dating the older man blogspot - Surface exposure dating
The camera that documented the famous meeting at Promontory Point, Utah of the tracks of the Central Pacific and Union Pacific railroads on was built to accommodate glass plates 10" x 13".
Flaws that were not obvious in the smaller cards now became very visible. Success in retouching led to innovations in the darkroom and at the camera.
Diffusion of the image reduced the need for retouching. Blue playing card stamps are known to have been used in the summer of 1866 as other stamps were unavailable as the levy came to an end.
Each eye looks at a slightly different image, and the fusion of the two images in the mind creates the illusion of depth. Sizes of stereo cards and slides: The typical mass manufactured stereo card of the period between the Civil War and WW I had a standard dimension: 3 1/2" x 7". The earliest of these cards were made on slightly curved mounts; later cards were made on slightly curved mounts that permitted greater clarity when they were seen in the stereopticon viewer.
A number of photographers, working with larger field cameras, created slightly larger cards of 4" x 7", 4 3/8" x 7" and 4 1/2" x 7".