ground glass Korona

[Pronier Jean Claude]

Bonjour... in french
Hi, I'm refurbishing a 8x10 Korona bough recently on e-bay.
I'm puzzled on a detail about ground glass. There is a clear circle area with cross reticule in the center. What is the purpose of such transparent zone?
regards
jean claude

[Michael Gudzinowicz]

The spot and reticule are for aerial focusing. To use it, you need to be able to focus your focusing loupe precisely on the reticule that lies in the plane of the ground glass. Then focus the camera on the subject. When the image appears to be in focus, shift your eye / viewpoint back and forth across the field of the focusing loupe. When the subject is exactly "in focus", the subject will not appear to move relative to the reticule. If it is not focused in the same plane, the image appears to move relative to the reticule as your shift your viewpoint. The same principle is use to focus an enlarger with a magnifier using an aerial image rather than ground glass.

[ronald moravec]

If you master this, it is the premium way to focus. Unfortunately landscape subjects with 8x10 are three dimensional, not two, so the point is moot.

[Pronier Jean Claude]

Agree with Ronald. I used aerial focusing for astronomical shooting using the Foucault method. It's good for ponctual object like shinning stars. But on a lanscape or portraiture, pretty hard to find the point!

[S. Preston Jones]

This is an interesting post. I had a friend who did an experiment by placing nine 1/4 " spots made by adding clear fingernail polish on the ground side of an 8x10 ground glsss (no edge cuts). Three spots each across the top, center and bottom of the glass to see if he could use the aerial (or simulated aerial) image for focusing. It was interesting in that as you did tilts and swings you could see them sharply. You always had a spot available whether you need it in the top, center or bottom of the image. I felt that 9 spots were too many for me, but it did seem to work. Since the polish was put on the ground part of the glass and focus seem to match the untreated part of the glass. Additionally, the spots were very bright.