Up to now, I was referring to a clear blue sky and direct sunlit scenes. Can we use the "sunny 16" rule under conditions other than sunny?
Yes, no problem at all!
Let's assume that for this discussion, it is mid-day and we are photography a frontlit scene. Also, we don't care about the D.o.F for the time being.
In full sunlight, as the rule suggest, we are now photographing with an aperture of f/16. This is very hard light, throwing very sharp shadows. Hence, whenever you see very clear and sharp shadows, you should consider using f/16. BTW, this light would be referred to a hard light.
With a light cloudy sky, the direct sunlight is somewhat attenuated and the contribution of scattered light increases. Shadows are a bit washed out under such lighting conditions. When I say washed out, I refer to clearly visible, having blurry soft edges. To compensate for this drop in illumination, we need to open up to f/11.
When the sky is heavily overcast, but still bright, w/o any direct sunlight, the only natural source of illumination is scattered light. Scattered light comes from all direction, hence, no shadows will be present. This lighting condition is referred to as soft light and would require to open up one stop to f/8.
With a rainy sky, a lot of light is absorbed. I figure it is not necessary to detect this condition by looking at shadows... to me the best indicator of rain is getting wet. The mentioned loss of light due to absorption requires us to yet again open up one stop to arrive at f/5.6.
Somewhat obvious, in heavy rain, we would yet again open up one stop to f/4. However, I figure it is pretty rare to go out shooting under such weather conditions.
I hope to be able to shoot some frames showing the different shadows of the first 3 conditions mentioned above. If so, I will show those in a future post.