Every camera lets the amount of light in that will let the average grey level of the picture equal 18%. You can buy an 18% grey card from Kodak to assist with setting exposures (I want to buy one) but more on that later.
If the subject matter is mostly dark, everything will get lighted up so the average shade is 18%. If the subject matter is mostly white everything is going to get darkened so the average grey shade is 18%.
The problem with this is that things that actually are white will not appear white. They will be a dirty grey. Black things won't appear black. They will tend towards grey.
To solve this problem some cameras (mine) have different white balance settings. Mine has white balance settings for the following situations:
Daylight
Outside shade
Cloudy, twilight, sunset
Tungsten lights
Fluorescent lights
Flash bulbs
These help, but it is not perfect.
Now for what I discovered today:
The camera doesn't know what things are actually white. You can take a picture of a pure white object in the lighting you are going to take the picture and tell the camera that is what white looks like under that light. Not only will the whites be white and the blacks be black, but all of the colors will be more accurate.
(Disclaimer: These pictures are for technical illustration purposes and are not intended to be family keepsakes.)
This is the picture I used to tell the camera what white looks like.
This picture was taken with the auto white balance mode. The camera can only guess what white is. Notice everything has a yellowish color to it. Her shirt is blue, but you can barely tell that by this picture.
This picture was taken using the custom white balance information gathered from the white napkin picture. Notice the blue shirt now looks blue. You have no way to know this because you are not here, but the colors are much more faithful in this picture than the first one, and the ugly yellow tint is gone.
Pretty neat, huh?
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