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Thread: Fresnel & Ground Glass - Which first?

  1. #1
    Sheldon N's Avatar
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    Fresnel & Ground Glass - Which first?

    I've seen reference to different cameras having the fresnel installed first then the ground glass on the exterior, and to some cameras having the ground glass first with fresnel on the exterior.

    Is there an inherent advantage to one approach or the other - in terms of brightness and ease of focusing with a loupe?

    The reason I'm curious is because I have a Wista DX that I purchased used, and it came with the ground glass installed first and the fresnel on the outside. The ground glass was also shimmed slightly. I recently changed out the ground glass for a Satin Snow and put it back together the same way (glass then fresnel). I tried taking out the shims, did some focus tests and determined that they were necessary and put new ones back in. The shims are actually about the same thickness as the fresnel. It works fine now and the focus is accurate.

    However, I've seen reference to new Wistas having the fresnel installed first then the ground glass. This seems to make sense based on the thickness of the shims that I had to install.

    Is there any advantage to changing my setup to a fresnel then glass orientation? I don't want to mess with it if there's no reason to, but if there's a benefit I wouldn't mind swapping them out.

    Thanks!

  2. #2

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    Re: Fresnel & Ground Glass - Which first?

    Sheldon,

    If it works the way you currently have it mounted, there would not be any advantage, at least none that I have seen to changing it.

    Others may have a different experience, but I always figure if it works don't change it.

    Dave

  3. #3

    Re: Fresnel & Ground Glass - Which first?

    Sheldon,

    I have a DXII which should have a back similar if not identical to your camera. Mine has an original Wista combination screen with a GG surface facing the lens and the Fresnel surface facing the rear cover glass. In that orientation, the Wista logo reads “correctly” from the rear of the camera looking at the GG. Since the screen and replacements are installed from the rear (away from the lens) and the focus plane position is established by the frame’s thickness, the GG focusing surface always should be the first surface facing the lens. Any additional or replacement components such as Fresnel screens or a cover glasses are placed behind the GG screen away from the lens.

    The wooden screen frame on my camera does not have a flat front surface. The corners are “raised” which permits the spring frame to exert more pressure on the corners of film holders. The depth from the GG surface to the surface of the frame measured in the corners is exactly the same as that from the surface of holders to the surface of an inserted sheet of film. The exact specs are available if you run a search. That is the only critical measurement, and if the values are equivalent, any focus errors have other causes.

    My camera has been in use for over 20 years, and the frame does not show any signs of wear that might affect focus. The camera and holders are cleaned, given a coat of bowling alley wax and polished every year, and paraffin is applied to the focusing tracks for lubrication. If significant wear should occur, I might consider giving the affected corners of the screen frame thin coats of varnish to restore spacing.

    The most common focusing error is using a loupe which is not focused precisely on the GG surface, but on the Fresnel or cover glass. That will result in an error which is equivalent to the distance from the GG surface facing the lens to the loupe’s point of focus, plus a bit more to compensate for the refractive index of the screens/glass. If the photographer switches between using/not using eyeglasses, the loupe should be refocused on the GG. Also, if the GG screen is inserted “backwards” facing the cover glass rather than the lens, an error will result. Other focusing errors may be due to warped or out-of-spec holders, lens focus shift when stopped down, bed or standard movement after focusing, buckled film, improper film insertion in the holder, failure to seat the holder’s lip, etc.

    Mike.

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    Sheldon N's Avatar
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    Re: Fresnel & Ground Glass - Which first?

    Thanks for the helpful information.

    I have my setup installed correctly per Mike's description of the manufacturer's setup - ground glass closest to lens with ground side facing toward lens, then fresnel on the outside with the fresnel also facing towards the lens.

    I use a Toyo 3.6x fixed focus loupe, and when I did my focus tests everything was very carefully controlled. I used a ruled scale at a 45 degree angle, heavy duty tripod, two different lenses and multiple exposures, all shot wide open to show exactly where the DOF was, good condition film holders, and focus was carefully checked both before and after each shot.

    I marked the bed of the camera with a bit of tape and adjusted focus by a couple mm of bed travel, then made note of how much effect that had on the point of focus on the scale. Once I got the negatives developed and scanned, I used that as a reference for how thick the corrective shim should be.

    The shots exhibited front focus, meaning that the lens and film plane were farther apart than they should be. By shimming the ground glass with a piece of light mat board I moved the ground glass away from the lens. This causes me to move the lens slightly closer in to the film plane to get perceived correct focus on the ground glass, thus shortening the distance between film and lens and moving the actual plane of focus slightly closer to infinity.

    Because the error was front focus I'm confident that I'm focusing on the ground glass surface and not the fresnel. If I was focusing on the fresnel (which is farther away from the lens than the ground glass) I would be introducing back focus. The prior owner of the camera had shimmed the ground glass as well, so I'm assuming that he experienced the same problem.

    I don't see any significant wear on the back of my camera, so my guess is that this is simply a manufacturing tolerances issue.

    Anyway, the focus is accurate now and it doesn't appear that there's any advantage to having the fresnel inside the glass, so I'll leave my camera as is.

    Thanks!

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    Re: Fresnel & Ground Glass - Which first?

    Just to add a little bit of information to this string, I decided out of curiosity to check my Toyo AII, and it has the plastic fresnel in front of the GG closer to the lens. The plastic fresnel lens part of the sandwich (no spacers) actually sits right against the GG. Hmmm, and just to mix it up a little more, the Wisner Site has a different arrangement:

    http://www.wisner.com/viewing.htm

    "What is the Proper Placement of the Fresnel? One subject which comes up often is the correct position of the fresnel lens. In years past, several arrangements have been used, including placing the fresnel behind the ground glass, in front of the ground glass, and incorporated into a ground glass made of plastic. However, there is only one correct arrangement. The ground glass surface should face the camera lens, and the fresnel is placed behind it, on the outside of the camera, toward the photographer. The textured surface of the fresnel should be placed against the ground glass. There is a particular reason for this arrangement. In manufacturing cameras and film holders, one overriding concern is the correct position of the focal surfaces of the respective parts. In the film holder this is the position of the septum, against which the film rests, and in the camera this is the position of the diffusion surface, or ground side of the ground glass upon which one focuses the camera. Nothing is more important than the proper registration and agreement of these two elements. If the fresnel were placed in front of the ground glass, interposed between the lens and the ground glass surface, even if it were still physically in the same position occupied by the film plane, a lack of registration can occur. This can be explained by considering the effect of a parallel sided glass plate such as a glass filter upon a beam of light. Rays passing through such a plate are displaced by about one third the thickness of the plate, and depending upon the angle when passing through the plate, will be displaced laterally. .Furthermore, the greater the angle, such as in a wide angle lens, the greater the effect, resulting in an apparent curvature of the image when no such real curvature exists. The result will be erroneous focusing of the edges of the image.
    T
    "Bright" Screens
    One device which gets a lot of press and is usually misunderstood is the so-called bright screen. These are usually proprietary ground glass type screens which claim to transmit a brighter image with greater contrast. The explanation is really quite simple. Going back to our discussion of the ground glass, we said that it possessed two characteristics, that of diffusion, and that of residual transmission. The less diffusion, the greater the transmission. This effect can be seen readily by smearing a little water on the ground side of the glass. The glass will become nearly transparent, and any image viewed through the wet area of the glass will appear to be brighter and higher in contrast. The hot spot will be greatly exacerbated. The less diffusion, as we have seen above, the worse the hot spot. Commercial bright screens used a permanent means of reducing the diffusion of the glass, usually by some sort of coating, thus creating a glass which has these characteristics. These glasses MUST be used with a fresnel lens because the hot spot is so much increased. Regarding the claims for brightness, no glass can be brighter than the original aerial image emanating from the camera lens. One proprietary focusing screen uses a micorcrystalin wax for the diffusion medium, and the result is a very sharp image with all the same qualities of the combined diffusion and transmission that a good screen requires. This screen uses a very thin cover glass as part of the wax lamination. In consideration foregoing discussion of interposed glass in the image path, this glass is probably too thin to make a difference, but you should be aware of the effect in the event of a critical application."

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    Re: Fresnel & Ground Glass - Which first?

    Yes, I am sure many of us have read Ron's views on fresnels, and there are a few of us that disagree in part with some of his opinions. This article of Ron's has been around for a great number of years now...

    Dave Parker
    Satin Snow Ground Glass

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    Re: Fresnel & Ground Glass - Which first?

    I saw Ron's information quite a while ago. All I can think is that he, perhaps, has never seen a Graflex product.

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    Is that a Hassleblad? Brian Vuillemenot's Avatar
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    Re: Fresnel & Ground Glass - Which first?

    Ron is right on this-mount the Fresnel on the outside of the groundglass, towards the photographer. If mounted on the inside, it will introduce a focus shift. Some cameras have it mounted on the inside, but the plane of focus has to be adjusted for it to work properly this way.
    Brian Vuillemenot

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    naturephoto1's Avatar
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    Re: Fresnel & Ground Glass - Which first?

    Quote Originally Posted by Brian Vuillemenot View Post
    Ron is right on this-mount the Fresnel on the outside of the groundglass, towards the photographer. If mounted on the inside, it will introduce a focus shift. Some cameras have it mounted on the inside, but the plane of focus has to be adjusted for it to work properly this way.
    Unless things have changed, as I recall the fresnel lens is placed on the far side of the photographer closer to the lens than the GG in the Technika cameras. On the other hand, Linhof changed this in the Technikardan 45 so that the Fresnel is closest to the photogrpher and the GG is closer to the lens. This makes it easier for the photograher to add or remove the Fresnel when wanted or needed, however care has to be taken when using a loupe to avoid damaging the Fresnel.

    Rich
    Richard A. Nelridge

    http://www.nelridge.com

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    Re: Fresnel & Ground Glass - Which first?

    Quote Originally Posted by Brian Vuillemenot View Post
    Ron is right on this-mount the Fresnel on the outside of the groundglass, towards the photographer. If mounted on the inside, it will introduce a focus shift. Some cameras have it mounted on the inside, but the plane of focus has to be adjusted for it to work properly this way.
    Ron is right, that is of course, unless the manufacture decided to set the camera up properly with the fresnel in front of the ground glass, and many have done this over the years.

    Dave

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