Fresnel placementgreenspun.com : LUSENET : Large format photography : One Thread
I have an older Toyo 5X7 field camera (configured to 4X5) and I only recently realized (yesterday and I know after owning it for 10 years a very big "duh") that I have a fresnel as well as a groundglass. The fresnel is positioned on the inside, closest to the film. My Toyo roll film back - it's specific to Toyo and has its own slider and groundglass/fresnel - is the same, groundglass on the outside, fresnel on the inside.
So are these the correct orientations? A friend who knows a lot more about LF than I do suggested that this might not be right and if it isn't my focusing has been saved only by depth of field!
-- David Grandy (firstname.lastname@example.org), November 27, 1999
If your images are sharp it is the correcet placement as the camera was adjusted for an internal fresnel.
If your images are not sharp it is the incorrect placement.
Most modern manufacturers prefer to place the frenel on the outside of the ground glass so it can be removed by the photographer if so desired. This also makes changing a ground glass and fresnel very easy as the film plane would require no re-adjustment for the new ground glass/fresnel.
However if your camera has been adjusted for an inside placement a camera repairman will have to reposition the ground glass if you want to change the fresnel's position.
-- Bob Salomon (email@example.com), November 27, 1999.
You could check it easily using the method described by Jack East in the May/June 1999 issue of Photo Techniques. He describes a very simple way to compare the position of the ground glass and the film in the film holder (harder to explain than to do). Ron Wisner also discussed the issue of relative placement of the fresnel & ground glass recently in the Q&A section of the Wisner web site (wisner.com).
-- Chris Patti (firstname.lastname@example.org), November 27, 1999.
The presence of a fresnel on the inside of the ground glass shifts the plane of sharpest focus by about 1/3 of the thickness of the frenel. If you believe what Ron Wisner writes on his web site, then this is nothing short of a sign of the coming apocalypse. Fortunately, he's wrong.
Most reputable camera manufacturers who position fresnels on the inside of the ground glass design their frames to offset the position of the ground glass to compensate, thereby entirely avoiding the focussing errors that Wisner rails against. If you're lucky, your frame is like that of my old Linhof Color Kardan, which incorporate shims that can be inserted or removed to give correct groundglass positioning with or without the fresnel, respectively. I'm nor sure if Linhof shipped it that way or if it was an aftermarket job, but either way it's slick and it works (by which I mean that focus doesn't shift when I remove both the fresnel and the shims, and that it's dead-on relative to my holders either way). I just wish that my new camera (an A-S F-line) incorporated something half as effective.
What I'm getting at is that with the fresnel removed, you'll want the groundglass positioned a bit closer to the lens than with it in. If you're adventurous, you might want to disassemble the rear standard of your camera and see if you can find some shims or other standoff which can be removed to accomplish that.
-- Patrick Chase (email@example.com), November 27, 1999.
With all due respect to Jack East and his photographic talent, I must take issue with his measurement technique as described in the article mentioned above. The "tools" he suggests for making these measurements are woefully inadequate for the job. You cannot expect to arrive at any meaningful conclusions using toothpicks, rulers and clamps to make comparitive measurements on the order of thousandths of an inch. Even if one could achieve this using the suggested materials, the method of measurement fails to take into account the entire bearing surface of the filmholder. Having built tools for making critical measurements of a similar nature, I have found it far easier to test, with film, using a specially designed target which is described in my article in the Nov./Dec 1996 issue of ViewCamera magazine. In it, you will find instructions for building a homemade test target. If you follow the instructions carefully, you will be certain that your camera's groundglass/fresnel are coincident with the film plane.
-- Robert A. Zeichner (firstname.lastname@example.org), November 27, 1999.
I sadly miss a Fresnel screen on my Nagaoka camera. Does anyone know where I can get one? Or how about replacing the GG with a Linhof Superscreen? I know from nothing about this, fellows. Thanks
-- Bill Mitchell (email@example.com), November 27, 1999.
I agree wholeheartedly with Patrick. I saw the Jack East article, promptly got out the rulers and toothpicks, and checked the groundglass on my year-old Toyo 45AX. I was shocked! The septum on the first holder I checked was deeper (farther away from the lens) than my groundglass. Then I checked another and another -- same thing. Same story for my Polaroid 545i and Fuji QuickLoad holders. So I decided that my groundglass must have been aligned improperly (even though I hadn't noticed any focusing problems).
So I decided to do a series of real-world tests. Shots with different holders with different focal lengths at different magnifications (including extreme closeups like a frame-filling watch face at more than 1:1)...and all wide open so that depth of field wouldn't mask focusing errors. The focus was dead on (judged with 8x, 10x and 15x loupes).
Then I figured out why my camera was focusing perfectly even when the ground glass/film holder measurements were different. My Toyo 45AX, like Patrick's, has the fresnel on the inside of the ground glass. That made the "ground glass" (actually the fresnel) measure closer than the film holder. But the thorough engineers at Toyo adjusted for it. I'd agree there's nothing to fear from a fresnel mounted on the inside, so long as the manufacturer knows how to adjust for it.
So, David, rest easy that the folks at Toyo didn't screw up when they mounted the fresnel on the inside. But if you're still concerned about your friend's advice, and fear that only depth of field has spared you from focusing errors -- try some shots wide open and find out. I'll bet you'll find that Toyo was doing it right ten years ago too.
-- Greg Lawhon (firstname.lastname@example.org), November 28, 1999.