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Stacking two fresnel lens together

Suk Jung Choo , Dec 05, 2010; 07:29 a.m.

When I purchased my 4x5 camera, I asked the dealer if he had any know hows regarding brigthening the focusing screen . He recommended placing two fresnel ground glasses together. I was skeptical but having little background in large format decided to accept his recommendation and let him install two fresnel ground glasses. My problems are

1. I am noticing dark rings in the ground glass especially when focusing with the light in certain angles
2. I have difficulty discerning the correct focus in the periphery (i.e., blurring and double images).

Could these problems be related to uising two instead of one ground glass? Looking at the pictures that comes back I am not sure if these problems actually affect the photographic end product. However, my suspicion is focusing and composition under the hood are made more difficult by this measure.
Any comments will be greatly appreciated. Thank you,
SJ

Responses


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Tom Mann , Dec 05, 2010; 07:45 a.m.

The dark rings are probably due to Moire rings formed by the similar, but not identical patterns in the two Fresnel lenses. Even if they are identical lenses, a small offset between them can cause this. For critical focus, I would recommend inspecting the image on the ground glass with a conventional magnifier.

Tom M

Frank Bunnik , Dec 05, 2010; 12:57 p.m.

I have 2 fresnel lenses on top of the ground glass in my cambo wide and I experience the same thing as you. The slides and negatives come back bitingly sharp however.

Suk Jung Choo , Dec 05, 2010; 04:50 p.m.

Thank you so much for taking the time to respond. Your answers have been very helpful and I have just what I wanted to know. I hope this will be helpful to other members coming across this subject. Regards,
SJ.

Bob Salomon , Dec 06, 2010; 10:50 a.m.

There are ground glasses and there are fresnel screens. The two together give the brightest possible and most even illumination. There is no reason why you would use two frenel screens together. You do have a ground glass don't you? Not just two frensnels?
If your brightest gg and fresnel combination seems too dark then you need a finer gg and, perhaps, a more modern fresnel. But as you get very much brighter you will lose the ability to have the image snap in and out of focus for you. It will be a more gradual difference between in and out of focus as the system losses its "tooth" by extreme brightening. Also, when you do movements it can become more difficult to use a loupe as your eye goes out of the optical axis with extreme brightness screen systems.
So, assuming that you have a gg and two frenels. Remove one frensnel. See how bright the gg amd fresnel are. Switch to the other fresnel. Is the first combination brighter or the second? Throw which ever fresnel is not as bright out.
Remember, fresnel screens are a lens, they have a focal length. They are not the focus suface, they exist to distribute the light across the gg not to form the actual image.
There have been special systems that combined the focusing surface and the fresnel in one piece, an exaample was the Linhof Super Screen. One side was the fresnel and the other side was frosted to form the image. You would put a cover glass on top of the Super Screen to protect the grooves but you would not use an additional fresnel screen.
There was also the Boss system which used two layers of glass with a layer of wax between them to form the image. This system also did not require a fresnel. But both the Super Screen and the Boss Screen are long gone from production.

Suk Jung Choo , Dec 06, 2010; 10:25 p.m.

Bob,
thank you so much for your detailed explanation. The dealer said that he sandwhiched the ground glass between two identical fresnel lenses. His intentions were honest although perhaps misinformed. Now I think I understand what my problems are. I had a feeling that what you had suggested was infact what I probably needed to do. Since both fresnel lenses were identical products, if I were to use only a single fresnel should it go in front or behind the ground glass? Thank you.

georges Giralt , Dec 07, 2010; 04:19 a.m.

Hello !
If I'm not mistaken, a Fresnel has to be put after the ground glass (as per the light travel path).
You can put it in front of the ground glass but you've to ensure that the frosted part of the GG still sit where it was before the addition of the Fresnel lens. It is on the frosted surface of the ground glass that the image form. So if you change the ground glass position, you'll change the focusing surface and get blurry pictures (because the film plane is meant to replace exactly the grounded surface of the ground glass...)
This is why you normally add the Fresnel AFTER the GG, to magnify the already formed image, and not BEFORE the ground glass where it would add another lens of unknown focal length in the light path before forming the image...
Some camera makers make a combined ground glass and Fresnel lens and they sometimes incorporate the Fresnel lens before the ground glass to protect the delicate rims of the Fresnel inside the camera bellow. But for this to work they have to make calculation and get the calibration of the frosted GG surface right...
Last but not least, piling up a couple of Fresnel lens is not a bad idea but you should pile them like condensers in an enlarger : plate face facing outward.Keep in mind that the resulting focal length will be the conjugate of the two focal length...
Hope this helps.

Bob Salomon , Dec 07, 2010; 04:38 a.m.

The position of the fresnel depends on the design of your camera. Normally, today, the fresnel goes on top of the gg but some cameras were designed for the fresnel to be placed in front of the gg.
It is critical that you know which way your camera was designed for because the gg ground side must lie at the proper point. By placing a fresnel in front of a gg you may move the gg out of the image plane resulting in your focusing at the wrong place. Some cameras have used both front and rear fresnel placement. For example, very old Technika cameras placed the fresnel under the gg. Current Technikas place the fresnel on top of the gg. Fortunatelty for Linhof owners the camera has adjustable shims so a technician can adjust the position of the gg to allow the fresnel to be placed on top, if desired.

Rodeo Joe , Dec 08, 2010; 07:49 a.m.

The purpose of the Fresnel screen is not to magnify the image, but to collimate the light coming from the GG into a tighter beam. This increases the brightness but narrows the viewing angle, so that the screen actually becomes darker if viewed from off-centre. Stacking two Fresnels will narrow the viewing angle even further, resulting in parts of the screen becoming dark when your eye is moved even slightly away from the focus of the fresnel lenses. Personally I can't see why anyone would want to stack Fresnels, since a good viewing hood or cloth is an easier solution, and the difference in brightness between a plain GG and a fresnel screen isn't that remarkable.

The most efficient use of a Fresnel is to place it on the inside (lens side) of the GG. However in this position the register of the GG needs to be altered in order to keep the focus accurate. If two Fresnels have been fitted then the only sensible positioning would be to have one on either side of the GG, so I hope your dealer knew exactly what they were doing, otherwise the focus will be out on your camera.

Bob Salomon , Dec 08, 2010; 12:54 p.m.

"The most efficient use of a Fresnel is to place it on the inside (lens side) of the GG."
Maybe you would like to explain that reasoning to Linhof and Sinar. Since at least the late 70s Linhof has positioned the Fresnel on top of the groundglass and Sinar has had it there since sometime in the 60s. The reason for that placement is that some people do not want to see enlarged Fresnel grooves when they are focusing with a loupe. So Sinar and Linhof have made systems where the Fresnel is easily removed and replaced when it is not wante. And this also makes gg changing very easy since the thickness of the Fresnel is no longer an issue.


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