Steven Pinker -- How the Mind Works - Hampshire High Italian ...

246 | HOW THE MIND WORKSUnfortunately, a given amount of reflected light could have come froman infinite number of combinations of matter and lighting. One hundredunits of light could have come from coal reflecting back 10% of the lightof 1,000 candles or from snow reflecting back 90% of the light of 111candles. So there is no foolproof way to deduce an object's material fromits reflected light. The lightness analyzer must somehow factor out thelevel of illumination. This is another ill-posed problem, exactly equivalentto this one: I give you a number, you tell me which two numberswere multiplied to get it. The problem can be solved only by adding inassumptions.A camera is faced with the same task—how to render the snowballas white whether it is indoors or out. A camera's meter, which controlsthe amount of light Teaching the film, embodies two assumptions. Thefirst is that lighting is uniform: the whole scene is in sun, or in shade,or under a lightbulb. When the assumption is violated, the snapshooteris disappointed. Aunt Mimi is a muddy silhouette against theblue sky because the camera is fooled by her face being in shade whilethe sky is lit directly by the sun. The second assumption is that thescene is, on average, medium gray. If you throw together a random collectionof objects, their many colors and lightnesses will usually averageout to a medium shade of gray that reflects back 18% of the light.The camera "assumes" il is looking at an average scene and lets in justenough light to make the middle of the range of lightnesses in thescene come out as medium gray on the film. Patches that are lighterthan the middle arc rendered pale gray and white; patches that aredarker, deep gray and black. But when the assumption is wrong andthe scene does not really average out to gray, the camera is fooled. Apicture of a black cat on black velvet comes out medium gray, a pictureof a polar bear on the snow comes out medium gray, and so on. Askilled photographer analyzes how a scene differs from the averagescene and uses various tricks to compensate. A crude but effectiveone is to carry around a standard medium gray card (which reflectsback exactly 18% of the light), lean it on the subject, and aim themeter at the card. The camera's assumption about the world is nowsatisfied, and its estimate of the ambient illumination level (made bydividing the light reflecting off the card by 18%) is guaranteed to becorrect,Edwin Land, inventor of the polarizing filter and the instant PolaroidLand camera, was challenged by this problem, which is all the more vex-