(This entry deals with gray cards and their use in digital photography. In order to get the most out of this post, you'll want to have a working understanding of white balance. If you need to brush up on white balance, read my previous entry.)
A gray card is a handy piece of paper or cardboard that serves as a reference middle point for cameras. The meter system in either a camera or editing program, recognizes the gray and understands what it should look like, despite it's surroundings. For example, yellow lighting indoors that makes the people in your photo appear like they've got a bad case of jaundice.
But maybe you don't run into that problem often. Maybe you're wondering, do I really need to use a gray card for photography?
No, you don't need one. You can work without one. I've shot with a lot of professional photographers and only a handful of them have whipped out a gray card. Most cameras have impressive auto white balance settings nowadays and what can't be perfected in camera can often be fixed in a post processing/editing program like Adobe Photoshop or Lightroom... But a gray card is a quick and reliable method of having consistent image exposure.
Your camera probably has auto white balance presets for various lighting conditions but technology will sometimes disappoint you. It won't always provide the consistency you might be expecting from photo to photo.
Gray cards can be used two ways. The first is using your internal camera system to create your own settings so that your camera recognizes the gray card neutrality and shoots every photo accordingly. Tthe second is to simply edit in a post processing program, like Adobe Photoshop or Lightroom.
Essentially how it works is when your system (camera or program) recognizes the gray card, no matter the variables of your photo, it will understand that the gray card is a neutral gray and work to tweak your white balance for you.
Gray Cards and Post Processing:
The easiest way to play with a gray card involves using a post processing program of some sort. This requires shooting in RAW format in order to customize your white balance in a "photoshop" program.
1. Set up your photo, including lighting.
2. Take a shot, including your gray card. (It sometimes helps to de-focus your camera when you take this photo, that way you're shooting the colour of the card and not the texture of it.)
3. Remove your gray card and take your photos as usual. Every time you change lighting, (for example going from being under a bridge to being in direct sunlight) go back to step one and repeat.
4. When you're importing your photos, use the gray card in your initial photo for a "sample" to set a custom white balance. Take note of the white balance settings it recommends after sampling so that you can alter all of the other photos to those specifications. Or if your program is capable of it, you should be able to highlight all of the photos you want to edit and customize the white balance all at once.
Gray Cards and Cameras AKA direct measurement:
The important thing to understand if you want to use a gray card with your camera, is that every camera is different. Each camera usually has a slightly different way of achieving the same action.
For example, on my Nikon D7000 these are the steps I would take to customize a white balance setting with direct measurement.
1. Set the exposure mode to a mode that offers WB customization, like P,S, A or M.
2. Frame your shot so that your gray card completely fills the viewfinder
3. Check the exposure and adjust your settings if need be.
4. Press the WB button while rotating the main command dial so you select PRE (Preset Manual) White Balance Setting.
5. Release the button and then press and hold it again until PRE begins to flash in the control panel and viewfinder.
6. Release the WB button and take a picture of the gray card before the PRE warning stops flashing.
7. Save your WB setting so that you can then select it from your WB options.
You can usually find step by step instructions that apply to your camera in your user manual. However, youtube has a plethora of tutorials that are just as simple. Typing in "-insert camera model here- gray card" into a search engine will probably come up with a handful of helping demonstrations.
