Color Cast Removal - Photoshop Tutorial

[Deval]

Hi everyone. Happy new year. I've learned so much from this forum since I've joined, I thought I'd share something if people are interested. I ran into a foolproof quick technique to remove color casts from any image in Photoshop using a fill gray layer and thresholds. I know a lot of tutorials are out there, but this is a mathematical foolproof way without the need for a real life grey card. I've tested it with Kodak Ektar 100, Portra, Fuji Velvia 100, digital. It should theoretically work with any type of file with a color cast. I've tested this technique versus "eyeballing it" in curves or mathematical curve adjustment...
I would like to put a video together to as a video is worth 10000 words, but I thought I'll start with this.

Step 1)Open the color image file.

Step 2)Step 2, Create a new layer(Shift-Command-N)

Step 3)Fill the layer with 50% gray(Shift-F5). Select 50% gray.

Step 4)Click on the gray layer and change the overlay method from normal to difference

Step 5)Select the gray layer(if not already selected) and create an adjustment layer for threshold. The image should look black and white.

Step 6)Drag the threshold slider all the way to the left until the image is completely white. Then slowly drag the slider to the right until the first black pixels start showing up on the image. These pixels should technically be the first 50% gray pixels. Click the zoom button and zoom until you can find individual black pictures. Click on the eye dropper tool and hover over any one of those black pixels and Shift-Click on the pixel. This will identify the 50% gray pixel of choice with a number(starting with one)

Step 7)Turn the Visibility of the gray and threshold adjustment layer off(eyeball button to the left of the image).



Step 8)Zoom into the saved image point of the 50% gray pixel you created in step 6. Select the image from layer and make a curves adjustment layer. On the left side of the curves are three eye droppers(Black point, gray point, and white point from top to bottom). Select the gray point eye dropper and click on the pixel that you put a target on step 6. That will get rid of the major color cast. If you are happy with the image, you can stop here. If you want, try step 10.



For those that already know a lot more about this than me, feel free to correct or comment. If this was helpful let me know as well. Thanks again to the forum as a whole for teaching me so much about the art of large format. If you have localized color casts you can use lumosity masks with this technique. You can also use threshold to find the black and white point from the original image to add points to select to from curves correction.

[Peter Gomena]

I learned a similar technique that is a bit faster but not foolproof. Create a duplicate layer of your background, go to Filter>Blur>Average. Create a curves adjustment layer and apply the gray dropper to the blurred image. Turn off or discard the blurred image. Use the layer sliders to fine tune. I find this technique most useful for pictures with consistent lighting over the whole frame that need a quick, minor global color correction.

[Preston]

Deval,

I just tried this procedure, and it does work, even though it's a bit involved. It does appear to be very precise. Good job on your write-up, Deval. I do have a couple of suggestions, though.

In Step 4: Change "overlay method" to "Blending Mode"
In Step 7: Better wording would be, "Turn the Visibility of the gray and threshold adjustment layer off (eyeball button to the left of the layer thumbnail image)".
In Step 8, you refer to "step 10". What is 'step 10'?

I have used Peter's 'blur average' method quite a bit. Like he says, it isn't fool proof. If an image is (say) predominately blue, then using the method he outlined will result in an image that will be too yellow. If that's the case, one can then adjust the blue channel in the curves layer to add some of the blue back in, use a Color Balance adjustment layer, or one can try Deval's method.

--P

[ThePhilosopher]

Here's a technique I use often: https://www.youtube.com/watch?v=09XC9WGTLyc

[Pali K]

OK that was pretty awesome! I just tried this on an image that was being problematic and it did a great job bringing the colors in the right range.

Thank you for posting this!

[Old-N-Feeble]

It's been years for me now... but I always just stuck to using the curves tool and the eyedropper to help guide me. Of course, this requires a well-calibrated monitor.

[Deval]

Preston, thanks for the correction. I typed it up pretty quickly. For some reason I can't figure out how to edit my original post. I was going to write about a step to identify white and black points on the curve as well, but it is not necessary. One thing that seems to work better is if the eye dropper is changed from point selection to 5x5 average. I seem to get more consistent results. Also don't be afraid to try different "gray points" to see if one works better than the other.

[djdister]

That sure sounds like a lot of work. All I do is open the image in Camera Raw (doesn't have to be a raw file) and use the White Balance tool on a neutral area in the shot. Done.

[Deval]

Dan, that is a viable option as well, and certainly should be part of the basic technique set, so thanks for adding that info. Many color films(especially negative) will still carry a cast after that adjustment usually because finding the neutral area in the shot is not always easy or possible just on visible inspection. Having a gretagmacbeth or neutral card in the image is one of the best ways but not always possible. Ektar 100 for example is one of the best negative films Kodak has produced, but unlike what is written on the box, is notorious for a difficult scan; I've found this works well really well with Ektar 100(amongst others) even after the camera raw eye dropper didn't work. The technique outlined only takes 30 seconds once tried and helps identify the neutral gray pixels in the shot. I've even created a photoshop action which turns it into a 10 second technique. There are many other techniques for fixing color casts, all of them worthwhile knowing with some working better than others depending on the situation and complexity of the cast;maybe this thread would be a good place to add them to.

[djdister]

Dan, that is a viable option as well, and certainly should be part of the basic technique set, so thanks for adding that info. Many color films(especially negative) will still carry a cast after that adjustment usually because finding the neutral area in the shot is not always easy or possible just on visible inspection. Having a gretagmacbeth or neutral card in the image is one of the best ways but not always possible. Ektar 100 for example is one of the best negative films Kodak has produced, but unlike what is written on the box, is notorious for a difficult scan; I've found this works well really well with Ektar 100(amongst others) even after the camera raw eye dropper didn't work. The technique outlined only takes 30 seconds once tried and helps identify the neutral gray pixels in the shot. I've even created a photoshop action which turns it into a 10 second technique. There are many other techniques for fixing color casts, all of them worthwhile knowing with some working better than others depending on the situation and complexity of the cast;maybe this thread would be a good place to add them to.

True, using the White Balance tool in Camera Raw doesn't work 100% of the time. Sometimes it skews the overall color balance in a particular direction, even while the object that was selected is made "neutral gray." And sometimes I can't find something that should truly be "neutral gray" in the shot, so that approach won't work. As they say, your mileage may vary...