Clear ground glass use scratch marks to focus??

[joho]

Due to a repair needed on a ground glass on a very old view camera, I came across some thing I had not seen in along time. the ground glass was not so frosted and is hard to focus -there is but some making as scratches . I remember once hearing that if the object was in focus the scratch marks would not move off the scratched spot that what was in focus???
O.K. -any ideas???

[mdarnton]

Yes. If the sharp image is in the same plane as the scratch when you move your head they will stay together. Otherwise they move relative to each other because they are different distances away. Similar to how someone in the doorway only moves relative to the door frame when you move yourself if the person is behind or in front of the frame. This is a very accurate way to focus if you can get used to it.

The term to search for is aerial focus..
https://www.largeformatphotography.info/forum/showthread.php?45251-aerial-focusing

[joho]

Yes. If the sharp image is in the same plane as the scratch when you move your head they will stay together. Otherwise they move relative to each other because they are different distances away. Similar to how someone in the doorway only moves relative to the door frame when you move yourself if the person is behind or in front of the frame. This is a very accurate way to focus if you can get used to it.

The term to search for is aerial focus..
https://www.largeformatphotography.info/forum/showthread.php?45251-aerial-focusing

Many Thank you for the info - have given the definition I was in need !!!