A very thoughtful article by David English at the Leica Camera Blog
We like to talk about the specs for cameras and lenses — this camera has a burst mode of 10 frames per second, and this lens has a maximum aperture of f/1.4 — it’s almost second nature for anyone interested in photography. But how do you talk about those aspects of photography that can’t be measured, such as the quality of light? There’s no internal meter that can evaluate a scene and give you a QL rating of 4.2 on a 5.0 scale. Yet the properties of the light can contribute as much to the image as the sensitivity of your camera or the distortion of your lens.