Replacing ground glass with

[ae5x]

Am I right in thinking that if I replace the gg in my camera with one from a 3rd party vendor, that the focal plane of the gg may no longer be the same? Besides the actual replacement, what procedures are involved to ensure what's in focus on the glass will also be in focus on the film?

Thanks,

John

[Bob Salomon]

The focal plane of a flat piece of glass is always the same. Fresnel screens are made for different focal lengths.

When replacing the ground glass on a camera the ground side always faces the lens. As long as you place the new GG on the same shims or supports as the old GG the GG will be properly positioned.

If your camera has a Fresnel beneath the GG and you are going to replace it with a newer one the new one must be the same thickness or the GG may need to have the supports it rests on re-adjusted.

If the Fresnel is on the back side of the GG no adjustment is necessary for a new GG.

[robc]

If the existing gg is removed from the out side away from the lens then providing that whatever it rests upon in the back doesn't move when you remove the gg, then replacing with another gg should not alter the plane of focus providing it rests on the the same support as the old one.

However, if it is removed from the inside then the thickness of the glass is a factor and will need to be compensated for.

Also if the existing glass has a fresnel on the inside then focus will be altered if the exact same fresnel is not reused or replaced with the same.

Measuring position is diffcult to do accurately but can be done. But basically you should not adjust the depth unless you are absolutely sure it needs doing. How it is adjusted depends on the back. Some require new shims and some have adjustment screws like the Linhofs.

See this thread (http://www.largeformatphotography.info/forum/showthread.php?t=15166&highlight=depth+micrometer).

If in any doubt then get an expert to do it for you.