This room is locked. A card with a number on its back is kept here. The front desk keeps the keys.

rey cards are the quietest object in photography. A standard one reflects about 18 percent of the light that falls on it. You hold it in the scene, take a light reading from the card instead of the subject, and the exposure comes out right for the middle of the tones.

Why 18. Brightness as we see it is not proportional to light. The CIE 1976 lightness scale, L*, runs from 0 for black to 100 for white and grows roughly as the cube root of luminance. Worked backwards, a lightness of 50, halfway, needs a relative luminance of about 0.184. So a card that reflects about 18 percent looks halfway. Through the sRGB curve, 0.184 comes out at about 119 of 255, hence the swatch.

Why a card. A reflected-light meter does not know whether it is pointed at snow or coal. It assumes it is looking at a middle tone and sets the exposure for that. Point it at the card and the assumption is true. Ansel Adams's zone system calls this tone Zone V.

Thresholds. The same grey turns up in the lab, where a faint mark on a mid-grey field is the classic thing to detect. The test patterns page has the screen version, and the measurement page asks what changes when someone looks.

In the tour, a card in the dark room has 0.18 on its back. That is the tour's story. The numbers above are not.