fresnel lens addition and focus accuracy

[Charlemagne]

I'm very new at LF, just looking at buying my first 4x5 camera.
there's something I don't quite understand about the ground glass and adding a fresnel lens to it.
when the ground glass is rear mounted in the back, how can one keep focus accurate when adding a fresnel lens (behind the ground glass, or as you wish, in front looking from the lens) to it? To me it seems the ground glass is set back by the thickness of the fresnel lens, and thus not in line with the film plane in the film holder anymore.

can someone explain?
I have been advised to look out for a camera that has a bright ground glass or one that has a fresnel added to it ...

thanks
Ludwig

[JMO]

No matter how thick your ground glass is, or whether you have a fresnel lens added (or not), you should adjust your focusing loupe carefully so that you are seeing most clearly and precisely the inside surface of the ground glass which the emulsion side of the film will replace when you slide the film holder in behind the ground glass. There are threads here on this Forum about choosing and using different focusing loupes, or making any adjustments to how the ground glass is mounted into the camera back, where Bob Salomon emphasizes this point as critical to proper focusing on the ground glass. All of the several focusing loupes I have allow a fair amount of diopter adjustment for the particulars of one's eye, and to accommodate varying thickness of the ground glass and any fresnel lens you might use.

[Bob Salomon]

If the Fresnel is closest to the eye it does not effect the focus. If you place it on the lens side of the gg then it will displace the focus plane by the thickness of the Fresnel and the back should be adjusted for this placement by a competent service center.

[JMO]

Mr. Salomon, thank you for your addition and clarification. It will help me since I recently purchased a spare back for my 4x5 Technika IV and TKS in good condition, but which lacks a ground glass (so I'll have to add). I am lucky to have Precision Camera nearby in Skokie IL in case I have any question about whether that new GG is properly positioned and aligned.

[Bob Salomon]

Mr. Salomon, thank you for your addition and clarification. It will help me since I recently purchased a spare back for my 4x5 Technika IV and TKS in good condition, but which lacks a ground glass (so I'll have to add). I am lucky to have Precision Camera nearby in Skokie IL in case I have any question about whether that new GG is properly positioned and aligned.

On the TK the Fresnel goes on top of the GG under the silver clips on the middle screws of the GG hold down clips.
On the lV it was usually positioned under the GG. Bob Watkins can guide you.