Do you have a gray card? A piece of foldable card that has a gray tone which is called 18% gray? If you do, there is a chance you have used it to set the white balance. But that is not where the 18% gray card is meant for.
A gray card is also called 18% gray. I have one in my bag, a foldable card from Lastolite. If white balance became critical, I used this card to have a reference point. I remember I tried to use it with the Lee Big Stopper also, to get rid of the infamous blue color cast. For that I took a second image with the card in view, so I could correct the color cast with a single click of a button in Lightroom.
However, did you know the 18% gray card is not meant for white balance? In reality it is a calibration value for the built-in light meter.
The Real 18% Gray
In reality 18% gray is a gray tone that reflects 18% of the light that falls on it. When you look at a gray scale, this gray tone is located exactly in the middle between black and white. From that point of view, this gray tone should rather be called 50% black.
Confused? You shouldn’t be. It becomes very clear if you look at the gray scale I made for you. The gray tone that reflects 18% of the light that falls on it, is exactly in the middle.
The Reference for a Light Meter Inside Your Camera
The light meter that is built into your camera is called a reflection meter. This meter will measure the amount of light that is reflected from your subject. It is not measuring the amount of light that is present. This is important to realize, because it tells us why the light meter will give a wrong value in some cases.
The reflection meter inside the camera is calibrated for 18% light reflection. It looks at the amount of light that is reflected from our subject, calculates the average and expect this to be 18%. In most cases this is very close to the reality, but not always.
What If the Reality Isn’t 18%
Not every object will reflect 18% of the light. Dark objects will reflect less light, while bright objects reflect more than 18% of the light. If you have mainly one of these objects in the frame, the average amount of light will deviate, and the light meter will give the wrong value.
If you have ever photographed in the snow, you know how wrong a light meter can read the scenery. Although the snow reflects a lot more light than 18%, the light meter will assume it is 18% and set the exposure accordingly. The result is an image which is one or two stops too dark.
This also happens when a lot of dark objects are in the frame. In that case the light meter will make the exposure too long. If you would look at both situations, the light meter tries to equalize the exposure, making it as close as possible to 18% gray.
A Test to See How the Light Meter Works
In normal situations we don’t think of it too much. We compensate the exposure according to the first photo, or the histogram that is visible on the LCD screen. It might even be difficult to see the effect of a lot of dark or light subjects, because the subject is almost never complete white, or completely dark.
Let’s do a test: photograph a white sheet and a dark sheet, with aperture priority in a controlled light environment. We’ll keep aperture at f/11 with ISO 200 and let the shutter speed be determined by the light meter. The results might surprise you.
When you look at the results, both images look very similar. The shutter speed is the only indication which photo is from the white sheet, and which is from the black sheet. Take a look at the histogram, and you see how the pixels are in the middle of the histogram, the location of the 18% gray tone.
How to Correct the Light Meter From a Wrong Exposure
The reason why this experiment has this result, is because of the light meter assumes it is 18% reflected light. In reality it is quite different. Black reflects much less light. White reflects a lot. How can we tell the difference if the image shows only gray?
If we use a subject that is reflecting exactly 18% of the light, the light meter will measure a correct exposure value. And yes, for that we must use the 18% gray card. If we have determined the correct exposure, we can use it for every photo we take, as long as the light remains exactly the same.
The Real Reason to Use a Gray Card
As you can see, the 18% gray card is meant to calibrate the light meter inside your camera. As a matter of fact, it is a sort of substitute for a hand held light meter, that measures the light that falls on the subject itself.
If you use the gray card for calibrating the white balance, just continue to do so. It works in most situations. But if you want to do it correctly, use a card that is made for white balance calibration. Often it is a white piece of card or, in my case, the back of the Lastolite foldable gray card. You can also use an Expo disk for that.
Just remember, the 18% gray card is not for white balance, but for a calibrated measurement of the exact amount of light.
Did you know the 18% gray card was not meant for white balance calibration? Or have you never used something to calibrate light or white balance? Please leave your experience in a comment below.