Colour Cast

[Stu Bowie]

Apart from actually noticing a colour cast in an image with your eye, is there a action/funtion in photoshop that one can check to see if the image has a colour cast. Futhermore, can this funtion show which colour it is. Looking forward from all you experts out there. Stuart.

[Fabs Forns]

Hi Stuart,

You can make a duplicate of your image and use Filter>Blur>Average. That would give you the color cast very clearly. Go to Color Balance and add the opposite color.
Hope this helps!

[Alfred Forns]

..... or get iCorrect Edit Lab 5.5 and the cast goes away with one click :) The plug in has excellent tutorials !! Nothing you can't do on your own in PS just faster and more convenient !!!

[Kerry Perkins]

Stuart, would add to Fabs' excellent suggestion that after you do the "average" of the colors it helps to have a reference to compare that against. A while back I posted a little jpeg file that has ten levels of gray for reference. It is very small and can be kept open as a floating window while you are working on your images. When you use the averaging technique this can be used to show the color cast very easily. Since these gray bars contain equal amounts of each primary color, they are by definition neutral. If your monitor is calibrated properly there will be no color component to this reference file.

You can download it here - http://www.birdphotographers.net/forums/showthread.php?t=35303

[Dave Phillips]

after completing Fabs' avg blur suggestion, simply use the color picker and see if you have
a neutral density gray......RGB, all equal values

[Charles Glatzer]

Here is another method-

Use the midtone eye-dropper in Curves/Level Adjustment layer, clicking on what should be a neutral area within the image to negate any overall color cast.

Chas

[David Thomasson]

When you know that some part of an image should be a neutral tone -- white, any neutral gray, even black -- bring up the
Info palette in Photoshop and hold the cursor over that spot and read the RGB values. If the area is in fact neutral, the RGB values
will be pretty close to equal. If not, you've got a color cast.

A white shirt collar should be neutral. Here it clearly isn't. The RGB values tell you there's a red/orange cast.

http://img15.imageshack.us/img15/2666/colorcast.jpg

[David Thomasson]

Here is another method-

Use the midtone eye-dropper in Curves/Level Adjustment layer, clicking on what should be a neutral area within the image to negate any overall color cast.

Chas

The gray dropper shouldn't be used on just any neutral area, but only those that are supposed to be 50% gray (128.128.128) or close to it.

Also, the gray dropper won't correct an overall cast. The gray dropper corrects a cast in the midtones by moving the RGB curves at a point near the middle (near 128.128.128); consequently, it has practically no effect on color casts in shadows and highlights near the endpoints of the curve.

[Stu Bowie]

Thank you all for your explaination regarding the colour cast.

David,

The diagram you showed with the RGB, the red (243)and green values ( 236) were closer to each other, and the blue was 228. You then said due to the values, there was a red/orange cast. Please would you kindly explain your deduction.

Does one hold the dropper on either a white or black area and then look at the RGB values. Is there any meaning of the values across from that, the CMYK?

Hopefully I will get my head around your explaination.

Regards,

Stuart.

[David Thomasson]

Thank you all for your explaination regarding the colour cast.

David,

The diagram you showed with the RGB, the red (243)and green values ( 236) were closer to each other, and the blue was 228. You then said due to the values, there was a red/orange cast. Please would you kindly explain your deduction.

Hi Stuart,

In that particular case, red is the highest value, so that means a slight red cast. Blue is the lowest, and since yellow is the complement of blue, that means a slight yellow cast combining with the red to create a red/orange cast. It's not a very bad cast, but there's enough of it that if the RGB values are neutralized, the difference will be noticeable. (It's a good idea to be able to say the complement pairs in your sleep: red/cyan, green/magenta, blue/yellow.)


Does one hold the dropper on either a white or black area and then look at the RGB values. Is there any meaning of the values across from that, the CMYK?When the Info palette is open, any tool will register RGB values; doesn't have to be a dropper. Check any area that you know should be neutral; that means anything on the gray scale from white to black and everything in between. The greater the gaps between RGB values, the greater the color cast. If you see 145.117.146, you can bet your grandfather clock there's a magenta cast.

Regarding CMYK, my advice is to forget about it unless you're doing color separations for an offset press. Notice on my Info palette I changed the CMYK readout to HSB (hue, saturation, brightness). Those are often useful variables to check, especially saturation and brightness.

http://img231.imageshack.us/img231/9711/infoptv.jpg

Here (http://forums.dpreview.com/forums/read.asp?forum=1006&message=21228863) is a note I posted on another forum showing how brightness readouts can help you add pop to an image. The same goes for saturation. When a subject doesn't jump out the way you'd like, it's quite often because of either of two things: the subject and background are too close to the same brightness, or too close to the same saturation (or both). With HSB as the other readout on the Info palette, you can quickly compare values just inside and outside the boundaries of a subject. After you consciously notice these differences a few times, you won't even need the Info palette. You'll just see the brightness problem of the saturation problem. Or the color cast. ;)

[Stu Bowie]

David, thank you for the explaination. It makes sense now, and I will endevour to absorb it all.

Regards,

Stuart.