Luft93 wrote:
This is the best response to the question. I learned the diagonal approximation in about 1946, have used and taught it for decades.
The essential factor is that distant objects appear at at the actual restive size, not closer or farther away. Be aware of single eye viewing.
I, like you , always referred to the diaganol of the 'Negative , would determine what the 'normal' focal length should be. But , because I have Macular degeneration in my left eye, I only view everything with with a Single eye. I am learning to cope with this. Those with good vision in two eyes, just count your blessings, and stop quibbling over small degrees of difference in vision versus lens focal length.