Once upon a time, “underexposure” and “overexposure” described mistakes.
Underexposed negatives lacked sufficient density. Overexposed negatives were too dense. Underexposed prints were too light. Overexposed prints were too dark. Underexposed color transparencies were too dark. Overexposed color transparencies were too light. Underexposure and overexposure were failings, embarrassments, proof of shameful incompetence.
The full moon not only is a splendid sight, it’s an object of known width that can be used to determine the focal length of a lens — or to verify the marked focal length of a lens (manufacturers sometimes round focal lengths to the nearest five millimeters, so a lens with an actual focal length of 178mm could be sold as a 180mm). And if the focal length of a lens is known, the size or distance of objects can be inferred from a photograph, so the exercise has practical applications.
TBL is at the same stage as my plum trees: starting to bloom.