Digital Outback Photo
- Photography using Digital SLRs

 

Workflow Technique #065

Color Correction using a Gray Card


by Brad Hinkel

 
 

I sometimes sound like I obsess about color balance; in many ways, color balance is one of the first steps to creating a great image; good balance makes the colors pop, makes the color contrasts stronger, and makes the image more pleasing. Color balance can be very challenging to get right; it can be hard to see what the color cast of a scene or image is, much less to know exactly how to balance it. Good color balance takes lots of experience to see and I am still improving my ‘vision’ of color both in the real world and on the screen.

In the real world, the lighting conditions are usually less than idea for color correction; outdoor or indoor lighting is seldom close to the photographic ideal color temperatures for ‘daylight’ or ‘tungsten’. This can be resolved fairly easily by capturing a photo of a gray card in the same lighting conditions and correcting to the gray.


The basic steps

Setup and shoot your first exposure normally; I typically just set my digital camera to use its auto white balance setting as this will not change between shoots. The auto white balance for most cameras does an ok job, but it only gets the color balance close and usually does not create a great color balance. If you are shooting hand held and are worried about small changes in the auto white balance, just set the white balance to its nearest manual setting (here I would use the ‘Shade’ white balance setting). If shooting film, I just shoot with daylight balanced film.


My Original Image showing the Color Cast of the Scene
(Download this image to follow these steps)

Next, place a gray card into the scene into the questionable light (this could be the overall light of the scene, or perhaps just the shadows). I suggest the official Kodak gray card, as these are designed to be neutrally balance in a wide range of lighting conditions. Make a second exposure. These two shots don’t need to be identical (ie. Shot on a tripod), but they do need to be as close to one another as practical. Read more about possible gray cards here.


The Same Scene with a Gray Card Added
(Download this image as well)

Process your images normally, just make sure that the original image and the gray card image are both processed identically.

In Photoshop, open the image with the gray card. Select the color sampler tool (it is hidden under the eyedropper tool); this tool places sample points onto the image that allow you to see the precise values for each point; click on the gray card with the color sampler tool, this places a sample point on the gray card, the color values for this point are shown in the info palette.
Notice that the R, G, & B values of the gray card are not equal; the card is not gray in this image.


A Color Sample Point on the Gray Card

To get the color balanced, we now need to adjust the color of the image so that the three color values for the gray card are returned to neutral. I typically pick the middle value for the three color points (RGB) and adjust the other values to match it. This produces a neutral gray without changing the density much.

To adjust the gray values, create a new Levels adjustment layer (called ‘correct gray card’). Within the levels adjustment layer, we can edit the midtone values (or the gamma) for the three individual channels to obtain a neutral value for the gray card. Once I have a neutral gray card, accept the Levels adjustment.

In this case, I adjusted the Red and Green channels to match the blue channel. The image now appears to have better color balance. I typically select the middle value (here: Blue 170) as the target value for the other two channels; this ensures that each channel is changed as little as possible.


Adjusting Levels to Get the Gray Card Gray Again

This should create a correctly color balanced image, but there are many times that we want the image color corrected, but wish to maintain some of the color cast of the ambient light. This helps make the image retain some of the environmental color. For example, we often wish to remove the Blue/Cyan color cast from shadows, but not remove it completely; otherwise the image might appear unrealistic. The easiest way to reduce the color correction is to reduce the opacity of the Levels adjustment layer; often a fairly small amount helps, perhaps reducing the opacity to 80% or 90%.

To finish, we just need to move the Levels adjustment layer that contains the correct color balance over to the image without the gray card. Open the image without the gray card in Photoshop, return to the gray card image, select the Levels adjustment layer(‘correct gray card’) and duplicated it use the Layer/Duplicate Layer. In the Duplicate Layer Dialog, chance the document to target the image without the gray card; this will duplicate the Levels adjustment layer onto the new image.


Copying the Adjustment Layer over to the Original Image




The Final Image

What about the Camera’s Custom White Balance

Many digital cameras include a custom white balance setting that does essentially the same adjustment as I do by shooting a gray card, so why not just use the cameras settings. If your camera does have this setting and you are comfortable with it, go ahead and keep using it. I still prefer to use shoot a gray card for a few reasons: First, the gray card works with any camera, film or digital. Second, if I don’t adjust the custom white balance, I don’t run the risk of shooting in a different light but forgetting to change the custom white balance; shooting with auto white balance gives me ok color balance most of the time. Third, shooting a gray card allows me to color correct for one part of the image with mixed lighting, for example, I can correct the shadows of an image by making an image with the gray card in the shadows and then correcting only the shadows in Photoshop. Finally, shooting with a gray card allows me to have the original image with the original color cast found in the real scene, the gray card allows me to correct for this color cast, but I have the option to only correct it most of the way towards neutral and retain some of the original color cast. For many images this retains the best overall look.


Advanced Technique (for Digital SLR Images)

I have been informed that the gamma correction used in the basic levels adjustment will product inaccurate color correction in the highlights for images captured with most digital SLR cameras. This is due to the non-linear response of the sensors in these cameras. The advantage to using the color correction using curves rather than levels is subtle but still noticeable.

The following steps use a more accurate curve adjustment instead of the levels gamma adjustment.

To adjust the gray values, create a new Curves adjustment layer (called ‘correct gray card’). Within the Curves adjustment layer, we can add a midtone adjustment point for two of the three individual channels to obtain a neutral value for the gray card. Select the first channel to edit, add a midtone adjustment point and move it up or down to match the value of the midtone for this channel to the target value.


Gamma Correction in Curves to Correct Midtones

To get a good overall color balance, the highlights will need to be shifted as well. Bring the highlight point down (or up/left) slightly so that the upper part of the curve is parallel to the center line of the curve; you may need to adjust the midtone adjustment slightly to keep the target value correct.


Highlight Correction in Curves

Change to the second channel that you wish to adjust and repeat. Once all of the channels have the same value at the target point then the gray card will be neutral. Hit OK to accept the Curves adjustment layer. Now the image appears to be more neutral overall.

(Thanks to Steve White at Microsoft for pointing out the problem with gamma corrections and digital cameras).

 
 
 

   

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