Comments on: Precise Color Cast Correction With Gray Fill Layers

By: IainC

IainC — Tue, 10 Jan 2012 19:40:29 +0000

As some posters have observed, the grey point determination using a 50% neutral grey layer set to Difference Blend Mode, doesn’t seem to make sense – although it may produce an image that is judged as ‘better’. Even so, this method is replicated on many websites.

To investigate this, here is what I did: I photographed a Grey Card in RAW (so that I could alter the WB & Exposure on my pc), then I processed the image, in Photoshop, through Camera Raw. I set the WB dropper (in Camera Raw) on the grey image and then opened the image in PShop; the grey read correctly (at 128/128/128), and a neutral grey layer in Difference Mode turned the grey to black (as it should). So far so good. I then went back to the original RAW file in Camera Raw and change the WB slider to some other color temperature to give the image a color cast. Opening this in PShop with the 50% neutral grey layer method, the color of the grey card darkened, but it did not turn to black (because the underlying color was not 50% grey – it was tinted (i.e. the RGB values were unbalanced)). Further, I then repeated the Camera Raw process, but, this time, after clicking the WB dropper on the grey image, I changed the Exposure before PShoping with the 50% layer: again it didn’t produce black pixels.

My conclusion: unless I know a neutral grey object in the scene (or have photographed it with a grey card or a Macbeth ColorChecker present), the 50%-neutral-grey-layer-difference-blend-method is at the mercy of any color cast or how the image is exposed and won’t actually work.

There are different methods to remove/alter color casts. I seriously recommend Katrin Eismann’s ‘Restoration and Retouching’. She doesn’t mention this 50% Neutral Layer trick.

By: Ohno

Ohno — Thu, 01 Jul 2010 11:30:22 +0000

I dont get it.
You find a pixel that is grey – it has no color cast.
Then you define it as beeing grey.
Why should this step make any difference to the picture?

What you wanted to do is tell the program what should have been grey, not what is already grey.
Unless grey is always grey no matter what color cast you have?But how do the program then know the difference from one picture to the next?

By: SMiGL

SMiGL — Mon, 21 Dec 2009 12:58:12 +0000

Great post. Thanks!

By: Landscape Prints

Landscape Prints — Tue, 07 Jul 2009 02:46:55 +0000

Thanks, your article is a fascinating read on colour correcting those annoying colour casts. Perhaps taking a pic of a colour chart before the other pic would help to take eyedropper readings with true white/black and neutral grey in the chart?

By: pixel hunter

pixel hunter — Thu, 11 Jun 2009 21:34:16 +0000

Think it thru. If you find a grey pixel and click on it, it is set to whatever the middle eyedropper is set to (default or custom setting).

Now, the problem with this method is that you find a pixel that is CURRENTLY grey. This may or may not be the pixel that SHOULD be grey. What needs to be done is to find a pixel that you know should be grey

By: PMLPhoto

PMLPhoto — Tue, 09 Jun 2009 23:09:57 +0000

@guarana Not sure I quite understand – do you fade the opacity if the levels layer after clicking on (and then deleting) the blured copy of the image? Once you move to doing it by eye though (also carlos garcia’s comment), there are plenty of ways, the ideal is to find an automated way though. So far, only shooting a grey card seems to do that.

@daniel You can try layer masks to selectively apply corrections, but Ive never been happy with the results. Better to embrace the colour shifts and add some geled lighting to get the final result you want.

By: guarana

guarana — Tue, 09 Jun 2009 20:45:57 +0000

> PMLPHOTO: I agree, there’s no fail-safe approach, though the techniques mentionned are giving me very good results (even with heavy color cast). Even with a blue sky if you play with the correction opacity (either one, match color or levels adjustment) it works good enough for me

By: Carlos García Illarxe

Carlos García Illarxe — Tue, 09 Jun 2009 19:24:15 +0000

I’m using the command “Layers” but using the red, green and blue histograms. First duplicate the layer and then move the controls of each of the layers. At last, you can darken or brighten the final image with the middle control.

By: Daniel

Daniel — Tue, 09 Jun 2009 17:59:16 +0000

Can just agree with everyone that question the soundness of this guide. The reason why it works most of the time is that most of the time you “know” where you have neutral grey and click there.

But the $64,000 question is rather, how to white balance an image which has multiple light sources with different color temperature (e.g. incandescent or fluorescent + window light)!

By: PMLPhoto

PMLPhoto — Tue, 09 Jun 2009 08:50:03 +0000

@guarana Does this appraoch only work if the image is reasonably neutral in colour overall. Say it was an image that was dominated by blue (blue sky and blue sea with maybe a model), then the blured copy would be pretty blue and the levels adjustment would adjust the blue to be a neutral grey which isn’t the right answer I think

I’ve pretty much come around to the idea that ther are no fail-safe shortcuts for this, and the only way to get it right is to do it right and shoot a grey card.