EM-5: Confused by Auto White-Balance results

Started Feb 12, 2013 | Discussions thread
micksh6
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Re: EM-5: Confused by Auto White-Balance results
In reply to Timur Born, Feb 12, 2013

Timur Born wrote:

assaft wrote:

Very noticeable difference between the shots. I don't recall seeing such cases with my E-PL2. I guess there isn't much you can do since you have no access to the algorithm.

One thing I am asking myself is how much other camera than the E-M5 would differ in these scenes or if there is something about the scenes/examples that leads to just the same expected outcome. I am struggling with flash and flash/mixed lighting anyway, but the first example was purely lit via daylight coming through the window.

There can't be pure daylight indoors, unless all objects inside completely lack colors. Part of the light is always reflected.

Another thing that may explain differences in the first shots - color response curves may vary for different ISO sensitivity. The second shot was made at ISO12800. At that ISO dynamic range and color depth are too low. Simply speaking, there aren't many colors left to choose from.

It would be interesting to see what a grey card sample would tell about these scenes. You can capture it from/with different angels/lenses/iso+aperture combinations to see if at least the light reflected from the card has changed and to what extent. It's a simulation of course, just in an attempt to rule out the effect of some parameters.

I hoped that someone else would already have gathered that intel about the E-M5's (+lens) WB behavior. Especially since I already analyzed so many aspects of this camera and shared the information that I thought this information might be available from others for a change.

I don't have intel, just some thoughts, which, I think, are logical,

When shooting in artificial light there are always two choices for AutoWB - (1) to adjust WB on photo so that skin tones looked as they were in natural light and (2) - to preserve light spectrum so photo would show what the light looked like in reality. Often there is no ideal WB setting, especially if your light has some color cast (and it always will have color cast because walls are rarely perfectly gray and the light is partially reflected from walls).
Camera may try to choose (1) but it may be too hard job because it doesn't know the exact light spectrum.

Now, you were saying that your halogen lights are rated at 2900-3000K. It's important to realize that this may not be the case after the lights were mounted in your particular room. As I tried to explain in my previous post the light spectrum will change after it's reflected from walls, ceiling, clothes and other objects that aren't neutral. The light mix will change after you change camera position resulting different WB value. This should explain inconsistency in Auto WB results.

Grey card may help getting better starting point but it's not always a panacea. If you take first shot with grey card and second shot without it, proportion between reflected and direct light may change because of different camera angle and this will affect WB. If you put gray card at the frame edge intending to crop it, the edge may be lit differently from your subject, again resulting different WB.

There is a benefit in having people wearing clothes with some white piece of fabric - you can use it as reference point. And even slightly pink dress may affect illumination of the face, making the job of AutoWB harder.

AutoWB works well in natural light, especially when you have all 3 RGB colors in the frame as a reference. But, it's hard to expect good results from AutoWB when light spectrum is shifted and you have one or two colors dominating the scene.

As an example, I think purple baby bath tub in your second two shots does a good job of confusing the camera. Decent amount of purple-colored light falls on the sides of the face, making it lit differently from face front. But, there is only one WB value applicable to whole scene. Camera tried to find compromise but it might be too hard job to succeed.

With that said, I think Auto WB works well on E-M5 and Pens. It is not perfect in difficult conditions, of course, but at least it mostly provides reasonable starting point. Compare E-M5 with Lightroom AutoWB. E-M5 in-camera AWB is almost always much closer to reality despite the fact that PC has more resources and time to estimate WB.

Another thing - E-M5 has much greater color depth than earlier Pens or Fuji X10. Because of that it will make slight WB errors more apparent.

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