Fresnel Question

[alanmcd]

I just want to get clarification about Fresnel screens

I have a plastic one for my Shen Hao.
I’ve watched Alan Brock’s video but I’m not outlaying for a Maxwell yet.
So if someone can let me know if I understand the following:
1. The screen needs to always be ribs to the lens – flat side to the eye.
2. If you put the screen under the GG, you may need to allow for focus since you are still focussing on the GG not the fresnel and the fresnel is now moving the focal plane 2 mm away.
3. If you put the fresnel on the eye side of the GG, the improved brightness may not be as good as inside the GG.
4. If I operate well within my DOF, the focus issue is not really in play
5. Obviously if I use tilt/shift movements, the DOF (hyperfocal distance) is harder to gauge and therefore moving the Fresnel to the outside would be better under these circumstances.

I’ll do some tests but I just wanted to know what to expect.
Thanks
Alan

[Huub]

I am afraid that your points 4. and 5. are not valid and the vocal issue will be in play, even when using larger appertures. Some of the other forum members will probably have some mathical formulas to illustrate the case.

The good news is that the Shen Hao has probably a feature that allows you to put the fresnel lens in front of the GG, without moving it's position. At least min has. When you take the GG of the camera you will see two slots that will allow for the fresnel lens and letting you put back the GG to it's original position.

[Huub]

I am afraid that your points 4. and 5. are not valid and the vocal issue will be in play, even when using larger appertures. Some of the other forum members will probably have some mathical formulas to illustrate the case.

The good news is that the Shen Hao has probably a feature that allows you to put the fresnel lens in front of the GG, without moving it's position. At least min has. When you take the GG of the camera you will see two slots that will allow for the fresnel lens and letting you put back the GG to it's original position.

[Bob Salomon]

1 all photo fresnels are always plastic.
2 if the fresnel is under the gig you have to have the gig focus positioned adjusted and, should you want to remove it for some reason, you can't without readjusting the gig.
3 the effect of the fresnel s the same, in fact, it is probably brighter as the fresnel correction does not then have to pass through the gig. And the fresnel may be removed at any time without changing gig position.
4 depth of focus changes with focal length of your lens, the shorter your lens the LESS depth of focus. So, since the fresnel has some thickness you cannot rely n depth of focus to correct for proper gig positioning.
5 the fresnel on top of the gig is far superior. One of the main reason for positioning original fresnels under the gig was that they were easily scratched. That s not as true today. And today means the last 3 or 4 decades.

[R.K]

On the Shen Hao cameras there are a special space provided for the Fresnel lens under the ground glass. If yours Fresnel made specifically for the Shen Hao, just remove ground glass, put Fresnel in with grooved side facing the ground glass, not the lens, put ground glass on the top of a Fresnel, place the screws back and you ready to go. If the Fresnel made specifically for the Shen Hao cameras and have a right thickness, the focusing of you camera will not be affected in any way.

[R.K]

If you want to know more on Fresnel's and how they work, follow this link http://www.fresneltech.com/pdf/FresnelLenses.pdf . Or open http://www.fresneltech.com/visible.html page and follow to the linked pdf file. I hope you will understand that grooves must be faced photographer and not the lens, and if you don't want grooves to be damaged because the Fresnel made from plastic and grooves usually very small and can be damaged easily, just keep your Fresnel under the ground glass.

[Jim Jones]

On the Shen Hao cameras there are a special space provided for the Fresnel lens under the ground glass. If yours Fresnel made specifically for the Shen Hao, just remove ground glass, put Fresnel in with grooved side facing the ground glass, not the lens, put ground glass on the top of a Fresnel, place the screws back and you ready to go. If the Fresnel made specifically for the Shen Hao cameras and have a right thickness, the focusing of you camera will not be affected in any way.

Beware, if the Fresnel is placed between the ground glass and the lens, it changes the image focus on the ground glass by roughly 2/3 the thickness of the Fresnel. This might not matter with very thin Fresnels and lenses well stopped down. The link provided above is informative about Fresnel lenses in applications other than the special case of camera ground glasses.

[Tin Can]

What I find amusing is my favorite camera maker Horseman, couldn't decide where to put fresnel. I have 2, which have the fresnel in each location. One front, one back of GG. They both look original and NOS.

Since I shoot them stopped way down I can't SEE the difference, but I
have poor vision. Ymmv

In a couple decades I'll have time to examine the problem.

[R.K]

Beware, if the Fresnel is placed between the ground glass and the lens, it changes the image focus on the ground glass by roughly 2/3 the thickness of the Fresnel. This might not matter with very thin Fresnels and lenses well stopped down. The link provided above is informative about Fresnel lenses in applications other than the special case of camera ground glasses.

So in accordance with post above, the Fresnel on the cameras is a special Fresnel, not just a regular Fresnel lens. Well, some people on this great forum like to make a posts even when they not sure are they right or not.
So, some information for the poster above. On the cameras the Fresnel lens used for the same purpose as on the light houses and any other lighting devises – to combine the rays of light falling on it, make those rays parallel to each other and perpendicular to the flat face of the Fresnel lens. As known the ground glass not only a focusing device, it is in the same time a good reflector and diffuser. So when the Fresnel placed under the ground glass it can combine all light falling on it and send a powerful parallel ray of light though the ground glass to make the focusing work much easier. Of course as any other surfaces the Fresnel also reflecting some light back, but because it is clear and made from the special optical material it reflect much less light then the ground glass. So when the Fresnel located under the ground glass it really help. Some people like to locate Fresnel on the top of the ground glass. This is possible to. But what the Fresnel really doing in that arrangement, almost nothing. In that arrangement the ground glass already reflect some sufficient amount of light back in the camera and this light will never any more reach the Fresnel and the photographer.

[Doremus Scudder]

So in accordance with post above, the Fresnel on the cameras is a special Fresnel, not just a regular Fresnel lens. Well, some people on this great forum like to make a posts even when they not sure are they right or not.
So, some information for the poster above. On the cameras the Fresnel lens used for the same purpose as on the light houses and any other lighting devises – to combine the rays of light falling on it, make those rays parallel to each other and perpendicular to the flat face of the Fresnel lens. As known the ground glass not only a focusing device, it is in the same time a good reflector and diffuser. So when the Fresnel placed under the ground glass it can combine all light falling on it and send a powerful parallel ray of light though the ground glass to make the focusing work much easier. Of course as any other surfaces the Fresnel also reflecting some light back, but because it is clear and made from the special optical material it reflect much less light then the ground glass. So when the Fresnel located under the ground glass it really help. Some people like to locate Fresnel on the top of the ground glass. This is possible to. But what the Fresnel really doing in that arrangement, almost nothing. In that arrangement the ground glass already reflect some sufficient amount of light back in the camera and this light will never any more reach the Fresnel and the photographer.

Most of the above quote is false and/or misleading. I'll try to address things in order:

First, although all Fresnel lenses work on the same principle, they are not all the same. Some are thicker than others and, more importantly, Fresnel lenses have a focal length, just like other lenses. For cameras, a Fresnel lens should ideally be matched to the focal length of the lens being used. This is rarely practical, but a Fresnel with an intermediate focal length works well with all but the shortest and longest lenses usually used on a specific format. Which focal length that is depends on the format, of course.

Yes, Fresnel lenses on cameras work the same as Fresnel lenses on a lighthouse in principle, however, it's function in a camera is slightly different, i.e., to take the angled rays coming from the lens and project them back toward the viewer's eye at an angle that lets the viewer see a larger and brighter image.

The diffusing and reflecting properties of ground glass don't change just because a Fresnel is placed between it and a lens (or between it and the eye of the viewer for that matter). Therefore, it doesn't matter really which side of the ground glass the Fresnel is placed on PROVIDED THAT allowance is made for the focus shift introduced by the Fresnel lens when it is placed between the ground glass and the lens. If this is allowed for, then the brightness and other viewing characteristics of the Fresnel/ground-glass combination are the same.

If you place a Fresnel between ground glass and lens, the position of the Fresnel must be slightly different than that of a plain ground glass without the Fresnel. Not adjusting for this will cause significant focus shift (approx. 2/3 of the thickness of the Fresnel screen) and result in unsharp photographs. Stopping down won't focus the lens in the right place, only reduce the size of the out-of-focus circles of confusion. As mentioned above, shorter lenses (although they have greater depth of field) have a shallower depth of focus and the softness will be most apparent when using these. However, long lenses aren't immune and, unless you're contact printing or only enlarging slightly, you'll be a lot better off having a Fresnel that is positioned properly.

For the OP: If your Shen Hao has a recessed seat for the addition of a Fresnel lens between ground glass and lens, you still have to be sure it is the proper thickness, i.e., the thickness that the manufacturer has designed the recessed seat to compensate for. If you aren't sure of this, then you'll be better off mounting the Fresnel between the ground glass and your eye. Often, in the latter configuration, a protective clear glass piece can be added to protect the Fresnel from scratches.

Best,

Doremus

[Bob Salomon]

What I find amusing is my favorite camera maker Horseman, couldn't decide where to put fresnel. I have 2, which have the fresnel in each location. One front, one back of GG. They both look original and NOS.

Since I shoot them stopped way down I can't SEE the difference, but I
have poor vision. Ymmv

In a couple decades I'll have time to examine the problem.

Not unusual. If you have very old Linhofs the Fresnel was in front of the gg, as mentioned earlier. If you have a later Linhof the Fresnel is placed behind the gg.

[alanmcd]

If you want to know more on Fresnel's and how they work, follow this link http://www.fresneltech.com/pdf/FresnelLenses.pdf . Or open http://www.fresneltech.com/visible.html page and follow to the linked pdf file. I hope you will understand that grooves must be faced photographer and not the lens, and if you don't want grooves to be damaged because the Fresnel made from plastic and grooves usually very small and can be damaged easily, just keep your Fresnel under the ground glass.

Wow! thanks - now I know much more about fresnels than I ever wanted to know :-)
Seriously - that's a good reference thanks

For other posters, I do see the top and bottom rail under the GG in my Shen Hao as lowered below the side rails which would allow my plastic fresnel to sit such that the GG is in the original spot. BUT the fresnel is the same size as the GG. It would appear that I need to trim the fresnel (left and right side, by about 3mm to allow the fresnel to 'sit down'. I shall then place it groove to me under the GG.

Alan

[alanmcd]

I just want to get clarification about Fresnel screens

I have a plastic one for my Shen Hao.
I’ve watched Alan Brock’s video but I’m not outlaying for a Maxwell yet.
So if someone can let me know if I understand the following:
1. The screen needs to always be ribs to the lens – flat side to the eye.
2. If you put the screen under the GG, you may need to allow for focus since you are still focussing on the GG not the fresnel and the fresnel is now moving the focal plane 2 mm away.
3. If you put the fresnel on the eye side of the GG, the improved brightness may not be as good as inside the GG.
4. If I operate well within my DOF, the focus issue is not really in play
5. Obviously if I use tilt/shift movements, the DOF (hyperfocal distance) is harder to gauge and therefore moving the Fresnel to the outside would be better under these circumstances.

I’ll do some tests but I just wanted to know what to expect.
Thanks
Alan

So to summarise:

1. The screen needs to always be ribs to the lens – flat side to the eye. - INCORRECT - ribs to the eye for a collimator fresnel
2. If you put the screen under the GG, you may need to allow for focus since you are still focusing on the GG not the fresnel and the fresnel is now moving the focal plane 2 mm away. - OK - see DOF in 4
3. If you put the fresnel on the eye side of the GG, the improved brightness may not be as good as inside the GG. - I agree with this because my gut is telling me that focusing the light before I diffuse it is better than diffusing it before I focus it.
4. If I operate well within my DOF, the focus issue is not really in play - I focus on hyperfocal distance and max out my DOF to infinity with plenty of room to move for my 65mm and currently my focus is good. I actually focus on the hyperfocal distance BEFORE I compose the shot. But I agree that with my 180mm lens, this will be a problem that I would rather avoid.
5. Obviously if I use tilt/shift movements, the DOF (hyperfocal distance) is harder to gauge and therefore moving the Fresnel to the outside would be better under these circumstances. - MOOT

I have used my extra sharp block plane to shave off both sides of the fresnel that i have and it now sits down nicely in the Shen Hao housing under the GG with ribs on my eye side. The fit isn't so tight (i.e. the thickness of the frenel is such) that the fresnel is held tight. It slides half a mm in it's housing but it confirms that it is not forcing the GG away from the lens. There is virtually no play/slopiness in/out on the focal line vector. So I hope I'm all good to go.
Thanks everyone
Alan

[R.K]

146846
Most of the above quote is false and/or misleading. I'll try to address things in order:

First, although all Fresnel lenses work on the same principle, they are not all the same. Some are thicker than others and, more importantly, Fresnel lenses have a focal length, just like other lenses. For cameras, a Fresnel lens should ideally be matched to the focal length of the lens being used. This is rarely practical, but a Fresnel with an intermediate focal length works well with all but the shortest and longest lenses usually used on a specific format. Which focal length that is depends on the format, of course.

Yes, Fresnel lenses on cameras work the same as Fresnel lenses on a lighthouse in principle, however, it's function in a camera is slightly different, i.e., to take the angled rays coming from the lens and project them back toward the viewer's eye at an angle that lets the viewer see a larger and brighter image.

The diffusing and reflecting properties of ground glass don't change just because a Fresnel is placed between it and a lens (or between it and the eye of the viewer for that matter). Therefore, it doesn't matter really which side of the ground glass the Fresnel is placed on PROVIDED THAT allowance is made for the focus shift introduced by the Fresnel lens when it is placed between the ground glass and the lens. If this is allowed for, then the brightness and other viewing characteristics of the Fresnel/ground-glass combination are the same.

If you place a Fresnel between ground glass and lens, the position of the Fresnel must be slightly different than that of a plain ground glass without the Fresnel. Not adjusting for this will cause significant focus shift (approx. 2/3 of the thickness of the Fresnel screen) and result in unsharp photographs. Stopping down won't focus the lens in the right place, only reduce the size of the out-of-focus circles of confusion. As mentioned above, shorter lenses (although they have greater depth of field) have a shallower depth of focus and the softness will be most apparent when using these. However, long lenses aren't immune and, unless you're contact printing or only enlarging slightly, you'll be a lot better off having a Fresnel that is positioned properly.

For the OP: If your Shen Hao has a recessed seat for the addition of a Fresnel lens between ground glass and lens, you still have to be sure it is the proper thickness, i.e., the thickness that the manufacturer has designed the recessed seat to compensate for. If you aren't sure of this, then you'll be better off mounting the Fresnel between the ground glass and your eye. Often, in the latter configuration, a protective clear glass piece can be added to protect the Fresnel from scratches.

Best,

Doremus


Doremus, I don’t know what kind of enlargement you can see on yours camera ground glass equipped with Fresnel lens. But on my Shen Hao and Ebony, both with Fresnel I don’t see any. What I do see that the image became much lighter after I add Fresnel to my Shen Hao.
Why this happens, because when the Fresnel installed under the ground glass the rays of light falling on the ground glass perpendicular to it surface. As you can see from the table I copied from the book on optics published by Dionysius Lardner in 1851 (this book available for free on Google, if you interested) the number of rays of light reflected and absorbed by the surface depending on the angles of incidence. And the surface absorbing more light rays when the angle of incidence close to 0. Because in the case with the ground glass we see the light what was absorbed by the ground glass the image we see becoming brighter. But this may happens only when the Fresnel installed under the ground glass and direct the light rays to travel perpendicular to the ground glass surface.
The camera focusing absolutely different issue. Of course the camera must be made with some space allocated under the ground glass for the Fresnel. And in case with Shen Hao it is exactly like this. You can find this information on the Shen Hao web site.

[alanmcd]

So in accordance with post above, the Fresnel on the cameras is a special Fresnel, not just a regular Fresnel lens. Well, some people on this great forum like to make a posts even when they not sure are they right or not.
So, some information for the poster above. On the cameras the Fresnel lens used for the same purpose as on the light houses and any other lighting devises – to combine the rays of light falling on it, make those rays parallel to each other and perpendicular to the flat face of the Fresnel lens. As known the ground glass not only a focusing device, it is in the same time a good reflector and diffuser. So when the Fresnel placed under the ground glass it can combine all light falling on it and send a powerful parallel ray of light though the ground glass to make the focusing work much easier. Of course as any other surfaces the Fresnel also reflecting some light back, but because it is clear and made from the special optical material it reflect much less light then the ground glass. So when the Fresnel located under the ground glass it really help. Some people like to locate Fresnel on the top of the ground glass. This is possible to. But what the Fresnel really doing in that arrangement, almost nothing. In that arrangement the ground glass already reflect some sufficient amount of light back in the camera and this light will never any more reach the Fresnel and the photographer.

Fully agree now - I can really see the difference between under the GG and on top of the GG. I can almost trust my focusing (on my longish lens) without a loop when it's under the GG. What a difference! I cannot imagine, now, how it ever works on top of the GG since the rays of light hitting the fresnel are already so scattered by the GG that the collimator effect must be almost totally lost in this arrangement.
thanks
Alan

[R.K]

So to summarise:

1. The screen needs to always be ribs to the lens – flat side to the eye. - INCORRECT - ribs to the eye for a collimator fresnel
2. If you put the screen under the GG, you may need to allow for focus since you are still focusing on the GG not the fresnel and the fresnel is now moving the focal plane 2 mm away. - OK - see DOF in 4
3. If you put the fresnel on the eye side of the GG, the improved brightness may not be as good as inside the GG. - I agree with this because my gut is telling me that focusing the light before I diffuse it is better than diffusing it before I focus it.
4. If I operate well within my DOF, the focus issue is not really in play - I focus on hyperfocal distance and max out my DOF to infinity with plenty of room to move for my 65mm and currently my focus is good. I actually focus on the hyperfocal distance BEFORE I compose the shot. But I agree that with my 180mm lens, this will be a problem that I would rather avoid.
5. Obviously if I use tilt/shift movements, the DOF (hyperfocal distance) is harder to gauge and therefore moving the Fresnel to the outside would be better under these circumstances. - MOOT

I have used my extra sharp block plane to shave off both sides of the fresnel that i have and it now sits down nicely in the Shen Hao housing under the GG with ribs on my eye side. The fit isn't so tight (i.e. the thickness of the frenel is such) that the fresnel is held tight. It slides half a mm in it's housing but it confirms that it is not forcing the GG away from the lens. There is virtually no play/slopiness in/out on the focal line vector. So I hope I'm all good to go.
Thanks everyone
Alan

You did it right. This is exactly the same what I did with my first Fresnel what I installed on the Shen Hao. Just planed out the edges to the correct thickness. After some time I purchased another Fresnel made directly for the Shen Hao and interesting that it was with same planed edges.

[Bob Salomon]

So to summarise:

1. The screen needs to always be ribs to the lens – flat side to the eye. - INCORRECT - ribs to the eye for a collimator fresnel
2. If you put the screen under the GG, you may need to allow for focus since you are still focusing on the GG not the fresnel and the fresnel is now moving the focal plane 2 mm away. - OK - see DOF in 4
3. If you put the fresnel on the eye side of the GG, the improved brightness may not be as good as inside the GG. - I agree with this because my gut is telling me that focusing the light before I diffuse it is better than diffusing it before I focus it.
4. If I operate well within my DOF, the focus issue is not really in play - I focus on hyperfocal distance and max out my DOF to infinity with plenty of room to move for my 65mm and currently my focus is good. I actually focus on the hyperfocal distance BEFORE I compose the shot. But I agree that with my 180mm lens, this will be a problem that I would rather avoid.
5. Obviously if I use tilt/shift movements, the DOF (hyperfocal distance) is harder to gauge and therefore moving the Fresnel to the outside would be better under these circumstances. - MOOT

I have used my extra sharp block plane to shave off both sides of the fresnel that i have and it now sits down nicely in the Shen Hao housing under the GG with ribs on my eye side. The fit isn't so tight (i.e. the thickness of the frenel is such) that the fresnel is held tight. It slides half a mm in it's housing but it confirms that it is not forcing the GG away from the lens. There is virtually no play/slopiness in/out on the focal line vector. So I hope I'm all good to go.
Thanks everyone
Alan

Alan, my answers below are correct. But I think that you are confusing some optical terms:

Depth of field is the area in front of the point that you focus on that appears sharp in front and behind that point. The amount of the acceptable depth of field depends on the circle of confusion that you chose to use, the smaller the CoC the narrower the depth of field will be. The shorter the lens you use the more depth of field will be. The larger the magnification of the print or projection the less the depth of field will be providing you are looking at it from the same distance.

Depth of focus is the area behind the lens that the film has to lie in and the ground side if your gg must lie in to achieve proper focus. The shorter the lens the less depth if focus you will have. The longer the lens the more depth of focus you will have.

Depth of field increases or decreases with the chosen f stop and the maximum depth if field is at the hyper focal distance. This has nothing to do with where the image plane is located behind the lens.

[alanmcd]

I do think I understand. I may not use the terms always correctly. What I mean with regard to this fresnel issue, is:
If I focus on my chosen hyperfocal distance with a proven and acceptable depth of field as per http://www.dofmaster.com/dofjs.html
and a circle of confusion 0.1mm, when NO fresnel is present, and THEN I place the fresnel under the GG, the point at which I focused does not change. It just appears on the GG to now be out of focus.

Typically with my 65mm lens, my workflow is to work with my aperture to select the most foreground object I want focused and decide my overall DOF. It most often includes a desire to have infinity in focus, but not always.
With this worked out and having accepted the shutter speed as desirable, I have a hyperfocal distance. I measure this distance to an object (with a tape or my scope) I can focus on easily and accurately. I frame this object in the middle of the GG and I focus exactly, and lock the focus knobs off. I then re-frame my shot ignoring any focusing. I know at this stage that infinity is in focus (if I planned it) and the nearest object planned to be in focus is also in focus.
I shut down and take the shot. I've worked in an abbreviated fashion like this for many years with my nikon 20mm manual lens by leaving the infinity marker of the focus ring wound over to the f22 or f16 stop mark. I always know what my nearest focus distance is and I never touch the focus again unless I need to check it or move up an f stop. In fact, I would never buy an autofocus 20mm lens I cannot see the need and my type of photography would never benefit from it.

Now (after placing the fresnel under the GG), if I change my focus (ON the chosen hyperfocal object only) to make it appear focused, the point of hyperfocus will change but given the latitude I am working on (i.e. the aperture giving me a total DOF desired), the photo will still be in the about same overall focus - less true for 180mm but more true for my 65mm. I could do some tests to see exactly how this effects my dofmaster spreadsheet but in any regard this point is moot for me now since I have the fresnel under the GG and it has not changed the position of the surface of the GG.

Alan

[alanmcd]

You know what else I could do. If I had no choice but to put the fresnel under the GG and thereby change the characteristics of the camera such that what appeared to be in focus was indeed not, would be to put a similar thickness of spacer under my film holders, such that when it was inserted under the camera back, it would present the film plane to the lens at exactly the same distance as the GG is now situated.

Alan

[R.K]

You know what else I could do. If I had no choice but to put the fresnel under the GG and thereby change the characteristics of the camera such that what appeared to be in focus was indeed not, would be to put a similar thickness of spacer under my film holders, such that when it was inserted under the camera back, it would present the film plane to the lens at exactly the same distance as the GG is now situated.

Alan

Alan, I think this is to much. I'm sure that it is always possible to find some way to place a Fresnel on the camera even if there no space provided under the ground glass. Especially if it is a wooden camera. Wood can be ease cutted and removed if nessesary and the size of a fresnel can be adjusted as required with simple woodworking hand tools. You just did today. So what for to make things more complicated.

[alanmcd]

Alan, I think this is to much. I'm sure that it is always possible to find some way to place a Fresnel on the camera even if there no space provided under the ground glass. Especially if it is a wooden camera. Wood can be ease cutted and removed if nessesary and the size of a fresnel can be adjusted as required with simple woodworking hand tools. You just did today. So what for to make things more complicated.

I said "if I had no choice". I have a choice and there is no need to do this. Just proving I understand the optics.
Alan

[Bob Salomon]

I do think I understand. I may not use the terms always correctly. What I mean with regard to this fresnel issue, is:
If I focus on my chosen hyperfocal distance with a proven and acceptable depth of field as per http://www.dofmaster.com/dofjs.html
and a circle of confusion 0.1mm, when NO fresnel is present, and THEN I place the fresnel under the GG, the point at which I focused does not change. It just appears on the GG to now be out of focus.

Typically with my 65mm lens, my workflow is to work with my aperture to select the most foreground object I want focused and decide my overall DOF. It most often includes a desire to have infinity in focus, but not always.
With this worked out and having accepted the shutter speed as desirable, I have a hyperfocal distance. I measure this distance to an object (with a tape or my scope) I can focus on easily and accurately. I frame this object in the middle of the GG and I focus exactly, and lock the focus knobs off. I then re-frame my shot ignoring any focusing. I know at this stage that infinity is in focus (if I planned it) and the nearest object planned to be in focus is also in focus.
I shut down and take the shot. I've worked in an abbreviated fashion like this for many years with my nikon 20mm manual lens by leaving the infinity marker of the focus ring wound over to the f22 or f16 stop mark. I always know what my nearest focus distance is and I never touch the focus again unless I need to check it or move up an f stop. In fact, I would never buy an autofocus 20mm lens I cannot see the need and my type of photography would never benefit from it.

Now (after placing the fresnel under the GG), if I change my focus (ON the chosen hyperfocal object only) to make it appear focused, the point of hyperfocus will change but given the latitude I am working on (i.e. the aperture giving me a total DOF desired), the photo will still be in the about same overall focus - less true for 180mm but more true for my 65mm. I could do some tests to see exactly how this effects my dofmaster spreadsheet but in any regard this point is moot for me now since I have the fresnel under the GG and it has not changed the position of the surface of the GG.

Alan

You don't understand. If you place the Fresnel under the gg then your gg will move out of the image plane unless the gg has been adjusted for that Fresnel to be under the gg. If you focus without the Fresnel under the gg and then take your back apart to place the Fresnel under it, and then reassemble your back then you have more then likely changed the image plane and you are not properly focused.
It is interesting that inexpensive, budget Chinese cameras place the Fresnels under the gg while top of the line European cameras like Linhof and Sinar place it on top of the gg. Why don't you investigate why the best cameras in the world place it where they do?

[Bob Salomon]

You know what else I could do. If I had no choice but to put the fresnel under the GG and thereby change the characteristics of the camera such that what appeared to be in focus was indeed not, would be to put a similar thickness of spacer under my film holders, such that when it was inserted under the camera back, it would present the film plane to the lens at exactly the same distance as the GG is now situated.

Alan

Alan, if all else fails you can just lie the Fresnel on top of your gg and hold it in place with strips of clear packing tape on the long edges.

[R.K]

[QUOTE=
It is interesting that inexpensive, budget Chinese cameras place the Fresnels under the gg while top of the line European cameras like Linhof and Sinar place it on top of the gg. Why don't you investigate why the best cameras in the world place it where they do?[/QUOTE]

Bob, maybe “crappy” Chinese manufacturers doing this because some good Japanese and American manufacturers did it this way. I mean Toyo, Vista, Ebony, Graflex. All of them having Fresnel below the ground glass with groves facing photographer.

[Dan Fromm]

Bob, maybe “crappy” Chinese manufacturers doing this because some good Japanese and American manufacturers did it this way. I mean Toyo, Vista, Ebony, Graflex. All of them having Fresnel below the ground glass with groves facing photographer.

Um, about Graflex. Pacemaker Graphics' ground glasses sit on raised bosses in the focusing panel. After Graflex Inc. made optional fresnels available they made two versions of the focusing panel. One with high bosses for GG with no focusing panel and one with low bosses for GG with focusing panel. They did this because putting a fresnel in front of the GG shifts the focal plane, end of discussion. Oh, and by the way, both types of focusing panel have the same casting number.

As for Shen Hao, if you search you'll find many complaints about 4x5 Shen Haos that don't focus properly. The reason they didn't turned out to be the fresnel. Crappy!

[R.K]

Um, about Graflex. Pacemaker Graphics' ground glasses sit on raised bosses in the focusing panel. After Graflex Inc. made optional fresnels available they made two versions of the focusing panel. One with high bosses for GG with no focusing panel and one with low bosses for GG with focusing panel. They did this because putting a fresnel in front of the GG shifts the focal plane, end of discussion. Oh, and by the way, both types of focusing panel have the same casting number.

As for Shen Hao, if you search you'll find many complaints about 4x5 Shen Haos that don't focus properly. The reason they didn't turned out to be the fresnel. Crappy!

So Dan, as per you post the Graflex was able to do low bosses for the optional Fresnel and place it under the ground glass. But it looks like nobody in this tread including you cant believe that the "pure" Chinese camera builders cant do the same thing. I don't understand, is it so hard to believe that Shen Hao manufacturer provided place for the optional Fresnel under the ground glass.

And I don't need to search on the Shen Hao. I have one. Agree, the original ground glass was not the best, but after I install the Fresnel and ground glass made by Steve Hopf the camera focusing became like on the any other cameras. The reason- Fresnel together with more finely grinded ground glass made image more lighter, clear visible and ease to focus. Even the Fresnel coupled with original manufacturers ground glass making a big difference.

So lets make a final conclusion. Manufacturer build a not bad camera and want to keep price low. So he didn't include the Fresnel in the package and selling it separately as an additional upgrade. I don't see nothing wrong with this. A lot of manufacturers do the same thing with their products.

[Bob Salomon]

So Dan, as per you post the Graflex was able to do low bosses for the optional Fresnel and place it under the ground glass. But it looks like nobody in this tread including you cant believe that the "pure" Chinese camera builders cant do the same thing. I don't understand, is it so hard to believe that Shen Hao manufacturer provided place for the optional Fresnel under the ground glass.

And I don't need to search on the Shen Hao. I have one. Agree, the original ground glass was not the best, but after I install the Fresnel and ground glass made by Steve Hopf the camera focusing became like on the any other cameras. The reason- Fresnel together with more finely grinded ground glass made image more lighter, clear visible and ease to focus. Even the Fresnel coupled with original manufacturers ground glass making a big difference.

So lets make a final conclusion. Manufacturer build a not bad camera and want to keep price low. So he didn't include the Fresnel in the package and selling it separately as an additional upgrade. I don't see nothing wrong with this. A lot of manufacturers do the same thing with their products.

RK, Linhof and Sinar also do not include the Fresnel with their cameras, they are an accessory.

Look, it is really very simple. A Fresnelon better cameras is placed on top of the gg because there are times when the photographer does not want the magnified groove sin the Fresnel interfering with the image while focusing and other times when it makes no difference. So the photographer can easily remove the Fresnel for those times. Since modern Fresnel screens made in the West are far more scratch resistant then early versions like the Kodak Ektalte Screen the manufacturers like Sinar and Linhof have not been bothered by modern Fresnels scratching and find the benefit of a user removable and installable Fresnel a big advantage.

[R.K]

RK, Linhof and Sinar also do not include the Fresnel with their cameras, they are an accessory.

Look, it is really very simple. A Fresnelon better cameras is placed on top of the gg because there are times when the photographer does not want the magnified groove sin the Fresnel interfering with the image while focusing and other times when it makes no difference. So the photographer can easily remove the Fresnel for those times. Since modern Fresnel screens made in the West are far more scratch resistant then early versions like the Kodak Ektalte Screen the manufacturers like Sinar and Linhof have not been bothered by modern Fresnels scratching and find the benefit of a user removable and installable Fresnel a big advantage.

This is a total nonsense Bob. I cant believe that the manufacturers like Linhof or Sinar selling their cameras tor a thooooousands of dollars, expecting the photographer somewhere in the field, say on the mountain peak to stay with screwdriver and removing or installing the Fresnel on the camera. This is a real nonsense. What a good manufacturer doing that providing a photographer with an optional fully assembled camera's focusing back equipped with Fresnel appropriate for the wide angle lenses. This is exactly what Ebony doing. Of course that additional back is an extra and cost $$$. But if photographer need it, he can have it. And if you want to verify that, just open the Ebony web site or visit their distributors in US page - Badger Graphics.

And the second thing about modern materials for the Fresnel. Unfortunately this materials same as before - it is plastic. And maybe that plastic a bit stronger then what used a years ago, but it still soft enough and easy can be damaged especially on the grooved side. I don't know Bob when you last time install the new Fresnel, but I got new Fresnel for my Shen Hao just a two years ago. And actually it was made not by pure Chinese manufacturer. My was made in Australia. Bob, the lens was brand new, just manufactured, from today materials and it was soft enough. I don't even can imagine that Fresnel to be on the front of my ground glass.

But I think I'm done with this tread. The OP did exactly what I suggest him and he is happy now with his Fresnel and camera. And you people just don't want or maybe cant listen to anything. I was trying to explain you the purpose of the Fresnel, explain how it works on the camera even provided you with a useful links from the Fresnel manufacturer. Still I cant change yours mind. So good luck to you. Continue to keep Fresnel's on the top of the ground glass on your cameras, and exchange it every 3-4 years when it gets scratched enough. Good luck.

[Bob Salomon]

This is a total nonsense Bob. I cant believe that the manufacturers like Linhof or Sinar selling their cameras tor a thooooousands of dollars, expecting the photographer somewhere in the field, say on the mountain peak to stay with screwdriver and removing or installing the Fresnel on the camera. This is a real nonsense. What a good manufacturer doing that providing a photographer with an optional fully assembled camera's focusing back equipped with Fresnel appropriate for the wide angle lenses. This is exactly what Ebony doing. Of course that additional back is an extra and cost $$$. But if photographer need it, he can have it. And if you want to verify that, just open the Ebony web site or visit their distributors in US page - Badger Graphics.

And the second thing about modern materials for the Fresnel. Unfortunately this materials same as before - it is plastic. And maybe that plastic a bit stronger then what used a years ago, but it still soft enough and easy can be damaged especially on the grooved side. I don't know Bob when you last time install the new Fresnel, but I got new Fresnel for my Shen Hao just a two years ago. And actually it was made not by pure Chinese manufacturer. My was made in Australia. Bob, the lens was brand new, just manufactured, from today materials and it was soft enough. I don't even can imagine that Fresnel to be on the front of my ground glass.

But I think I'm done with this tread. The OP did exactly what I suggest him and he is happy now with his Fresnel and camera. And you people just don't want or maybe cant listen to anything. I was trying to explain you the purpose of the Fresnel, explain how it works on the camera even provided you with a useful links from the Fresnel manufacturer. Still I cant change yours mind. So good luck to you. Continue to keep Fresnel's on the top of the ground glass on your cameras, and exchange it every 3-4 years when it gets scratched enough. Good luck.

RK, one last time, you aren't reading. Both Sinar and Linhof place the Fresnel on top of the gg and neither company requires any type of tool to install or remove the Fresnel at the users choose.

The last Fresnel was when I stopped being the Linhof Product Manager/Sales a Manager fir the USA a year ago after 35 years of doing so. The last time I showed a photographer how to install and remove the Fresnel from a Linhof was last October at the NY Photo Show where I was working in the Linhof and Rodenstock booths. I also, in the 70s was the Sinar salesman and in the early 60s used both Sinar and Linhof in my studio. Even back in the mid 60s both of the companies placed the Fresnel on top of the gg.

[djdister]

Well since we're just arguing now, how about what Mike Walker did: "The focusing screen is a one piece acrylic screen with etched surface on one side and Fresnel on the other, with a separate cover glass which has a grid pattern plus markings for roll film."

[Bob Salomon]

Well since we're just arguing now, how about what Mike Walker did: "The focusing screen is a one piece acrylic screen with etched surface on one side and Fresnel on the other, with a separate cover glass which has a grid pattern plus markings for roll film."

You just described the old Linhof Super Screen and glass overlay grid. In theory it works fine but we had to discontinue it as, depending on the environment the camera is used in and how it is transported, it was possible for it to warp. And that led to out of focus images. We had the screen material changed by the manufacturer, Fresnel Optics - they owned Beatie, but were never was able to find one that would solve the warping problem. Even a thin glass plate under the gg and having the gg position adjusted by service to compensate for the bottom glass, did not solve the problem. The closest cure that we could find was placing a match stick under each long side of the screen to prevent it from sagging. But that was only stop gap and did not really solve the problem. Then the Linhof factory changed their gg to a much finer grained one and supplied brighter Fresnel screens and there was no longer a need for the Super Screen anyway.
The Super Screen was only sold in the USA during the mid 80s to the early 90s.

[alanmcd]

You don't understand. If you place the Fresnel under the gg then your gg will move out of the image plane unless the gg has been adjusted for that Fresnel to be under the gg. If you focus without the Fresnel under the gg and then take your back apart to place the Fresnel under it, and then reassemble your back then you have more then likely changed the image plane and you are not properly focused.
It is interesting that inexpensive, budget Chinese cameras place the Fresnels under the gg while top of the line European cameras like Linhof and Sinar place it on top of the gg. Why don't you investigate why the best cameras in the world place it where they do?

No - in my case, the GG stays exactly where it was before the fresnel was installed. There is a place inside the camera back which allows the fresnel to sit, between lens and GG which does not alter the GG image plane.
I'd like to know why they place it above because it's nowhere near as effective in providing brightness to the GG.
Alan

[alanmcd]

Bob, maybe “crappy” Chinese manufacturers doing this because some good Japanese and American manufacturers did it this way. I mean Toyo, Vista, Ebony, Graflex. All of them having Fresnel below the ground glass with groves facing photographer.

And what about the Maxwell fresnel/GG combinations? Doesn't he make it so the fresnel side is TO the lens?
Alan

[R.K]

And what about the Maxwell fresnel/GG combinations? Doesn't he make it so the fresnel side is TO the lens?
Alan

Yes Alan this is possible. Read again that brochure what I recommend yesterday. They having some explanations on this type of Fresnel to.

[alanmcd]

I'm sorry we can't all agree.
I've just done some tests with my camera. There is absolutely no question that the screen is brighter with the fresnel on the lens side than it is on the eye side of the GG. As long as the ribs are pointing to the eye.
I have also tested the focus. I can focus with the fresnel then take it away and see absolutely no need to re-focus. I can focus on a different subject without the fresnel, then place it in it's slotted, designed position and again have no desire to re-focus.
I'm leaving it under the GG for the next few live sessions and I will watch closely to any focus issues compared with the shots I have already done without the fresnel in place.
Alan

[alanmcd]

Yes Alan this is possible. Read again that brochure what I recommend yesterday. They having some explanations on this type of Fresnel to.

Yes - that's what prompted me to look at Alan Brock's video:
https://www.youtube.com/watch?v=j_2kvBDg63s&list=PLURXjukJmKt13LhUtnVgWSedDx4PqWHJ6
Except that he's not absolutely clear on the manufacture of the screen he likes and I cannot find drawings of it
Alan

[R.K]

Yes - that's what prompted me to look at Alan Brock's video:
https://www.youtube.com/watch?v=j_2kvBDg63s&list=PLURXjukJmKt13LhUtnVgWSedDx4PqWHJ6
Except that he's not absolutely clear on the manufacture of the screen he likes and I cannot find drawings of it
Alan

As I understand the direction of grooves depends on the actual design of the Fresnel. And in the most simple standard design situation groves must face the photographer. And I already checked that the Linhof providing Fresnel on the top of the ground glass. But well, for the money what Linhof charging, they probably can do whatever they want. Don't know maybe their ground glass so bright that the Fresnel can stay on the top of it and just direct the light rays absorbed by ground glass to the photographer and this is more than enough for fine focusing. But we are not talking about the Linhof or Sinar. We talking about simple less expensive cameras, and in this cameras in most of the cases I have seeing the Fresnel below ground glass with grooves facing photographer.

[alanmcd]

Yes Alan this is possible. Read again that brochure what I recommend yesterday. They having some explanations on this type of Fresnel to.

RK, the paper from fresneltech describes Positive/Collector, Positive/Collimator and Negative/Diverger
I know my fresnel is positive because I can see it under my loupe. I can't actually believe that someone would use a negative fresnel in any of the positions I mention below so I assume negative fresnels aren't supplied to LF cameras at all.

I don't know if the Maxwell fresnel is positive or negative but Alan Brock's video shows dramatic fall off when you are not viewing in the sweet spot. He also documents that the ribs are placed pointing the lens. This suggests to me that the Maxwell is positive and being used as a collector. Alan also comments that the focusing task with the Shen Hao fresnel is hampered by the ribs. I can testify that when the ribs are against the GG, their tops are more in focus and therefore more visible. If the ribs point to the lens, the ribs are not as visible during focus but the brightness almost 'switches off' as you move your eye from centre spot. This also goes to confirm in my tests that if you prefer less of this immediate switch-off, and you can tolerate the ribs being more visible to the focus task, then you place the ribs pointing to the eye. If you wish to remove the hard rib lines from the focus task (they are softer when pointing to the lens side), and you don't mind the light switching off a lot when you eye moves from the centre, you can place the fresnel with ribs to the lens.
Either way, in my tests, as long as the GG is constantly in the same place, focusing still takes place on the GG and focus does not change with or without fresnel in place.

I think for people who don't understand I am enclosing a photo of my Shen Hao back showing the ledge for the GG and the fresnel on its own ledge under the GG.
146896

You can also place the fresnel on the eye side of the GG. It makes a difference in brightness but no where near as much a difference as it does on the lens side of the GG. And, if the fresnel is on the eye side of the GG, you can probably, on some cameras, more easily remove and replace it. But not on my camera. It's the same screws.

This probably all explains why some cameras place it on top, some below, some ribs in, some ribs out. All the positions have some merit and some trade-off. You choose what's best for your taste. I can at least now say that I understand the trade-offs and can choose which way I prefer it.

For the moment, I'm leaving it lens side, ribs pointing to the eye. Maybe I'll report back sometime if I decide to change that setup and the reasons why.

[Whir-Click]

I purchased a Maxwell screen for my 8x10 Kodak 2-D. In the customary pre-purchase discussions with Bill Maxwell, he explained that not only would mounting the fresnel under the ground glass provide protection, it would also provide superior brightness. I can't remember the technical explanation, but he's the optical physicist and I was happy to take his word. He also cautioned me that the fresnel ribs are extremely delicate and must not be touched.

For installation, I followed Bill's recommendation and sent the camera back, fresnel, and gg to Bill Moritz of Pro Camera in Chatlottesville, VA. Bill milled the camera back to allow the insertion of the fresnel under the ground glass and checked alignment of the film and focus plane. I looked at the fresnel just now to confirm that the smooth side faces the lens and the ribs are in contact with the ground glass, facing the photographer.

I have been fully satisfied by the quality of both Maxwell's screen and Bill Moritz's installation. The Maxwell screen is an utter joy to use.

[alanmcd]

I purchased a Maxwell screen for my 8x10 Kodak 2-D. In the customary pre-purchase discussions with Bill Maxwell, he explained that not only would mounting the fresnel under the ground glass provide protection, it would also provide superior brightness. I can't remember the technical explanation, but he's the optical physicist and I was happy to take his word. He also cautioned me that the fresnel ribs are extremely delicate and must not be touched.

For installation, I followed Bill's recommendation and sent the camera back, fresnel, and gg to Bill Moritz of Pro Camera in Chatlottesville, VA. Bill milled the camera back to allow the insertion of the fresnel under the ground glass and checked alignment of the film and focus plane. I looked at the fresnel just now to confirm that the smooth side faces the lens and the ribs are in contact with the ground glass, facing the photographer.

I have been fully satisfied by the quality of both Maxwell's screen and Bill Moritz's installation. The Maxwell screen is an utter joy to use.

Fascinating! Alan Brock definitely went against my expectation and placed the ribs of his Maxwell screen pointing to the lens (as show in his video). But you confirm that Pro Camera did it the other way. I can only assume that Pro Camera did that under Bill Maxwell's instruction.
What a wonderful story this all tells.

I'm sure Bill's screens are far more delicate than my plastic screen but I'll stop touching it from here on anyway.
Alan

[alanmcd]

I purchased a Maxwell screen for my 8x10 Kodak 2-D. In the customary pre-purchase discussions with Bill Maxwell, he explained that not only would mounting the fresnel under the ground glass provide protection, it would also provide superior brightness. I can't remember the technical explanation, but he's the optical physicist and I was happy to take his word. He also cautioned me that the fresnel ribs are extremely delicate and must not be touched.

For installation, I followed Bill's recommendation and sent the camera back, fresnel, and gg to Bill Moritz of Pro Camera in Chatlottesville, VA. Bill milled the camera back to allow the insertion of the fresnel under the ground glass and checked alignment of the film and focus plane. I looked at the fresnel just now to confirm that the smooth side faces the lens and the ribs are in contact with the ground glass, facing the photographer.

I have been fully satisfied by the quality of both Maxwell's screen and Bill Moritz's installation. The Maxwell screen is an utter joy to use.

As another aside, the Bill Maxwell screens are quite thick, (again as I see them in Alan Brock's video) compared with 'standard'? plastic ones. I'm sure that's the reason why the camera back need to be milled. IN my situation, with a cheap plastic fresnel, the Shen Hao is already milled to accept the plastic and it sits very nicely without effecting the focus plane. Since the GG doesn't move the film plane will be unaffected.

Alan Brock, on the other hand has bent his clips up to accommodate the extra thickness or the Maxwell screen and I can't help feeling that by doing this, his focus will be out. He should have milled the ledge down so the GG remained in the same place.

Alan

[R.K]

As another aside, the Bill Maxwell screens are quite thick, (again as I see them in Alan Brock's video) compared with 'standard'? plastic ones. I'm sure that's the reason why the camera back need to be milled. IN my situation, with a cheap plastic fresnel, the Shen Hao is already milled to accept the plastic and it sits very nicely without effecting the focus plane. Since the GG doesn't move the film plane will be unaffected.

Alan Brock, on the other hand has bent his clips up to accommodate the extra thickness or the Maxwell screen and I can't help feeling that by doing this, his focus will be out. He should have milled the ledge down so the GG remained in the same place.

Alan

So looks like we all now on the same page. And I think we are correct. But just for curiosity I suggest you guys to visit the Linhof site. I was wery surprised that Linhof located fresnel on the top of the GG. Why, I don't know.

[alanmcd]

So looks like we all now on the same page. And I think we are correct. But just for curiosity I suggest you guys to visit the Linhof site. I was wery surprised that Linhof located fresnel on the top of the GG. Why, I don't know.

Light from the camera lens is travelling, by design, in a direct path from the lens to the GG. The rays are diverging from the lens aperture but they are not being scattered by the lens to any larger degree than the lens coverage area. On the other hand, light coming through a GG plate is being scattered. The rays are coming out in lots or strange directions. If you place the fresnel on top of the GG you will only catch and take benefit of the significantly reduced number of rays which are aligned with the desired angle of incidence falling on the fresnel.
Possible Reason 1: Putting the fresnel on top of the GG provides ease of installation. You still get a slight increase in brightness and depending on the quality of the fresnel, it could still be better than a much cheaper plastic fresnel placed under the GG.
Possible Reason 2: You are also not seeing the ribs while focusing since the ribs aren't near the film (focal plane). The focal plane being the lens side of the GG, the ribs are now 2mm? away.
Again - it appears to be the Linhof engineer's personal preference mixed with a reluctance to provide for an internal installation alternative.
Just guessing.
Alan

[Greg]

I have owned and used a Nikon Multiphot for the past 30 years for photomacrophotography and photomicrography. This instrument's alignment tolerances are second to none. Not a 4x5 view camera out there that could match its alignment tolerances. Anyways, took it apart to clean it a few years ago and then reassemled it all back together. Shot some 4x5 images with the Multiphot over my Nikon Ske microscope and the images were out of focus. FYI: images from microscopes exhibit literally no of depth of field... if the objectives had a diaphragm, stopping them down by 1 f/stop.... diffraction would render the image unsharp (I may exaggerate a little here, but not by much) Turns out I had put the fresnel lens behind the ground glass. Correct orientation is the fresnel lens slips in first and the ground glass above it. Have been told that Nikon's reasoning to put the fresnel lens in front of the ground glass was to be able to view the optimal high mag image through their 4x5 reflex finder.

[Bob Salomon]

I have owned and used a Nikon Multiphot for the past 30 years for photomacrophotography and photomicrography. This instrument's alignment tolerances are second to none. Not a 4x5 view camera out there that could match its alignment tolerances. Anyways, took it apart to clean it a few years ago and then reassemled it all back together. Shot some 4x5 images with the Multiphot over my Nikon Ske microscope and the images were out of focus. FYI: images from microscopes exhibit literally no of depth of field... if the objectives had a diaphragm, stopping them down by 1 f/stop.... diffraction would render the image unsharp (I may exaggerate a little here, but not by much) Turns out I had put the fresnel lens behind the ground glass. Correct orientation is the fresnel lens slips in first and the ground glass above it. Have been told that Nikon's reasoning to put the fresnel lens in front of the ground glass was to be able to view the optimal high mag image through their 4x5 reflex finder.

Greg, nothing strange here. If the device was originally sold with the Fresnel in front if the gg then when you put the Fresnel behind the gg the gg positioning has to be adjusted. That was mentioned several times in this thread. Also, in order to use a reflex viewing device or a focus metering bellows on a Linhof you also need to use a Fresnel, otherwise you would not see the edges and corners due to the fall off normally seen on a gg without a Fresnel.

[Doremus Scudder]

I've been following this thread with interest since I posted, especially the notion that positioning the Fresnel between ground glass and lens results in a brighter image. That may well be to some extent, however I've just examined all the factory Fresnel sandwiches that I own (two factory Wista Fresnel screens and one from a Graphic View II) and all of these are arranged in the following way: First in the path of the light from the lens is the ground glass, i.e., the surface upon which the image from the lens is projected and viewed, with the ground surface facing the lens. Then comes the Fresnel screen with the rings facing the ground glass. Finally there is a protective cover of clear glass.

The Wista screen is known for its brightness and ease of viewing. For my part, I often focus without a darkcloth when using the Wista screens and the Graphic View as well. If positioning the Fresnel first in the light path makes a difference in brightness, it can't be much.

In any case, if you install a Fresnel between the ground glass and the lens without repositioning the ground glass, there will be a discrepancy in focus between the ground glass and the film. Alan, if I were you I'd do the classic focus test using your new set-up: lay a ruler down on the table, point the camera at it at a 45° angle and focus on the 6" mark. Make a negative up close and wide open with the shortest lens you have (shallowest Depth ofFocus) and see if the 6" mark is indeed in focus in the negative. If so, you're good to go, if not, you'll likely have to do some shimming of your ground glass.

Best,

Doremus

[Maris Rusis]

Try these numbers:

Assuming an acrylic fresnel 2mm thick and of a refractive index 1.49 is placed on the lens side of a ground glass. The optical path length of such a fresnel is pretty nearly 3mm compared to its physical path length of 2mm.
The effect is that the ground glass surface now has to be racked back an extra 1mm to show a sharp image.
When the film holder is placed in the camera the film finds itself 1mm too far back for sharp focus of the original subject.
If the fresnel was now, in theory, placed on the film surface sharp focus (ignoring fresnel aberrations) would be restored. But this is not what happens.

How bad is the error? If the camera back is not re-engineered to allow for the fresnel between lens and ground glass a 150mm lens focussed on apparent infinity would actually be focussed on about 23 metres as far as the film is concerned. Depending on lens aperture and depth of field this might not be obvious but it is an error all the same.

[alanmcd]

Try these numbers:

Assuming an acrylic fresnel 2mm thick and of a refractive index 1.49 is placed on the lens side of a ground glass. The optical path length of such a fresnel is pretty nearly 3mm compared to its physical path length of 2mm.
The effect is that the ground glass surface now has to be racked back an extra 1mm to show a sharp image.
When the film holder is placed in the camera the film finds itself 1mm too far back for sharp focus of the original subject.
If the fresnel was now, in theory, placed on the film surface sharp focus (ignoring fresnel aberrations) would be restored. But this is not what happens.

How bad is the error? If the camera back is not re-engineered to allow for the fresnel between lens and ground glass a 150mm lens focussed on apparent infinity would actually be focussed on about 23 metres as far as the film is concerned. Depending on lens aperture and depth of field this might not be obvious but it is an error all the same.

OK - I appreciate this info.
DOF may be disguising my observations.
This photo taken yesterday f/22 has the plant in the foreground in pretty good focus. It appeared in focus on the GG and the fresnel is lens side of the GG. I did have rear tilt on this photo so there was very little DOF on the plant relative to the whole scene while I was focusing at f5.6.
https://www.flickr.com/photos/metadigital/24789841239/
I also concede that Alan Brock adjusted his clips for the thickness of the protective glass over the Maxwell Screen. The screen was, however fitted by him with no re-engineering of the camera back. Perhaps the Maxwell screen is manufactured "knowing" the camera back and ensuring in the screen that the focus plane is where it needs to be.
I'm going to test lens side of GG/ribs forward next. And I'll try a wider aperture,
Alan

[schafphoto]

FWIW, I was reading this thread for info about placing a Cambo fresnel on my camera. I found the Cambowide manual online which states:
Fresnel Lens Screen (C-158) The Fresnel lens, which is smaller than your ground glass, fits over the ground glass, with the smooth side facing you, and the etched side towards the ground glass. The Fresnel lens fits between the
retainer clips, not behind these.