Fresnel & Ground Glass - Which first?

[Sheldon N]

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!

[Dave Parker]

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

[Michael Gudzinowicz]

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.

[Sheldon N]

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!

[toyoman]

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."

[Dave Parker]

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

[BrianShaw]

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

[Brian Vuillemenot]

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.

[naturephoto1]

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

[Dave Parker]

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

[neil poulsen]

Arca Swiss also has the fresnel placed closest to the lens.

[Doug Kerr]

It may be interesting to reflect on the theoretical basis for the use of Fresnel "field lenses" in connection with viewing screens. There are in fact two distinct objectives involved, which explains the fact that sometimes the field lens is placed ahead of the ground glass screen (the "lens" side) and sometimes behind it (the "viewing" side).

1. If the ground glass screen were a perfect transmissive Lambertian diffuser, then:

a. All the light that landed on any tiny area of it (from the lens) would result in that region exhibiting a certain luminance as seen from behind it, unaffected by the angle at which the rays struck the "front" of the screen.

b. The luminance of the region would be the same from any angle of view. (The distribution of luminous intensity leaving the "back" of the screen would vary as the cosine of the angle from which we view the screen, but since the "projected" area of any region also varies as that cosine, the luminance is constant.)

However, the typical ground glass screen is not a perfect transmissive Lambertian diffuser. As a result, the distribution of light emitted to the rear is skewed in a direction corresponding to the angle of arrival of the rays at the front. Thus, to a viewer observing the screen "head on", there is a greater falloff of brightness toward the edges of the image than there would be otherwise, and the distribution of brightness varies as one's angle of observation changes, neither of these being desirable.

This situation is mitigated by placing a "field lens" in front of the ground glass screen. At regions away from the center of the screen, it refracts the arriving rays so they strike the screen at more nearly a right angle ("normal" incidence", in technical terms). Thus the luminance of the screen to the observer is more uniform over the whole image, and if the screen is observed from an angle, the luminance declines but still remains fairly uniform across the image.

This is the situation of the Fresnel field lens used in modern Graflex focusing plates. SLR cameras such as the Canon EOS series also have a Fresnel field lens in front of the ground glass plate (actually molded integral to it.)

2. If in fact the ground glass were a perfect transmissive Lambertian diffuser, as mentioned before, the light emitted from any small region of the screen is, as mentioned before, is distributed over a full range of viewing angles in a "cosine" distribution. Only a small portion of that light enters the iris of the viewer's eye (or, in an SLR camera with a prism viewfinder, enters the eyepiece lens to be relayed to the eye).

Improved luminance of the image can be achieved by placing a field lens just behind the ground glass screen. This serves to concentrate the distribution of light emitted from any small region about the "normal" (perpendicular) direction, so that a larger fraction of the light enters the iris of the viewer's eye (or, in an SLR camera with prism viewfinder, enters the eyepiece lens to be relayed to the eye.

Thus, SLR cameras such as the Canon EOS series have a Fresnel field lens mounted behind the ground glass screen (as well as one in front of the screen, mentioned above).

Evidently, from the discussions in this thread, both uses of Fresnel field lenses in connection with ground glass viewing are found over the range of large format view cameras.

Best regards,

Doug

[Rider]

Anyone know which ground glass has the proprietary "micorcrystalin wax"?


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."

[Dave Parker]

Anyone know which ground glass has the proprietary "micorcrystalin wax"?

If I am not mistaken, that is the BOSS screen you are talking about.

Dave

[seawolf66]

So I guess the bottom Line on this matter is How the manufacture sit it up, and if the manufacture did not have a fresnel lens G.G set up and some one put it on the camera ,{THEN] what is the proper way to set up this combination up of these two items: I have Linhof technika IV [1957] when I bought it it came with both:: just simple person who likes things to be right on his equipment:

[Bob Salomon]

So I guess the bottom Line on this matter is How the manufacture sit it up, and if the manufacture did not have a fresnel lens G.G set up and some one put it on the camera ,{THEN] what is the proper way to set up this combination up of these two items: I have Linhof technika IV [1957] when I bought it it came with both:: just simple person who likes things to be right on his equipment:

Linhof has over time positioned the fresnel in front of the gg (closer to the lens) and on the back of the gg (so it can be easily removed if the fresnel grooves present a problem when focusing with a loupe).

On your camera the fresnel was positioned under the gg. Service can re-set the gg shims and supply the current fresnel clips if you want to have your fresnel on the top of the gg.