What is ground glass focusing system?

The ground glass viewing system is the most accurate and is usually associated with view cameras like  8x10 or 4x5. The viewing and focusing takes place on a piece of finely ground glass that sits exactly in the plane of the film.

Once the focusing is complete a cut film holder, which is a light tight container where the film sits, is then placed where the ground surface of the glass was. A dark slide is removed for the exposure and replaced after this.

In actual fact 35mm SLRs, 120 SLRs and Twin lens reflex cameras also have ground glass focusing screens, but rather than looking directly at the screen you look through a viewfinder onto the screen.