I just bought gray cards.  I had a gift certificate, I needed them, and it’s probably good to buy gray cards if you’re going to be using them a lot.  They’re low-reflectivity, they’re sturdy, and they hold their color.  But it hurts me to pay money for carefully colored cardboard, and I probably wouldn’t have purchased any without a gift certificate.  But not white-balancing will hurt your photography.  What to do?  You could always hack your own… the results will almost certainly not be as good as a professional gray card, but they will be free.  This is a photochop tutorial, so here goes.

Now, I know that gray cards are usually called 18% gray cards.  And I know that they are 18% gray because, when you take a scene that is equal parts 0% black and 100% white, the 18% gray is the resulting color.  I don’t know why that is–ask a physicist.  And I don’t know enough about how printing works to do anything useful with that 18% number, so I’m going to cheat.

1. Make a new file, 2px by 2px.  May as well set the background color to be white.

2. Zoom in to the maximum extent allowed (do this automatically by double-clicking the hand tool).  You’re going to need to see what you’re doing.

3. Using the Rectangular Marquee tool, mask off the top left pixel and fill it with black.  Repeat for the bottom right pixel.

4. Select the entire image (Edit > Select All) and turn your image into a pattern (Edit > Define Pattern).  Name it whatever you’d like.

5. Create a new file, the actual dimensions you want to make your gray card.  I find that it’s helpful to switch to “inches” here from pixels… for example, 4″ x 6″

6. Select your entire new image (Edit > Select All); we’re going to fill it (Edit > Fill…) with the pattern that you defined in step 4.  Select “Pattern” from the “Use” drop-down list, and then choose your pattern from the “Custom Pattern” drop-down list. Note!  If you are using an analog monitor, the resulting image may exhibit frightening moire effects.

7. We have cleverly constructed an image that consists of equal parts 0% black and 100% white.  If only there were some way to average the two… (Filter > Blur > Gaussian Blur…) Oh look! 18% gray… see what I mean by cheating?

8. For added utility, I like to add strips of white and black at the ends of the card.

9. Our histogram shows we’re ready to go out and kick some butt!  Just make sure to print this on a printer configured for ColorSync (if you can) and on the least glossy paper you can find (important!).

Some day soon I’ll have a tutorial about what to do with a gray card.