Makes sense, Roger. I wonder if most people intuitively look from this 'magic' distance as they move in and out from a gallery photo.

The strange thing is it implies very close viewing distances for wide angle shots and strangely far viewing distances for telephoto shots. I mean if you use a 47mm lens on 4x5 and enlarge to 8x10, that means you're looking at an 8x10 print from 3 1/2 inches away. Or if you use a 600mm lens on 35mm, a 4x6 print would be viewed from 8 feet away.

Seems to be a bit impractical except for normal lenses

There must be a similar phenomenon for drawings and paintings, assuming any perspective is consistent across the picture. I mean the print doesn't know if it's passed through a lens or not.