Metering with an 18% Gray Card is a method a lot of people do not use now a days, but one that can help you understand how your camera’s light meter works.
Your camera’s light meter is calibrated to measure the light in the camera and produce a medium gray tone (18% gray), the same tone of that of a Gray Card.
An 18% Gray Card represents the value your camera’s light meter is calibrated to.
Knowing that your camera is calibrated to Middle Gray helps when you are shooting scenes that are lighter than Middle Gray like Snowy Landscapes and Beach Scenes. The Autoexposure system in your camera doesnt realize that the scene is too bright and will produce and image that is too dark.
This means that if you have an object against a white background your camera will under expose the image. Your camera is taking a light reading of everything is in the viewframe and comparing it against the 18% Gray it is calibrated to and under exposes the entire shot.
The images below were shot against a white background, on the left the image was metered by the camera, on the right, the image was metered using a Gray Card.
To do this it is best to set your camera on Manual Mode so that you keep your aperture and exposure settings. Hold the Gray Card so that it is parallel to the front surface of your lens and close to the subject. Next make sure the entire viewfinder is taken up by the Gray Card and meter accordingly. Step back and take the shot of the subject.
If you do not have a Gray Card, another way you can do this is to just walk up or zoom into the object that you want to meter, take your light reading, zoom or walk out and then take the shot.
By closing in and filling your viewfinder with only the object you want to meter, you expose correctly just for that object.
Then zoom out and take the shot, and your exposure should be correct.
If you want to use your Histogram to correctly expose the Gray Card, take a shot of your Gray Card and make sure that the bulk of the Histogram, or the Histogram Peak is in the middle of your Histogram. This way ensures that your camera is reading the light source ase Middle Gray.
Below is another example of how you can use close up metering out in the field.
Against the bright sky the man is under exposed. Zoom in on the face, meter that and zoom out.
The same is true for landscapes or cityscapes. If you meter with the sky the landscape or cityscape will tend to be underexposed. Tilt down and only include the landscape and meter. Use this in combination with a polirizing filter and your landscapes will be exposed correctly.
Below is a Black and White Gray Scale, as a rule of thumb its good to know this so you know how to expose correctly and when to override your camera’s autoexposure’s/
You can also get creative with metering and combine it with basic design elements like Repetion to create pleasing compositions like Rusell Lee’s photograph below. Notice how he purposefully turned off the light bulb in the middle of the scene and used his camera’s flash to provide flat even lighting.