fresnel lens mountinggreenspun.com : LUSENET : Large format photography : One Thread
I have a Linhof Tech III: It came with the fresnel lense mounted to the front(toward film holder) side of the ground glass. The question is does this effect the focus as you have now changed the relationship of the GG to the film plane?
-- ronald lamarsh (firstname.lastname@example.org), July 28, 1999
Ron - This is the correct placement of the fresnel lens. The focus should be perfect.
-- Richard Fish (email@example.com), July 28, 1999.
Ronald, A fresnel screen is a lens. As such, it shifts the plane on which an image is focused back toward the filmholder by approximately a distance equal to 1/3 of the thickness of the screen! When you place the fresnel between the lens and the ground glass, the camera had better well be designed for this placement or your negatives will be out of focus! On the other hand, if you hang the fresnel on the viewing side of the ground glass, this shift will be compensated for by your eyes. I would suggest finding out from Linhof, exactly how this camera was originally delivered and use it that way. Linhof employs some very clever miniature jackscrews to precisely adjust the the ground glass position on many of their cameras. It is most likely possible to set your camera up for a fresnel brightener between the g.g. and lens. If you would like to know how to test the set up to confirm all is aligned, read my article in the Nov./Dec 1996 issue of ViewCamera magazine. It describes a test target you can build to prove your ground glass is properly aligned. Please feel free to contact me if you have specific questions.
-- Robert A. Zeichner (firstname.lastname@example.org), July 28, 1999.
There are 2 correct placements for a frenel.
On old cameras the fresnel was commonly placed closest to the lens and covered by the groundglass after the groundglass position was adjusted for the added fresnel. This protected early fresnel surfaces which were very prone to scratching.
There are 2 major problems with this placement.
1: Fresnel screen thickness varies from unit to unit so whenever the fresnel had to be replaced the ground glass needed to be adjusted. Replacement in this case is only by a repair man. 2: When using a loupe the circles on the fresnel could be annoying and some photographers wanted to be able to view without the distraction of the rings.
The more common placement is on top of the groundglass.
This has 2 major advantages.
1: The photographer can add or remove the fresnel at will.
2: The fresnel has no effect on focus.
Most modern cameras use the fresnel on top.
If you have a camera with the fresnel on the bottom it must be installed by a repaiman. In some cases the repairman can adjust the ground glass so the fresnel can be used on top and for that reason fresnel holding clips are available from Linhof to easily mount or remove the fresnel on top of the ground glass. They may not work on a III,
-- Bob Salomon (email@example.com), July 29, 1999.