Today’s Question: In Tuesday’s answer you cite a 50% gray value for the exposure metering target. I thought the calibration was for 18% gray. When did that change?
Tim’s Quick Answer: While it may sound a little confusing at first, the “50% gray” and “18% gray” here actually refer to the same shade of “middle gray”.
More Detail: In a recent edition of the Ask Tim Grey eNewsletter about metering and exposure compensation, I made reference to the fact that a camera’s exposure meter is trying to set an exposure so that the area you metered off of will appear with a brightness equivalent to 50% gray in the photo. Many photographers are of course familiar with the concept of using an “18% gray card” as the basis of exposure metering. So it is reasonable to be confused by these two different percentages.
The two shades of gray are in fact the exact same shade of middle gray, just being described in a different way.
Middle gray, or 50% gray, is perhaps the easiest concept to understand here. This is a shade of gray that is halfway between white and black. In Photoshop, for example, this is the shade of gray that would have a brightness value of 50%, or for an RGB image in the 8-bit per channel mode that would mean a value of 128 for red, 128 for green, and 128 for blue.
A standard “18% gray” gray card is also intended to be middle gray, but with a different way of describing the shade of gray. Specifically, an 18% gray card reflects 18% of the light striking the surface of the card.
To slightly complicate things further, many cameras have their exposure meters calibrated for 12% reflectance rather than 18% reflectance, which means the exposure for an 18% gray card won’t necessarily be an accurate exposure. However, if you tilt the 18% gray card away from the camera at a 45-degree angle while taking a meter reading, you’ll have about 12% reflectance and therefore an accurate metering. The result will be that the gray card would appear middle gray in the final image.
Needless to say, even simple concepts can get a little bit confusing when they are described in different ways, without it being clear that there is a difference.