For me, a lesson on Perspective by C.B. Neblette, in his book, Photographic Lenses 1965. Paraphrased --- If you stand before a glass window, with wax pencil, you can trace on the glass, the outline of objects. This sketch is the “human perspective”. With camera, you can replicate this perspective. Replace the eye with camera. Take a picture of this vista. Make an image for display. Regardless of format size or focal length, if you view the resulting picture from a viewing distance that is the same as the focal length, the image replicates the “human perspective”. This is for a contact print (image same size as camera format. If an enlargement, the viewing distance is focal length multiplied by degree of magnification.
Example a 35mm full frame is used with a 50mm lens. The viewing distance for a contact print is 50mm. Not practical so you make a 8 x 12 inch display image. To make this size image, 8 ½ X magnification was applied. The viewing distance becomes 50mm X 8 ½ = 425mm = about 16 ¾ inches.
The lesson: Follow these simple steps and the resulting image matches the human perspective regardless of the focal length of the taking lens. Additionally, most images need not replicate the ‘human perspective” (portraits generally sell better if they do).
Thus the real lesson is art has no rules, you are free to follow your heart.
A tip of the hat to Neblette Past Dean of the College of Graphic Arts and Photography at the Rochester Institute of Technology