We’ve been intrigued of late by two questions: (1) What is the brightness (reflectance) of an average scene? and (2) How many scenes qualify as average? The obvious answer to the first question is that average scenes are 18% reflective—after all, Kodak’s ubiquitous 18% gray card has served as proxy subject matter for decades. But the card has an odd feature: Sometimes the instructions that accompany it tell a photographer to use camera settings directly as read from the card. Sometimes, however, they say to note those settings, then open up half a stop. That curious proviso, when it is included in the instructions, suggests the 18% gray card is not dark enough. If it were a half-stop darker, which would make it 13% gray, there would be no need to open up that half stop; the reading could be used directly. So perhaps an average scene is only 13% reflective, not 18%? If you Google “average scene reflectance,” you are rapidly immersed in a debate about this very question.
To satisfy our curiosity we turned to Photoshop. Every pixel in an 8-bit digital image has a brightness value (BV) between 0 and 255 assigned to it. Photoshop reports the average BV for all the pixels in an image under the heading “Mean” on the expanded view of its histogram palette. We opened 150 digital photos we had taken outdoors in the Galapagos Islands and noted the average BV for each image. Brighter, more reflective scenes give rise to larger average brightness values and vice- versa—but how can one convert between BV in the image and reflectance (R) in the scene?