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Thread: White balance drift: How do I prevent this?

  1. #1
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    Question White balance drift: How do I prevent this?

    I'm recording a series of lectures with a Canon XH-A1s. It is in the evening with about 90% incandescent light and 10% outdoor light (very few windows beginning near dusk).
    At the beginning of the 1st lecture I used auto white balance and auto gain. About 1/2 hour into the recording, it was pretty dark outside (100% incandescent).
    This light was pretty poor/dim. The resulting video is bright enough but the color started good but drifted pretty red. The 2nd lecture I used manual white balance set to K=3200.
    Gain was still set to auto. The while balance still drifted to the red, and somewhat noisy due to poor light. Shouldn't a fixed white balance prevent drifting?
    The speaker doesn't move and is under dim incandescent floods. Kind of a pain trying to fix in post. Thanks for any thoughts.

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    You appear to be suffering from the differences in light-sources...
    If the light is changing, then it is recording reality.
    I can't say I've experienced the same situation as most of my filming is in short bursts (if only to protect against long files). How long are yr Lectures?
    Are you sure there isn't a feature to force a WB during continuous filming?
    How important is it to have continuous filming . . . . if you check with the Lecturer, is it not possible to arrange to have a brief break? "Please turn to your Book, page xxx" - this brief interruption should cover Stop/Start with a quick WB.

    I'm unclear about "Gain set to Auto" - surely this the electronic equiv to the Iris? This means the camcorder can cope with changes in light-levels.
    Another thought is why is the colour a problem? You can adjust the WB in Edit "key-framing" although it might be a tad tedious. . . . as you highlighted.
    Further, you could add pale red filter(s) to raise the red-level (reduce degK) and then remove them as the room-lights prevail. I don't really like this fix - but it depends on how much the colour-shift really matters. Those artificial light are likely to create video "noise" as it will be a much lower level than daylight....as you say.

    I note that this Model was first available c.2001 - so I'm believing this will have CCD sensors - perhaps you could try a modern camcorder (OR Bridge camera?) that uses CMOS which will usually be much more sensitive than yours...? As it is a well-respected model, you might be able to sell it and put the money towards something else, although you probably want a simple fix and would prefer to keep much-treasured kit....

    However, you should make sure the "Colour" instructions are being followed correctly - and - try a simple test, using an incandescent light free from daylight - film a grey-scale which should remain consistent.... if not, then maybe there is drift in yr camcorder - is there any sign of overheating? Is the camcorder being heated by proximity to something?

    Hope that's some use.
    Last edited by vidmanners; 03-20-2014 at 04:09 AM.

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    Yes, the light does change but so little I'm surprised it is that noticeable on the video. I think the XH-A1 is ~ 2001 but I have the XH-A1s ~ 2008 but still a 3CCD. I also have a T3i which I believe is a CMOS but I have the 4 GB limit to deal with (but I do have Magic Lantern which may help). When you say 'more sensitive' do you mean to light or color?

    <<Are you sure there isn't a feature to force a WB during continuous filming?>>
    Are you saying Auto WB only adjusts on a stop/start? I do go non-stop until a tape change is needed. I may be able to work in a pause with the speaker if that would help.

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    Quote Originally Posted by Steve221 View Post
    Are you saying Auto WB only adjusts on a stop/start?
    It certainly does on my camera. Simple enough to test. Shoot outside, then move indoors into tungsten (or whatever) without pausing.

    As I'm sure you're aware the issue is that the colour temperature of the light does change over time and so your camera is simply recording what is actually happening. Our brain white does a sort of variable white balance to compensate. Bear in mind that even with clever keyframing it is unlikely you'll get the whole image at the start and the end to match. Concentrate on the important parts (normally the flesh tones) and simply accept some of the background may change colour a tad.
    Tim

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    <<Simple enough to test. Shoot outside, then move indoors into tungsten (or whatever) without pausing. >>

    Mine does seem to adjust without the need to pause. It is gradual over ~ 8-12 seconds.
    I wonder if the problem is the very low overall light that makes it hard for AWB to be accurate?

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    What is the difference between using the camera's own "white balancing" system - as opposed to performing the balancing in post, using video software?

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    Quote Originally Posted by TimAndrews View Post
    What is the difference between using the camera's own "white balancing" system - as opposed to performing the balancing in post, using video software?
    Adjusting poor WB in post is never as good as getting it right in the first place. You can fix one colour but another will still look bad or even worse.

    Auto WB can work well or can be terrible depending on the scene. Canon typically employs a shockless WB adjustment (switchable on some models) to prevent instant changes. This may be what you want as you walk about, but if you go from outside in to inside (tungsten) then it can be problematic to get right in post.

    If you're in a mix lighting setup, e.g. some daylight and some tungsten and then maybe some fluorescent (I've had this in several locations) then you either put up with it or you end up with lots of power windows spread over the scene to try to fix up what you couldn't get right on the day. Adding gels to lighting (and even entire windows) can make a big difference but it's not always practical or even possible.

    Quote Originally Posted by Steve221 View Post
    I'm recording a series of lectures with a Canon XH-A1s. It is in the evening with about 90% incandescent light and 10% outdoor light (very few windows beginning near dusk).
    At the beginning of the 1st lecture I used auto white balance and auto gain. About 1/2 hour into the recording, it was pretty dark outside (100% incandescent).
    This light was pretty poor/dim. The resulting video is bright enough but the color started good but drifted pretty red. The 2nd lecture I used manual white balance set to K=3200.
    Gain was still set to auto. The while balance still drifted to the red, and somewhat noisy due to poor light. Shouldn't a fixed white balance prevent drifting?
    The speaker doesn't move and is under dim incandescent floods. Kind of a pain trying to fix in post. Thanks for any thoughts.
    To come back to the OP, yes, setting a fixed WB should prevent drifting provided you have a constant lighting source, but you didn't. Let's say you had 90% tungsten (3200K) and 10% daylight (say 7500K in the evening) then your actually light is going to vary from maybe 4000K down to 3200K overall, so even though the camera is set to a constant 3200K the actual light temperature being captured varied.

    If you set a constant WB within a scene where the light temperature also remains constant then there would (should) be no drift.

    But, you then added yet another variable in to the mix, that of auto gain. The bad news is that adding gain to any signal will cause additional colour drift, whether you like it or not. Obviously perfect gain wouldn't, but we don't have perfect noise free gain on any camera built today, let alone 6-8 years ago. So you have the compound problem of changing light temperature within the scene being recorded plus variable colour reproduction coming from auto gain.

    How to fix this in post?

    When ever I've had changing light temperatures light this over the course of (say) 30-60 minutes I've created a second layer (a copy) of footage that is colour corrected to the final scene and then key frame the opacity between the two as needed. This should get you close to what you need with the added benefit that you can also add noise reduction (e.g. Neat video) to the second layer that is also gradually introduced as you reduce the opacity of that layer. You can then vary the position of the opacity keyframes as needed to get the effect you want and the change will be so gradual that no one will really notice it.
    Last edited by David Partington; 03-25-2014 at 10:09 PM.

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    Quote Originally Posted by David Partington View Post
    When ever I've had changing light temperatures light this over the course of (say) 30-60 minutes I've created a second layer (a copy) of footage that is colour corrected to the final scene and then key frame the opacity between the two as needed. This should get you close to what you need with the added benefit that you can also add noise reduction (e.g. Neat video) to the second layer that is also gradually introduced as you reduce the opacity of that layer. You can then vary the position of the opacity keyframes as needed to get the effect you want and the change will be so gradual that no one will really notice it.
    Useful solution, David, filed for future reference. Thanks.
    Tim

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    Quote Originally Posted by David Partington View Post
    I've created a second layer (a copy) of footage that is colour corrected to the final scene and then key frame the opacity between the two as needed. This should get you close to what you need with the added benefit that you can also add noise reduction (e.g. Neat video) to the second layer that is also gradually introduced as you reduce the opacity of that layer.
    Good tip, this worked great for me!

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