Fresnel lens placementgreenspun.com : LUSENET : Large format photography : One Thread
I am new to the large format photography and I have a question regarding the placement of the fresnel lens. In the instructions it says to place the fresnel lens in front of the focusing screen. Is the front of the focusing screen the side that is closes to the lens or the side closes to the photographer? Thanks for your help!
-- Allan Jones (firstname.lastname@example.org), March 24, 2002
What type of camera are you using? Ron Wisner makes a compelling argument in an article on his website for placing the Fresnel, ridges towards the inside of the camera, against the smooth side of the ground glass, for optimum optical performance. But the fact remains that many older cameras, including those using the fairly standard Graflok back, are designed to focus properly with the Fresnel on the inside of the camera, ridges towards the etched side of the ground glass. You don't want to tamper with this arrangement, unless you use some sort of shim of the right thickness to take the place of the Fresnel, otherwise you won't be focusing on the same plane as your film. A quick way to figure which way things work on your camera is to measure from the inside of the camera back to the ground glass, then put in a film holder, pull the dark slide (no film necessary!) and take the same measurement again. Ideally, the distances should be the same. If inverting the order of the Fresnel and ground glass makes it work, you have your answer. In any event, the ridged side of the Fresnel always faces the ground glass, and obviously the etched side of the ground glass faces the inside (front) of the camera.
-- Stephen Longmire (email@example.com), March 24, 2002.
A Fresnel Screen IS a lens. It typically shifts the focus plane rearward (toward your eyes) by a distance roughly equal to 1/3 the thickness of the Fresnel screen. That said, if your camera was not designed to utilize such a screen in the first place, it is better to install it behing the gg (between the gg and your eyes) so that this shift will automatically be compensated for by your vision.
Some cameras were designed to give you a choice of whether or not you use a Fresnel. In these instances, there are some shims that go between the gg and the mounting pads against which the gg is clamped down. These will normally be of a thickness equal to about 1/3 of the thickness of the Fresnel. The Fresnel gets "hung" in front of the gg between it and the camera lens, not sandwiched between the gg and the mounting pads! If it were sandwiched, that would "double shim" the gg. This hanging is usually accomplished with some long metal clamps that are u-shaped in cross-section. At least this is what Horseman does on their 45FA. If you were to decide not to use the Fresnel, you would remove it AND the shims and now you would have just a gg to focus on. If a camera was made to use a Fresnel, but had no shims (in other words, the gg and Fresnel were sandwiched together and right up against the pads, the only way you could remove the Fresnel and make the gg work right would be to machine away some of the camera!
As far as making measurements of gg and film holder depth, I would forget it unless you are a machinest and have a THOROUGH understanding of what it is you are measuring as well as the CORRECT tools. The ANSI standard for film holder depth in 4x5 is .197"+/-.007" tolerance. A piece of film is .003" to .005" thick and so measurements must always be made with film in place. In an optimal situation, you want the gg to be right at the correct depth (=/-0)for the type of film you use. This way, the film holders depth can vary from holder to holder (within the specified tolerance) and not negatively impact the sharpness of your results.
When you are all done making whatever changes you make, you should test the final arrangement with film. And for this, I will email you my article on testing your gg, in which I describe a home made test target. Keep in mind, many cameras and lenses have been sold and or traded in because their owners thought they weren't as sharp as some other camera or lens, when in reality, the gg was misaligned (usually because someone thought they could improve brightness by installing a Fresnel).
-- Robert A. Zeichner (firstname.lastname@example.org), March 24, 2002.
Allan: The above is sound advice. No matter what you do, when you're done and you think you've done it right, check it with film. Put a row of objects on a fence, mark one of them, take a picture with the lens wide open, and develop the filme to make sure the plane of focus onthe ground glass is the same as it is on the film. Better to discover it this way than in the darkroom after a trip. Depth of field can mask significant errors and bite you when you least expect it. Polaroids, of course, make this quick and easy, but it is worth the trouble to consume one sheet of film for the test.
-- Kevin Crisp (KRCrisp@aol.com), March 24, 2002.
All of the answers above are right on the money.
Both Kevin and Robert, above, have assisted me in my efforts to make my old Crown Graphic as sharp focusing as possible. My thanks to both.
You'd do well to follow thier advice.
-- Steve Feldman (email@example.com), March 24, 2002.
Could you tell the rest of us where we might find your article on ground glass testing? Thanks
-- jason (firstname.lastname@example.org), March 24, 2002.
The groundglass testing article appeared in the November/December 1996 issue of ViewCamera magazine.
-- Robert A. Zeichner (email@example.com), March 25, 2002.
If you want complete piece of mind and don't mind spending aboutg $75 to get it, send the camera and fresnel to Steve Grimes, let him install it, do the measurements, testing, etc. I've done that three times now with a Bosscreen and I'd do the same thing with a fresnel lens if I ever bought one. When you start talking about distances and mesaurements like .007 inches you're talking about something I have no interest in doing myself.
-- Brian Ellis (firstname.lastname@example.org), March 25, 2002.