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.
Steve.
And the amount of the subject in shade, which is what seems to me is often left out by the Sunny 16 rule. Sure the main subject may be in full sun, but it is generally agreed (I'm sure not by every single person though) that you expose negative film for the amount of shadow detail you need. To me, Sunny 16 is more a useful snapshot rule where the nuiances of shadow detail may not matter, or for transparnency film. 1/500 @ f11 is about the least exposure I ever use for Tri-x (ISO 400) in full sun, though I can picture situations where f16 would be adequate. So my rule of thumb is Sunny 11. Doesn't have the same ring to it, but it gives me exposures I like better. I actually am often likely to give more than that in many cases. And more diffusion often doesn't change that exposure for me since it often doesn't change the shadow level.
As has been pointed out, the differences between the old and new shutter speeds are small and ignorable. Thinking in half stops for aperture is perfectly adequate. For me, when in between, give move the dial in the direction of exposure rather than less, at least if using Sunny 16.
To be clear, I'm not disagreeing with anyone, just expressing my feeling about the adequacy of the Sunny 16 rule, and how that impacts the OP's question.
