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An 18% gray card is a handy accessory that every serious photographer should keep in their bag. It doesn't cost much and it barely takes up any space. If you encounter a situation where you have mixed lights, this unassuming piece of plastic helps you determine the white balance. It can also be used to determine the correct exposure.
I hear people saying, "wait, what? It does both? But my camera has a light meter, auto white balance and a bunch of other white balance settings. Do I still need it?"
Maybe. Leaning toward yes. If you’re the type that enjoys spending a lot of time post-processing or flying by the seat of your pants and eyeballing it, this is probably not for you. On the flip side, if you care about getting the white balance and exposure right, then yes, definitely.
It does have to be a precise shade of gray. There are DIY print-your-own 18% gray card, but your printers are likely not accurate enough to print an even, precise 18% gray. You can try, but your results may vary. Here’s a rundown on how an 18% gray card helps.
Camera meters are set to expose for 18% gray, this is a standard in photography. However, if you point a camera at a mostly white subject, say, snow, a white wedding dress or a giant pile of sugar, the camera meter will underexpose the image because it’s trying to make the image gray. Likewise, it does the opposite for very dark subjects. Heavy shadows, a black-haired dog, or a black backdrop: the camera will overexpose these.
To use it to establish correct exposure using the card, you’ll need to shoot in manual mode so that you maintain control over the holy trinity, aperture, shutter speed and ISO.
Put it in the same light as your subject and fill as much of the frame as you can. If your card is too small to fill the frame, as mine is, and still get the same light as your main subject, use spot metering.
Study your camera’s light meter and make the adjustments to the settings as needed to make sure it's exposed correctly. Proceed to shoot your subject using these settings sans the gray card.
While you could let your camera’s Auto White Balance determine the WB, cameras are, as a rule, not smart and can produce some weird results in mixed lights. The above image is lit by both indirect daylight from a nearby window and an incandescent bulb.
Suppose you took several photos at an event hall where they often mix orange and green florescent and halogen lights. How long would it take for you to go through all the images in an image editor to correct the WB? You could avoid this post-processing step by taking 30 seconds to do a test shot with an 18% gray card.
Setting Custom WB
Step 1. Check your manual and see if you have the option to set a custom WB. (If your camera doesn’t, skip ahead to the next section.) Read up on how to find it in your camera’s menu and how to set it.
Step 2. Get your lights set up if you’re using artificial ones like Strobes, flashes, etc.
Step 3. Find the option for custom WB. Refer to your manual if you're not sure where it is.
Step 4. Hold your 18% gray card out in front of the camera, and make sure all the lights you’re using fall on it, and hit the shutter button for your test shot. You may need to try a few times to get a good position if your card is small.
Step 5. Get back behind the camera and check the display. You may need to move the ‘box’ into the position. Your manual should have instructions on how.
Step 6. Proceed to shoot without the gray card. Keep in mind that if you change your light position, power level, add or subtract a light, it may be necessary to set a new custom WB.
That’s it. You’ve just saved yourself a step in your post-processing workflow. And your WB should be much improved, if not perfect.
Correcting White Balance in RAW
If your camera didn’t have an option to set the custom WB, you’re not missing out on too much. You will, however, need a program that can edit Raw files – Gimp, Photoshop or something similar. Here, I am using Adobe’s Camera Raw Editor that’s packaged with CS4.
Step 1. Highlight all the pictures in a batch, including the one of your 18% gray card, and make sure the image with the card is visible.
Step 2. Find the icon that looks like an eyedropper (if you hover the cursor over it, it should say "White Balance Tool") or press the letter "I."
Step 10. With the eyedropper, click a spot on the gray card. Voila! All of the images that you highlighted should now have corrected white balance.
Correcting White Balance in JPEG
If you’re not shooting RAW, there are a few different methods you can use to correct the WB.
You can open the JPEG images in Camera Raw Editor, and batch-edit the images. I recommend this if you prefer shooting in JPEG.
Open Photoshop without opening any files, go to "Edit" dropdown menu and choose Preferences -> Camera Raw. Find the "Jpeg" dropdown menu, the default is "Automatically open JPEGs with settings" and change it to "Automatically open all supported JPEGs."
You should now be able to open all JPEG images in Camera Raw. Open the JPEGs you want to edit, including the image with your 18% gray card, and follow the steps outlined earlier in this article.
There you have it. Get perfect exposure and white balance using one simple technique. Get it a try on your next shoot.