Quote Originally Posted by David Lyga View Post
Well, the amount of change in intensity might well be off.

Actually this discussion might be at least somewhat analogous to a situation whereby flash exposure depends upon the reflectance of surroundings. Take, for example, the following: you wish to photograph a grey scale that is exactly 10 feet from the camera. The environment is in complete darkness. With flash on manual, you judge the proper aperture and fire. Now, if you are in a small room that has white walls and white ceiling, you are going to get a different rendition of the grey scale than if you were outside, at night in an open field with nothing to reflect upon. The grey scale in that outdoor instance would be considerably underexposed.

The following attachment shows part of page 416 of my (desk edition) FOCAL Encyclopedia of Photography under the category of 'exposure'. - David Lyga
Hi David,

Yes, this is fairly self-evident. One factor that you haven't touched on is that the spectral composition of sunlight is also influenced by Rayleigh scattering of light, and the frequency affected by scattering depends on the particle size of the matter doing the scattering. The blue colour of the open sky is caused by light scattering by oxygen and nitrogen molecules. We are actually seeing the light scattered away from the direction it is moving in. In early or late hours, the distance the light travels through air is sufficient to deplete the blue spectrum enough to turn the light substantially yellow, hence the "golden hour". Golden hour is not quite relevant to your discussion, but it is worth noting that latitude has a marked effect on spectral composition for the same reason. Smoke, dust and mist in the higher layers of the atmosphere may have similar effects on spectral composition, and may vary due to changes in weather, volcanic erruptions, seasons etc. All things considered, "Sunny-16" is a guideline with a narrow set of parameters, and probably never was intended as a rule or a law.