GH2 White balance

Posted on 16 November 2012

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White balance

Red channel Cr problems are reported with a lot of high frequency noise in the red channel (Cr). This is a different problem then the lower Chroma resolution due to the 4:2:0 sampling and the CUE bug. The big reason is that it seems like 3200K is the native color temperature of the sensor, so in 5600K light the red channel will be digitally gained to match the green and blue channels.

So digging a little bit more into white balance, the first question is: Why not do it in post? You could set the camera to 3200k and then amplify the reds in post. The reason is that the recording is done in 8 bits only, but mostly because it is recorded in YCC color space: Cb and Cr are not the same as Red and Blue: amplifying reds later will also touch the greens, yellows, magenta etc. You will find that it is very difficult (if not impossible) to get skin colors right in post (it will end up with a mix of too green, yellow or magenta). That’s why you want it done in camera, before converting to YCC color space. There is a good explanation of white balance and RGB channels at Silicon Imaging.

The exception to this rule is low-light situations: if you are in low light daylight it might be better to keep the camera around 3200 K as you might very likely not see the difference and this will result in a better recording with less noise. For normal light situations there have been tests with 85 filters but I don’t know yet.

Hot vs cold

So how to remember it: if colors are shifted towards red -> the colors are recorded too ‘hot’ so set camera to a lower color temp. This results in less red amplification (as the GH2’s native color temp is low) cq more blue attenuation  (for most other camera’s as the native color temp is high). This is the opposite of the cultural associations attributed to colors, in which “red” is “hot”, and “blue” is “cold”:

  • film in sunlight (about 5600k) while camera on studiolight (about 3200k) -> blue picture.
  • film in studio (about 3200k) with camera on daylight setting (5600k) -> yellow/orange/red picture.

Setting white balance

There are a few preset white balance settings, like tungsten (2900k ?), sunlight (5600k), shadow (7100k), cloudy (8100k) and bright sky (9800k). Why not use auto white balance (AWB)? Because this might deliver two shots that are in the same room but have different white balances. So is there a ‘hold auto white balance’ button? No. But there is the nice manual white balance (white set) option to tell the camera that ‘this object is white’ by just pointing at a grey card and pressing the menu button. You can setup four (!) presets this way. Just make sure you don’t over-expose your white card as this will lead to a green/yellow cast.

After that you can also make small corrections by hitting adjust and make fine-adjusting it to more amber/blue and/or green/magenta. This could be very useful to correct for the color casts of some film modes. I just don’t really understand why it’s not just a blue/red slider?

To adjust multiple camera’s to the same color temperature the only option is using the ‘set color temperature manually’ option. You can first do a manual white balance and as it’s next to the ‘manual color temp’ you can easily try to get it as close as possible. I found that you will also need the ‘adjust’ menu to get it right (green +3), which means that the manual white balance has a much finer scale then the manual color temp option. It’s a bit of a hassle but works ok.

Color shift

The only downside is that you can’t readout the value of your manual white balance to check if it makes sense. The thing you can do though is make a short recording and then hit the play button. By pressing display you can see the RGB histogram and check if the levels are o.k. The other good reason to do this is that there is a serious color shift when you hit record. This is sensor mode, but it might have something to do with live-view mode being full-swing and AVCHD recording being studio swing. Here and here are nice read-ups on TV safe colors.

What I think is happening is that live-view sends RGB (without gamma) directly to screen from sensor and that recording sends the preview through YCbCr system, but without properly expanding it, showing the super-whites and super-blacks. Gradient banding appears now because of the resampling to the YCC format, which means less bits for the same information. Is that right? Question remains: because the HDMI out is also showing this gamma shift, is it RGB?

White balance limitations

Auto white balance is limited to 2800-8000K and setting the color temperature manually is limited to 2500-10000K. So to set it lower then 2500K you will have to use the manual white balance or select either the candle-light (1500K) or sunset (2300K) preset. If 10000K is not heigh enough you could also try the manual white balance as this might get up to 11.000k.

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Posted in: GH2