One such person who suggests that the whole grey card percentage is off is the well respected Thom Hogan.
His contention is that the roots of 18% grey being 'half way between white and black' goes back to the print industry. That the printing industry determined that visually, a block printed with 18% black coverage looked half way between white and black. That's fine. We can assume that's true.
He also contends that Ansel Adams was the major mouthpiece many years ago convincing Kodak to use 18% for their grey cards instead of the hotly debated 15% and 12% grey.
what does all this mean to you? Well it depends. If you go with the flow of the world, it doesn't mean a lot. 18% will get you close to a correct exposure regardless. You'll be slightly underexposed but that's not the worst thing in the world. Of course, 18% could be the way to go and all the other talk is wrong.
I mean, can you even tell the difference between the three?
So, with that bit of uncertainty out of the way, why do you need a grey card? Your camera sensor sees in luminance. Luminance is defined as:
Optics. the quantitative measure of brightness of a light source or an illuminated surface, equal to luminous flux per unit solid angle emitted per unit projected area of the source or surface.
Whats that mean? well, everything (nearly) reflects light. Light from the sun.. Light from a flash.. Light from a street lamp.. Before the advent of color film, black and white film only measured luminance of objects. Red and Green could have the same luminance and would look identical on the black and white photo. This is similar to what your sensor does when it determines what the luminance of a scene is. It takes all the color in the scene (as long as you're using matrix metering) and figures out how to make that exposure fit the typical scene programmed into its memory. According to generally accepted thought, most scenes in nature average out to an average luminance of 18% grey. (Thom contends that Japanese engineers who design and build Nikons disagree, but that's beyond the scope of this entry)
Now, will all scenes you photograph average out to 18% grey? NO! Will the camera try to tell you that you should expose every scene to 18% grey? YES! The camera is a 'dumb device'. It can't think. It can't evaluate a scene. It doesn't even know what the subject of your photo is. Therefore, left to its own devices it will do what it is told to do. And unless you intervene, it will decide that a scene should be 18% grey and will suggest an exposure to fit that range.
This thinking by your camera sensor is a problem in a number of scenarios. One of these scenarios is snow. A scene with a large amount of snow (something that is nearly white) makes the camera think that, in general, the scene is very bright. It will suggest you either, raise the shutter speed, stop down the aperture, or lower the ISO (in digital cameras). Should you? maybe. Doing what the camera thinks you should do will cause the parts of your scene that aren't snow to be underexposed. They will be darker than they should be and the detail of those items could possibly be lost. This goes back to our discussion of dynamic range. You, the living thinking human, have to decide what is important and what is not. If you want white snow, take the exposure that makes your snow white. This will undoubtedly blow out the highlights in the snow. But, this is the trade off you have to make. If you want to show the details in the snow, you will have to underexpose the snow to make it more grey. The trade off, as we mentioned, its an unnatural darkening of the non-snow items in the scene.
The same sort of rules go for night exposure too. Lets say you wanted to shoot a photo of the night sky. If you pointed the camera up at the sky, it would see a majority black. It would determine that, for a proper exposure, you would need to open the shutter more, have a longer exposure, or raise the ISO. If you wanted to show the black of the night sky as black, the camera's thinking would be incorrect. If you figure that 10% of the scene is brightness from stars and 90% of the scene is black, then you would have to compensate for that in your exposure, despite what the camera tells you is 'right'.
So how can you tell the camera to expose a scene correctly? One way that's been used for many many years is the grey card. If you point the camera at the grey card (and fill the viewfinder with said grey card) your camera's brain will think, OK, how do i make this scene 18% grey. That's a good thing because the card IS 18% grey. Now once you get that exposure, you can use that setting to accurate render the real scene behind the grey card.
Is it really that easy? almost. The only caveat is that the grey card has to be getting the same sort of luminance from whatever light source is lighting your subject. If its a landscape photo, you will be using the sun as your light source. So the grey card needs to be in the sun getting the same exposure as the scene. If your subject is in the shade, you will need to have the grey card in the shade.
Do you point the card directly at the camera? Not really. General consensus is to split the difference on the angle between the camera and the light source. If the sun is 90 degrees to the left of your scene, and you are looking at the scene from straight on, then you would position the grey card at 45 degrees to get your reading.
Hopefully this clears up a little bit about how the camera's sensor works and now you can understand how sometimes the camera is wrong and how you, as the human operator, need to make decisions about the scene that the camera simply cant do. This is yet another reason why you need to get the camera out of automatic mode and take control! Your photos will thank you!
