It's exactly what it sounds like, it's simply a bit of cardboard printed a special shade of grey. This special colour is known as 18% grey, since it reflects precisely 18% of the light falling on it.

Interestingly, the average scene that you will point your camera at will have the same reflectance as that bit of grey to the left (or thereabouts, it depends on your monitor settings, but it's pretty close).

Since the average scene reflects this amount of light, your in camera light meter is calibrated so that it will expose the average scene properly, that is, to the 18% grey tone.

The key is that your camera is calibrated for an average scene, not for what you may really encounter. Manufacturers like to tout cameras as having advanced metering systems, but all at a basic level rely on this 18% grey assumption.

Therefore if you point your camera at something that is substantially different from a normal tone, it will attempt to make it look like a normal tone, and improperly expose the picture.

The best example of this is a snowy field on an overcast day. Your camera isn't smart enough to know this is a bright scene, it just thinks there is a lot of light flying around, and will let in less light to make what it thinks is a proper exposure. In reality, it is underexposing. Your snow will look grey and ugly.

A snowy scene such as this is probably 1.0-2.0Ev (stops) brighter than 18% grey. However, this can get confusing very fast with automatic cameras. Automatic cameras are programmed to know that the brightest light they will see is broad daylight, also known as sunny 16 conditions (sunny 16 means in broad daylight set your aperture to 16 and shutter speed to 1/ISO). So if you come across a snowy field on a sunny day and then think you ought to be adding a couple of stops of light, your exposure will be 4Ev brighter than 18% grey and your shots will be ruined. This depends on your camera and how it is programmed.

Of course, the opposite can happen too, that is, you want to shoot a dark subject but your camera goes and makes it too bright.

Avoiding this whole fiasco is a lot easier than you might think, and fortunately for us students, a lot cheaper. You can buy professional light meters with an incident light meter built in. They measure the light hitting the scene rather than what is being reflected and therefore are not influenced by the reflectivity of the scene. But do you really want to spend hundreds of dollars when you have a perfectly good meter built into your camera? I surely hope not!

You can do the exact same thing by buying a grey card. Any decent camera store should have them, mine were $23 for one 4x5 I carry everywhere and 2 8x10's I never use. Yes, that's a lot, but it is much cheaper and more important than many other accessories. When you just need to get your exposure right you'll be glad you have one.

Using it is easy, just place it on the spot that you want properly exposed (or if you are in sunlight or overcast conditions it doesn't matter since light is uniform everywhere). Then take your meter reading off of that, make sure it fills the frame but that you are not shading it with your camera.

By pointing your camera at this average toned card, you are showing it a "scene" of average tone in the difficult light you are using. Your snow or coal pile or whatever should now be appropriately light or dark in the final picture. I find my grey card useful in all sorts of difficult lighting situations, including backlit subjects and when shooting in pools of light at night. I can make sure that everything under the streetlight is properly exposed while the rest fades to black.

For the Truly Frugal

If you are really cheap and feeling ambitious you can actually use the palm of your hand to accomplish the same thing; obviously this will depend on your skin tone. An average white person like me usually has to add about one stop to the reading off my hand. You will likely differ, trial and error it until you get it right.

Article written November 16, 2005