Quote Originally Posted by benjiboy View Post
+1, The human eyes and the brain are very poor instruments to evaluate light because they react too quickly and imperceptibly to changes of light intensity for us to notice them.
I agree that the human eye cannot detect actual light levels due to the 'automatic exposure' mechanism in the eye. However, we can judge contrast very well by the distinctiveness of shadows.

As the sun is a constant, the only variable during daylight hours is the amount of diffusion given by the clouds and this can be judged very well by eye.