View Full Version : Fresnel Lenses: Someone Please Clarify This For Me...

Clark King
6-Oct-2001, 22:20
I have heard that if the fresnel lens is behind the gg(lens side) that the image s will be out of focus. Then also I have heard (from this board) that it does n ot matter on some cameras. I have asked this queation before but no one gave a real solid answer. I am new to LF and have been making test exposures on pol. 5 59. I have noticed some softness while using nikkor 210 and at a subject distan ce of about 9 inches. While the image is very sharp on the gg. I am wondering i f the softness is not just a product of the polaroid film. Would the same resul t from neg or trans film?

Sorry about the newbie type questions, thank you very much!

Clark King
6-Oct-2001, 22:27
I have thought of another question. The gg and the fresnel on the camera back have a small amount of play, meaning that with the camera back in a vertical position (on the camera) if you touch the gg it moves back and forth toward the lens. The focus does not change when it is moved. But is this okay?

Thanks, CK

Robert A. Zeichner
6-Oct-2001, 22:38
If a camera was designed to use a Fresnel between the gg and the lens, removing the Fresnel will cause focus problems unless the gg is repositioned. Conversely, if a camera not designed to use a Fresnel is fitted with one between gg & lens, once again, the gg must be repositioned to compensate for the rearward shift of focus. A Fresnel screen is a lens, essentially. It shifts the image rearward by an amount roughly 1/3 of the thickness of the screen. Some cameras are designed in such a manner that will allow the user the choice of Fresnel or no Fresnel. This is accomplished by 1. "Hanging" the Fresnel off the gg and not between it and the mounting pads of the camera back and 2. by putting shims equal in thickness to 1/3 the thickness of the Fresnel between the gg and the mounting pads. With Fresnel and shims in place, the image that comes into focus on the gg will be in focus on the film. If you were to remove the Fresnel, you would need to also remove the shims. This would move the gg forward toward the lens by a dimension equal to 1/3 the thickness of the now removed Fresnel screen. Again, image on the gg would be coincident with what would strike the film. If you want to test a paricular camera, you might read my article in the Nov./Dec. 1997 issue of ViewCamera. I describe a home made test target and a procedure that will confirm proper alignment.

Clark King
6-Oct-2001, 23:22
After further investigation I have found that on my camera back(horseman 4x5 approx. 14 years old), there are shims on the top and bottom that are placed on the photographer side of the back. Because these shims have been eroded away because age and the material. It seems that I can simply unscrew the screws and replace the shims but can I do this without precision equipment to measure this? The placement of Fresnel lens is lens side to the GG and the smooth side of the Fresnel lens is toward the lens board. I think can you actually feel the rings on the fresnel. I have shot some poaroids and they seem to be soft at f/5.6. I have wondered that this might be because the polaroid film is not as sharp as neg or trans film. But could it be because of the gg/fresnel placement. As I've mentioned before I use a 10x loupe and the image is very shrp on the gg but not on Polaroid test shot. Also because the shims have been "eroded" could this "play" or movement be causing a shit in focus?

Thank you very much for bearing this message I know it's long but I'm trying to learn so that I can truely enjoy the fruits of this wonderful labor.

Clark King

andrea milano
7-Oct-2001, 07:22
In the case of your Horseman, if I were you, I would get in touch with Jin Yamaguchi at Horseman in Japan, he will be able to advise the best procedure. As far as I am aware , Horseman cameras mount fresnel at the back of the GG, therefore facing the photographer, just slightly unscrew the retainers and slip the fresnel underneath, then screw tight and voila'! At least this is what I did on my Horseman EM, other models might have a different construction.

Film and GG allinement have been in the past widely discussed onn this site and you should find in the archives all the necessary information.

Concerning the poor grammar and typing skills of the contibutor of any site, which another contributor to this thread quoted, I am afraid that an International and well-meaning array of contributors produces contributions to the best of their abilities, in other words the content is more important than its form.

andrea milano
7-Oct-2001, 07:25
OOOOPS! it was another thread which spoke about poor grammar and typing skills, the point is valid anyway!

Bob Salomon
7-Oct-2001, 07:30
Where you place the fresnel does not matter. As long as the ground glass is adjusted to lie in the film plane.

When the camera is set up for the GG to be placed closer to the lens you cannot switch the position and put the GG behind the Fresnel unless the shims the GG sit on are re-adjusted for the shift in GG position.

It is usually better to place the fresnel closer to the eye so it can be removed when the lines in the fresnel are bothersome or when it makes focusing difficult with some length lenses.

Doremus Scudder
7-Oct-2001, 08:24
Clark, There is a quick test you can do to determine if the film is really in the same plane as the groundglass. Set up a close-up shot of a ruler lying flat or at a nearly horizontal angle. Choose a lens (longer is better), aperture (i.e. wide open) and subject distance (closer is better)that will give you the minimum depth of field. Focus carefully on, say, the six-inch mark, noting how much the other marks in front of and behind the one you focused on are out of focus. Expose a couple of sheets of film and develop them. A quick examination of the developed negs will tell you if your focusing plane is the same as the film plane. If the mark you focused on is exactly as you focused it, you are in great shape.

If closer marks are in focus, the film plane is behind the focusing plane (this happens only rarely, and not because of the fresnel, but because of some other mechanical positioning error such as too many shims or faulty back). In this case, you would have to remove shims or otherwise find a way to move the ground glass/fresnel sandwich closer to the lens.

If marks farther away than you focused are the sharpest, then the film plane is closer to the lens than your ground glass (this is the most common case, especially when adding a fresnel screen for the first time). In this case, you would have to add shims or otherwise find a way to move the ground glass/fresnel sandwich farther from the lens. Usually the error is around 1/3 the thickness of the fresnel.

I usually do these tests while I am printing and have a tray of print developer handy. I expose two sheets (one holder) of B&W film, take them to the darkroom, unload the holder and toss them in the print developer for 3 minutes or so (safelights off, of course). Time isn't critical, you only need an image to check for focus. If adjustments are necessary, these can be made and subsequent tests done quickly (much faster than normal film developing).

After making the adjustments, you must test again. Sometimes it takes two or three tries to get things just right. I had to add shims to a factory-designed gg/fresnel made especially for my camera. Apparently some camera manufacturers don't take this focusing problem into account when designing!

Polaroids should work for this, but if the sharpness of the Polaroid material is in question, I would recommend using conventional B&W film

Hope this helps, ;^D)

Trevor Crone
7-Oct-2001, 15:57
Clark I have an Ebony wide angle fresnel permanently mounted between the GG and protective glass on my Ebony SW45. No problems negs are as sharp as a pin. Regards,

Clark King
8-Oct-2001, 00:27
Thanks to all who have responded, I appreciate the information!