Now you are asking ... "Why is he reviewing a piece of cardboard? The simple answer is that an 18% gray card is a valuable photography accessory! I'll explain.
A note for beginners ... The gray card is close to what your camera is trying to make your subject look like. The camera actually meters closely to 13% gray - meaning you need to add about 1/2 stop to an exposure setting based on a gray card. If you are photographing a light colored subject such as a wedding dress, the camera will try to make it darker - 13% gray. You need to add positive exposure compensation. If you have a dark subject (such as a black tux), the camera will attempt to make it lighter - 13% gray. You need to add negative exposure compensation. As for the rest ... Beginners might want to bookmark this page and come back later.
Our eyes are incredible creations. The specific incredibleness I'm referring to now is our eye's ability to make colors look correct under varying lighting conditions. Indoors, outdoors, shade, sun, electric lights, etc - the light sources all have different color characteristics. For example, tungsten lights often give off more red light. Our eyes simply adjust and send the right "white balance" to our brains.
Our cameras are not quite as incredible. The auto white balance feature works reasonably well in many conditions, but it works very poorly in others - especially under artificial light sources such as incandescent and fluorescent light. Canon Digital SLRs have various pre-programmed white balance modes (such as flash, sun, shade ...), but they are just approximations of the actual color of the light.
The Wallace ExpoDisk attempts to mix all of the colors of the light together through a filter to simulate a gray card. This determines which colors are present more/less strongly in the light source. The ExpoDisk should be aimed at the source of light to work best. I found the results of using the ExpoDisk to be hit or miss. The ExpoDisk works well when you have a single light source and little reflected light. The ExpoDisk shot is then used for the basis of a custom white balance.
The 18% gray card takes reflected light into consideration. The camera and the computer can determine which colors of light are being reflected more strongly by the card - and can make adjustments accordingly using a custom white balance. I find my white balance results to be excellent when using the gray card. And you could buy a few cases of gray cards for the price of one ExpoDisk!
To use a gray card, simply take a picture of it in the same lighting your subject is (or was) in. Use auto white balance and P (Program) mode for the picture. You can either use the resulting picture as the basis for a custom white balance setting, or you can use the gray card picture during post processing for a custom white balance.
To use the gray card as the basis for an in-camera custom white balance, make sure enough of the center of the frame is covered by the card in your picture to be properly read for white balance. If I'm using my 8x10 card, I usually fill the frame with the card. Next, go through the menu to select that picture for your custom white balance. Finally, set the camera to the custom white balance setting.
If you are using the gray card picture as the basis for your post processing white balance setting, the card does not have to take up much of the frame and does not need to be exactly centered. Simply select a point on the card in the gray card picture from your post processing software (such as Canon's File Viewer Utility) to be the basis for the custom white balance setting for all pictures taken under the identical lighting conditions.
gray card pictures can be saved for later use as well. If you take pictures in the same lighting at different times, you can use an old gray card shot for future photo sessions. For example, keep a gray card shot of your church sanctuary to use each time your kids are performing. Keep a shot of your high school gymnasium to use for all of your high school basketball photography. Get the picture?
While the post-processing gray card method works well, I prefer using the in-camera custom white balance method. First, post-processing work is reduced. More important is that getting the right exposure settings in-camera is easier when shooting under strong non-white lighting conditions if a custom white balance is used. For example, the lights in our church sanctuary have a very strong red color tone. If I shoot using auto white balance and use the histogram for exposure, I find that my shots are significantly under-exposed when I later color-correct the image - removing the red that pushed my histogram to the right. If I would have shot with a higher exposure, I would have blown the red highlights.
There are situations that a gray card cannot handle. If you are in an audience shooting a lighted stage, court, rink ... you cannot simply pull out your gray card to get an accurate custom white balance shot (unless the lighting at your position happens to be identical to the subject's lighting - and it rarely is).
In this situation, you have a couple of options.
I often use a K (Kelvin) white balance setting. This generally requires some trial and error to get the right temperature. I take a couple of test shots until I dial it in (using the LCD for my estimation). I like to use skin tones and hair color for this purpose. Some DSLRs have a 3-color channel histogram that can be help in this situation. Practice will help you get the right temperature setting quicker. I will then fine tune the colors in my post-processing. I will take note of the K setting I end up with (after post-processing) and record it somewhere for the next time I am shooting at the same venue.
If the venue has a true neutral color (such as a gray wrestling mat) under the subject lighting, shoot an out-of-focus picture of it. Try using that for a rough custom white balance.
Can you get permission to shoot your gray card shot on stage (court, rink ...) before or after the event? Or even days prior to the event. Can you hang over the stage (court, rink ...) far enough to get the gray card shot without getting the boot? If you can get the gray card shot one time, you can save it on a CF card and re-use it each time you are at the same venue.
The ExpoDisk might be the right solution for these situations. If you have a good view of the subject lighting's source that is.
There are some situations that become extremely difficult For example, a church with a combination of light types and natural sunlight coming through a colored window. It becomes almost impossible to get the white balance right in all parts of the picture.
In these cases, you have to make the most of what you have to work with. If you can move your subject(s), have them stand in a location that limits the lighting types illuminating them - especially their faces. You can also use a flash set bright enough to just overpower the various colors of light on your subject. Use the custom white balance of the flash in this circumstance.
I bought a couple of 8x10 gray cards. They are very inexpensive. I cut one into 4 pieces and I carry a piece in each of my camera bags - I am never without one. I use an 8x10 card around the house or when I am packing lots of gear.
For what it is worth - Sports Illustrated photographers use the gray card approach for setting their in-camera custom white balance.