fresnel in relation to GG on Cambo 8x10

[Robert_5479]

Probably a newbie question, but should the fresnel lens go in front or behind the gg in my newly aquired Cambo 8x10 Legend. I received it with the fresnel on the lens side of the gg but I'm thinking that if one chose to use just the ground glass, the focus would be different than what it is now. However, if the fresnel, which is a soft plastic and not exactly flat, were behind the gg it would get scratched fairly quickly. What is the normal configuration of these two? Thanks!

[Michael S. Briggs]

I don't know what the factory configuration of your Cambo is supposed to be, but you are correct to be concerned about a focus shift if you removed the Fresnel lens. A Fresnel lens in front of the ground glass (i.e., lens side) causes a focus shift relative to the same thickness of air. Plus if the Fresnel is resting against the camera, removing it will change the position of the ground glass. Hopefully someone will know whether the current configuration of your screen is the factory configuration. It seems fairly common for owners to alter their focusing screens in ways that reduce focusing accuracy, so you may want to carefully check the focusing accuracy of your camera.





Here are some previous threads on these topics: Shim thickness to replace fresnel at
http://largeformatphotography.info/lfforum/topic/499244.html,
Is my ground glass mis-seated? at http://www.largeformatphotography.info/lfforum/topic/497848.html
and Ground Glass focusing errors at http://www.largeformatphotography.info/lfforum/topic/118997.html.

[John Cook]

Since a Beattie screen is essentially a fresnel plus a groundglass, their online installation instructions may be applicable to your installation:

http://www.intenscreen.com/technical.htm

Failing that, I'd recommend a telephone chat with Calumet's excellent service department.

1-800-CALUMET (225-8638)

[Brian Ellis]

Why not call Calumet and ask them? 1-800-CALUMET in the U.S.

FWIW I don't think you have both a Fresnel lens and a gg (ground glass). The Fresnel lens usually replaces the ground glass. I'm not familiar with your particular camera or setup but what you likely have is a Fresnel lens and a piece of clear glass or plastic of some sort that serves to protect the Fresnel from scratches and may also have a grid etched or painted on it. The usual configuration is Fresnel on the lens side, protective sheet on the photographer side. The Fresnel probably has concentric circles on one side and a rough or ground look on the other side. The rough side of the Fresnel usually faces the lens. The protective sheet probably has one side that is smoother than the other. The smooth side faces the photographer. But some cameras and setups are different plus I'm not the world's greatest expert on this so I'd call Calumet and ask about your particular camera to be sure.

[Michael S. Briggs]

Robert can look at his screen and determine whether the ground surface is on the plastic part or the glass part. I think that integrated over the history of LF cameras the configuration of ground glass plus plastic Fresnel lens is more common than Fresnel lens with integrated ground surface plus protective cover glass. The latter is a recent innovation. But we don't know which Robert has. But since he describes his plastic part as "not exactly flat", it would seem to be a poor choice to have the reference ground surface.

For the ground glass plus Fresnel lens, a correct setup can have either the Fresnel lens closer to the lens or closer to the photographer. In either case the ground surface of the glass faces the lens. The advantage of closer to the lens is that the plastic Fresnel lens is protected by the glass. The advantage of closer to the photographer is that the Fresnel lens can be removed without effecting the focus. For the design of ground glass plus Fresnel lens, the configuration of ground glass closer to the lens, Fresnel closer to photographer is the more common one. But again, it can be done either way. And you can't switch between these configurations without effecting the focus.

There is also no guarantee that Robert's camera has the factory original screen. If it were my camera, I would make a careful photographic test of the focus accuracy.

[Robert_5479]

Michael is correct when stating that the Fresnel is plastic and seperate from the Cambo brand Glass gg with its grids. The Fresnel is rather beatup and I'll eventually replace it. But for now, it'll have to do as they are a bit pricey for my current needs. Also, Michael's belief that the ground glass being closer to the lens makes more sense than the way mine is configured (Fresnel closer to lens) which is what led me to ask since the arrangement seems at odds with practical use.

I'll have to do some testing (eventually) but being lazy and short on time I was curious how these may have shipped as I'm led to believe that they came with the Fresnel installed. I'll give Calumet a call...someday :) Thanks for all the great responses!

Robert