Originally posted by weaponx525 With respect to Peterson, in his Understanding Exposure book, he doesn't mention North facing blue sky.
I don't know about north, but he most certainly mentions blue - he calls this "brother blue sky". And as I said before, he also specifically recommends this for bright sunny days, and really, it's designed to make subjects *in direct sunlight* look well-exposed, while making subjects that are in shadow *look* like they are in shadow. It is a good technique *if* you pay attention to how it is supposed to be done, but it is also true that if you take any old scene in any old light and then meter off any old part of the sky, you will almost always get pretty bad underexposure. It really has to be a subject in direct sun, and it really has to be blue sky. That's he also has although those other sky brothers and uncles and so forth - to handle the other kinds of lighting conditions.
Here's important info to keep in mind:
All objects we can see are illuminated by something. And nothing can ever be brighter than the light source illuminating it (well, there is the concept of fluorescence, which can sometimes create that illusion, but we can ignore that here).
On a sunny day, the sun is what is doing all the illuminating. Nothing lit by the sun is going to brighter than the sun. We all recognize that instantly. However, we need to also keep in mind that for objects in shade, the sun is not lighting the objects *directly*. The sunlight that illuminates the object is actually first reflected off the sky. That is, the sky is the main source of illumination for objects in shade, although nearby sunlit objects can also help fill in the shade too. Because objects in the shade are lit by the sky, with help from nearby sunlit object, *nothing* in the shade can ever be brighter than the sky and/or the directly lit objects on the ground that are helping fill in the shade. That is so important it bears repeating - *nothing* in the shade can ever be brighter than the sky or the directly lit objects on the ground that are helping fill in the shade. Even a white piece of paper in the shade is darker than the sky, and is usually darker than just about any object in the light (even most objects we think of as "black").
Now, the sky is directly illuminated by the sun, and is actually about "average" in terms of how it compares to other directly lit objects on the ground. Meaning the sky is lighter than a dark colored object that is directly lit by the sun, and darker than a light colored object that is directly lit by the sun. So metering off the sky produces an exposure in which light colored objects in the sun look light, and dark colored objects in the sun look dark. But objects in shade - whether light or dark - will *also* be darker. So metering off the sky will always produce underexposure for objects in the shade (assuming you didn't *want* them to all look dark). It only produces normal looking exposure for objects in the light.