that I noticed a regular, periodic shift in colors. For example, a blue surface was shifting to pink every few seconds, then back to blue.

I was worried there was something wrong with the camera. Next time I went to the rink, I was careful to set the WB against a very bright white sheet of paper and only tweaked the AE shift up about .25 - .50. I did not see any color shift on those tapes.

So if I had to wager a guess, I'd say it's related to overexposure and/or perhaps the HID (metal halide?) lighting at the rink.

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Old February 20th, 2005, 07:25 AM   #6
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Rember that if you overload any sensor (R,G,or B) balance is finished and something that was purple, for example, may shift towards pink. This will only occur in highlights, of course. This might be what you were seeing. The zebra stripes are a great help in seeing what is over exposed or close to being over exposed. I don't know anything about these lights but it may well be that they had something to do with it.

Something you might want to consider is setting the balance to the light, not something illuminated by the light. There is a doodad called an Expodisk which is basically a piece of frosted plastic in a filter holder. It is designed to be absolutely neutral and to diffuse incoming light over the entire frame. You place it over the lens, point at the light source, set exposure to a reasonable level and then push the balance button. It's not a bad idea to pull a couple of seconds of tape with it on. These can be used to check neutral in post. After the disk is removed the camera is balanced for anything illuminated by this light source. If this idea is intriguing you can try it with a bleached coffee filter which does exactly what the Expodisk does but is less expensive if somewhat less robust.
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Old February 21st, 2005, 03:47 PM   #7
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JUst to clarify what I was doing and how the exposure affected it.

Outside, Midday, clear. No clouds.

XL2 with manual 16x lens. Both ND filters engaged.

The Xl2 set to Tv mode so that I can test my new manual lens and let the brains of the camera handle the iris for today.

I white balances using the default outdoor WB setting.

The problem: Dark blue Jackets and blue jeans seem to be turning purple as kids and moms move from bright area to shaded areas on the playground.

I have reviewed the tape and watched the datacode and here is what I find.

In the sunlight....most of the time....the iris goes to f4.0-5.6 and the colors appear more accurate and close to normal. The bluejeans seem ok but the blue in the jackets still seem to lean just a tad towards purple.

When the kids move to the shade the iris opens to 2.8-1.6 range and here the blues just become purple and the jeans start to look purplish.

What does this mean? And why is the iris affecting colors so much? I know how it affects DOF but I wasn't aware of a color issue. Also.....do these iris numbers seem accurate? Or should I be using only 1 ND filter and let the iris close more in the sunlit area so when in the shade it is around 4-5.6 range? Does any of this make sense?

Am I going crazy? WTF???
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Old February 21st, 2005, 03:55 PM   #8
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Marty,

There is a color difference between shade and direct sunlight.

That's what you are seeing.
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Old February 21st, 2005, 03:59 PM   #9
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Okay. I am willing to believe this to some degree. But this was ridiculous! I have been shooting for ten years and a large amount of it has been outside. Why all of a sudden is it behaving like this? I mean the colors of the jackets and jeans are blatantly wrong and the skin tones and other colors seem to still be okay.

Anyone else have any theories?
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Old February 21st, 2005, 04:53 PM   #10
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Yes, but what you are seeing is exactly the opposite of what should happen in this case. If you balance for daylight and move into shadow the picture should take on a bluish overall cast i.e. shift towards blue. When blue shifts towards purple that means more red is being recorded and shadow has less red than sunlight. Something is not kosher here but at this point I can't tell what. About the only thing I can think of is that the automatic gain control might be doing you a mischeif. Try this again with the gain set to 0 dB (i.e. not to A). No promises here but worth a shot. I do what you are doing all the time and all I ever notice is a blue shift. As you say you have been doing this for years and never seen anything like it nor should you with this camera. One suspects there may be a problem with your XL2.
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Old February 21st, 2005, 09:00 PM   #11
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The Auto gain was off. So gain was at 0db. The iris was set to auto....meaning the camera judges the proper exposure and adjusts accordingly. The only other issue I can think of is that I may have had a custom preset chosen by accident where the RGB channels were tweaked. The reds were @+1 and the blues -2.

Would that cause this type of effect? Again I am not even sure that I had that preset chosen. I am just guessing. And the data code does not store preset information.

I should be clear that when I said shadows on the initial thread I do not mean "complete" shade. I am talking about a wooden jungle gym that has some thick bars that occassionally obstuct a large portion of the person on screen. The XL2 opens the iris to adjust for this and the purples come up.....even on the coat and pants that are in direct sunlight. So the objects themselves can be illuminated directly from the sun yet have a red cast.

I am still testing. I believe this is some strange anomaly and not a defect. I have shot too much other stuff with the XL2 for this not to be some form of "user error"....although I am not quite sure what yet. Is there any chance the ND fitlers on the manual lens color the image differently? That is the only "new" variable in the equation.

THanks all!
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Old February 22nd, 2005, 07:53 AM   #12
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Tweaking the individual color gains could have something to do with it I suppose. The tweaks you describe, boosting the red and suppressing the blue would push everything towards red. Blue would go to purple. Adjusting the red and blue does certainly effect the color balance. Neutral density filters are exactly that: neutral. If they do introduce a color cast it should be a very very minor one. The iris should not have an effect on color balance either but if you have tweaked color gains far enough it may be possible for one channel to saturate before the others do and this can cause a color shift.

I guess if I were in your position I would try restoring the camera to the factory presets, put it in manual, set gain to 0 dB and do some experiments moving from sunlight to shadow with various color balance settings including ones obtained with the coffee filter method I mentioned earlier in this thread. You might want to get a Macbeth Color Checker and shoot that in order to have a reference to refer to as you look at the video. On Mac computers there is a digital color meter that you can use to actually measure the color of pixels on your screen. You can use that to compare the on screen color of the gray patches on the Macbeth card under different ligthing and color balance conditions. I don't know if PCs have such a utility but as they usually eventually imitate the Mac's capabilities I would expect they do (sorry - know I shouldn't but it's a compulsion). It seems to me you need to reassure yourself that things are normal with your camera and this is my guess as the quickest way to get there.
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