A gray card in photography refers to a flat object of neutral gray used as a reference object for exposure and color. If you don’t own a gray card and keep the camera’s white balance set to auto. It’s easier for most people to keep the white balance set to auto as lighting conditions change quickly when photographing outdoors with natural light. Certain types of light will tint photos is an unwanted way, giving them a color cast. This can, and should be, fixed in post processing by correcting the white balance to give an image true color.
Adjusting white balance in Adobe Photoshop
In Adobe Camera Raw and Lightroom there is a one step tool to correct white balance if there is something in the image around middle gray or a gray card. In Photoshop you can use the center eye-dropper tool on the Curves adjustment layer to correct the white balance. If isn’t any obvious gray in an image there is a way to find it.
Find the correct grayscale
The first step to finding our middle gray is to add a Solid Color layer above the background layer with a 50% gray fill.
Change the gray layer’s blend mode to Difference. This blending mode uses the difference of the base and blend pixels as the resulting blend. It does a selective inversion where black never gets inverted, white inverts absolutely, and all of the other luminance levels invert based on their brightness on a channel-by-channel basis.
Add a new Threshold adjustment layer above the gray layer. Threshold renders all the pixels as either black or white. The histogram in the box shows the images luminance levels. By moving the Threshold slider to the left until there is only a little bit of black showing in the image it reveals where the neutral gray areas are in the image.
Zoom in to the image on an area of black pixies and use the eye-dropper tool from the tool menu and while holding down the shift button mark the spot.
With that spot marked you can remove or hide both the Threshold and Solid Color gray layers. Add a Curves adjustment layer and use the middle eye-dropper and click on the marked spot. To remove the eye-dropper/picker tool marker hold down the shift tool, grab the marker and drag it off your image. Zoom back out and see the color difference.
To save time when editing a series of photos in the same light conditions that need color correction save the Curves adjustment layer by clicking on the upper right hamburger icon, choosing ‘Save Curves Preset…’ and naming the preset. This one of many tips useful for repetitive tasks in Photoshop. On edit each new image find the newly saved Curves Preset in the drop down list. Don’t forget to go back when you are done and delete it.
Some photographers like to accurately recreate the image as they saw it at the time of taking the photo. Others want accurate true color. Removing unwanted color cast by correcting the white balance is a great place to start in the editing process. From here you can push your editing process any direction you like.