Fresnel lens in front of the ground glass or behind the ground glass?

I've been shooting LF for a few decades and I've only seen fresnel lenses in front of the ground glass, the side facing out. First first experience with a fresnel lens was when I bought my Sinar F. Before then, I had a Cambo that didn't have one. I saw you YouTube a person mounting the fresnel on the bellows side.

Are there benefits of mounting the fresnel lens on the inside facing the bellows vs mounting it facing out?

If your camera does not have a fresnel on the lens side now you will create a focus shift if you put one their now, unless someone adjusts the cameras film plane.
Putting it on the lens side eliminates that problem.

Bob S said:

If your camera does not have a fresnel on the lens side now you will create a focus shift if you put one their now, unless someone adjusts the cameras film plane.
Putting it on the lens side eliminates that problem.

Click to expand...

+1

A fresnel smooths out hot spots making the shadow detail easier to see and focusing easier.
The ribbed side of the fresnel always goes against the ground glass.
A fresnel is a lens and as such the ground glass has to be moved back the optical distance of the fresnel when the fresnel is mounted on the same bosses in the frame as the ground glass.
IF you mount the fresnel in front of the ground glass (lens side) without moving the ground glass position then no compensation is needed. This means you have to suspend the fresnel in front of the ground glass or modify the camera so that the fresnel mounts to the focus panel without changing the ground glass position. I was able to do this on a Meridian 45B a few years ago.

I never had a fresnel on 4x5's, but they were invaluable on TLR cameras. They went on the bottom of the GG (w/ the fresnel closest to the mirror/lens), and of course this would result in having to reset the focus a little. On old TLR's, replacing the mirror, adding a fresnel and swapping out the standard issue GG for a brighter one made a huge difference in image brightness on the GG.

Bob S said:

If your camera does not have a fresnel on the lens side now you will create a focus shift if you put one their now, unless someone adjusts the cameras film plane.
Putting it on the lens side eliminates that problem.

Click to expand...

Are you sure the statement in the second sentence is correct in reference to what is stated in the first? .....fresnel on the lens side will create a focus shift .....putting it on the lens side eliminates that problem...

Ron (Netherlands) said:

Are you sure the statement in the second sentence is correct in reference to what is stated in the first? .....fresnel on the lens side will create a focus shift .....putting it on the lens side eliminates that problem...

Click to expand...

hmm, it loos like a typo to me. Putting the fresnel on the side away form the lens seem to be the approach that avoides the focus shift.

Ron (Netherlands) said:

Are you sure the statement in the second sentence is correct in reference to what is stated in the first? .....fresnel on the lens side will create a focus shift .....putting it on the lens side eliminates that problem...

Click to expand...

If the gg was not positioned for the fresnel on the lens side and you put one there you will create a focus shift as the fresnel will position the gg in the wrong plane.

If you place a Fresnel screen between lens and the ground glass, the whole sandwich has to be repositioned in order to focus correctly. Just placing the frosted surface of the ground glass back where it was originally will not do the job; the Fresnel screen changes focus depth by about 1/3 of its thickness too.

Non-OEM Fresnel screens are better placed between ground glass and the eye.

Doremus

Doremus Scudder said:

If you place a Fresnel screen between lens and the ground glass, the whole sandwich has to be repositioned in order to focus correctly. Just placing the frosted surface of the ground glass back where it was originally will not do the job; the Fresnel screen changes focus depth by about 1/3 of its thickness too.

Non-OEM Fresnel screens are better placed between ground glass and the eye.

Doremus

Click to expand...

Since the OP is talking about 4x5 large format camera does it matter if there is a focus shift since focus is obtained by moving the lens ot obtain a sharp focus on the GG or Fresnel. Or are you saying that even if the scene is in focus on the GG or Fresnel it will still be out of focus?

The ground glass and the film must be at the same distance from the lens.

If a camera was designed without a fresnel in mind, and you place a fresnel between the lens and the ground glass, the ground glass has now shifted relative to the film plane, and your resulting photo will be out of focus, even though the image on the ground glass was in focus.

If you place the fresnel between the ground glass and the viewer (you), then you haven't moved the plane of the ground glass, and the film and ground glass will both still be at the same focal length from the lens.

beemermark said:

Since the OP is talking about 4x5 large format camera does it matter if there is a focus shift since focus is obtained by moving the lens ot obtain a sharp focus on the GG or Fresnel. Or are you saying that even if the scene is in focus on the GG or Fresnel it will still be out of focus?

Click to expand...

The scene on the negative will be out of focus. That is the whole point of all the posts. The Fresnel lens must be on the outside of the ground glass or the camera focusing will have to be readjusted.

It depends on whether the camera designers factored in existence and position of the fresnel when they designed the back.
I could be wrong, but I think most fresnels are behind the ground glass, but they don't have to be.

MattKing said:

It depends on whether the camera designers factored in existence and position of the fresnel when they designed the back.
I could be wrong, but I think most fresnels are behind the ground glass, but they don't have to be.

Click to expand...

True, however the question posed was for cameras which were designed without the Fresnel lens. It is the addition after the fact is the question in play.

IF you place a fresnel between the lens and ground glass on a camera that never had a fresnel THEN if you move the position of the ground glass to mount the fresnel then the focus at the film plane will be off unless you modify the back to put the ground glass back to the optically correct position, BUT if you can mount the fresnel between the lens and ground glass without moving the ground glass position then no adjustment to the ground glass is needed.
It is indeed easier to put the fresnel between the ground glass and viewer.

grat said:

The ground glass and the film must be at the same distance from the lens.

If a camera was designed without a fresnel in mind, and you place a fresnel between the lens and the ground glass, the ground glass has now shifted relative to the film plane, and your resulting photo will be out of focus, even though the image on the ground glass was in focus.

If you place the fresnel between the ground glass and the viewer (you), then you haven't moved the plane of the ground glass, and the film and ground glass will both still be at the same focal length from the lens.

Click to expand...

Guess I don't it. If the Fresnel moves the plain of focus back 1/32" then by my way of thinking moving the adjustable bellows on the view camera to move the lens board back 1/32" would maintain focus. And as long as focus in sharp on the GG then the negative will be sharp since the film sets behind the GG. And no matter where the Fresnel lens is you are always focusing on the GG. For what it's worth the directions for my Toyo 45AII has the Fresnel between the GG and the lens board.

beemermark said:

the film sets behind the GG

Click to expand...

????

beemermark said:

And as long as focus in sharp on the GG then the negative will be sharp since the film sets behind the GG. And no matter where the Fresnel lens is you are always focusing on the GG. For what it's worth the directions for my Toyo 45AII has the Fresnel between the GG and the lens board.

Click to expand...

The film does not sit behind the ground glass-- you'd be taking a picture through a frosted piece of glass, and while that might look nifty, it's never going to be sharp or clear.

Yes, you focus on the ground glass. Then you insert a film holder, which pushes the ground glass out of the way, and sets the film at the same plane of focus as the ground glass-- but that's a predetermined location, and the film holder has no way to know that you've actually moved the ground glass from it's original plane of focus by inserting a fresnel (or any other shim, spacer, etc.) in the way.

Take a 4x5 camera, with a 150mm lens on the front, with no fresnel lens. The ground glass, at infinity, will be 150mm from the focal point of the lens. Ordinarily, when you put the film holder in, the film will also be 150mm from the focal point. But if you put in a 2mm thick fresnel, now the ground glass surface is 152mm away, and you have to shorten your draw by 2mm to compensate. But when you switch to the film holder, it's now 148mm from the focal point, and your photograph is out of focus.

Cameras like your Toyo that's designed to have the fresnel between the lens and the GG will set the film plane back by 2mm to compensate, so that you're at 150mm for both.

grat said:

The film does not sit behind the ground glass-- you'd be taking a picture through a frosted piece of glass, and while that might look nifty, it's never going to be sharp or clear.

Yes, you focus on the ground glass. Then you insert a film holder, which pushes the ground glass out of the way, and sets the film at the same plane of focus as the ground glass-- but that's a predetermined location, and the film holder has no way to know that you've actually moved the ground glass from it's original plane of focus by inserting a fresnel (or any other shim, spacer, etc.) in the way.

Take a 4x5 camera, with a 150mm lens on the front, with no fresnel lens. The ground glass, at infinity, will be 150mm from the focal point of the lens. Ordinarily, when you put the film holder in, the film will also be 150mm from the focal point. But if you put in a 2mm thick fresnel, now the ground glass surface is 152mm away, and you have to shorten your draw by 2mm to compensate. But when you switch to the film holder, it's now 148mm from the focal point, and your photograph is out of focus.

Cameras like your Toyo that's designed to have the fresnel between the lens and the GG will set the film plane back by 2mm to compensate, so that you're at 150mm for both.

Click to expand...

I wanted to post that the ground glass is moved out of the way for the film holder, but @grat made a better write up.

Remove the back from the camera. Insert a film holder with a scrap sheet of film in it and the darkslide removed. Lay a straight edge across the back, film side, on edges that are the same thickness. Measure from the straight edge to the surface of the film. Now remove the film holder and measure to the ground surface of the ground glass, it should be identical to the film surface distance, if not shim the ground glass until it is.
Insert a fresnel and you now have to compensate for the fresnel's optical distance which may not match the fresnel's physical thickness.
Most focus panels of the camera's back do not allow you to mount a fresnel without moving the ground glass.
A fresnel on the viewing side of the ground glass will function as well as one in front of the ground glass.

grat said:

The film does not sit behind the ground glass-- you'd be taking a picture through a frosted piece of glass, and while that might look nifty, it's never going to be sharp or clear.

Yes, you focus on the ground glass. Then you insert a film holder, which pushes the ground glass out of the way, and sets the film at the same plane of focus as the ground glass-- but that's a predetermined location, and the film holder has no way to know that you've actually moved the ground glass from it's original plane of focus by inserting a fresnel (or any other shim, spacer, etc.) in the way.

Take a 4x5 camera, with a 150mm lens on the front, with no fresnel lens. The ground glass, at infinity, will be 150mm from the focal point of the lens. Ordinarily, when you put the film holder in, the film will also be 150mm from the focal point. But if you put in a 2mm thick fresnel, now the ground glass surface is 152mm away, and you have to shorten your draw by 2mm to compensate. But when you switch to the film holder, it's now 148mm from the focal point, and your photograph is out of focus.

Cameras like your Toyo that's designed to have the fresnel between the lens and the GG will set the film plane back by 2mm to compensate, so that you're at 150mm for both.

Click to expand...

grat said:

The film does not sit behind the ground glass-- you'd be taking a picture through a frosted piece of glass, and while that might look nifty, it's never going to be sharp or clear.

Yes, you focus on the ground glass. Then you insert a film holder, which pushes the ground glass out of the way, and sets the film at the same plane of focus as the ground glass-- but that's a predetermined location, and the film holder has no way to know that you've actually moved the ground glass from it's original plane of focus by inserting a fresnel (or any other shim, spacer, etc.) in the way.

Take a 4x5 camera, with a 150mm lens on the front, with no fresnel lens. The ground glass, at infinity, will be 150mm from the focal point of the lens. Ordinarily, when you put the film holder in, the film will also be 150mm from the focal point. But if you put in a 2mm thick fresnel, now the ground glass surface is 152mm away, and you have to shorten your draw by 2mm to compensate. But when you switch to the film holder, it's now 148mm from the focal point, and your photograph is out of focus.

Cameras like your Toyo that's designed to have the fresnel between the lens and the GG will set the film plane back by 2mm to compensate, so that you're at 150mm for both.

Click to expand...

Now it makes sense. Thanks.

Some years ago I stayed in southern Turkey at a member here's apartment. He had bought a new Chamonix 5x4 camera they’d just started adding a Fresnel to their backs under the GG screen (lens side). He was doing quite close portraits and wanted shallow DOF and couldn't get sharp where he wanted it in his images, it turned out the company hadn't adapted the backs for a fresnel, they did rectify the situation.

One disadvantage of a fresnel under the GG screen is it can make focussing harder with quite wide angle lenses, it's less of a problem when rear mounted. I seem to remember at least one company offering two different fresnels one petter optimised for wide angle use.

Ian

Ian Grant said:

Some years ago I stayed in southern Turkey at a member here's apartment. He had bought a new Chamonix 5x4 camera they’d just started adding a Fresnel to their backs under the GG screen (lens side). He was doing quite close portraits and wanted shallow DOF and couldn't get sharp where he wanted it in his images, it turned out the company hadn't adapted the backs for a fresnel, they did rectify the situation.

One disadvantage of a fresnel under the GG screen is it can make focussing harder with quite wide angle lenses, it's less of a problem when rear mounted. I seem to remember at least one company offering two different fresnels one petter optimised for wide angle use.

Ian

Click to expand...

I never considered that a wide angle lens and a fresnel lens would introduce would introduce a new set of problems.

Wow, this has sure spawned a lot of misunderstanding.

First things first. Focusing the image on the ground glass doesn't guarantee that the image will be in focus on the film. If the ground glass has to be in the correct location for that to happen. With a plain ground glass, it is a simple matter of making sure the frosted surface of the ground glass is in exactly the same place as the surface of the film relative to the seat in the camera back.

In the case of introducing a Fresnel lens between the lens and the frosted surface of the ground glass, there are two considerations that dictate the correct position for the ground glass: First, you obviously have to compensate for the thickness of the Fresnel screen by modifying the camera back so that the whole sandwich sits closer to the lens. However, just returning the frosted surface of the ground glass to its original location will not give you correct focus. That's because the Fresnel lens itself shifts the focus by about 1/3 of its thickness. So, the frosted surface of the ground glass will have to be "off" from its original position by whatever that 1/3 of the thickness of the Fresnel screen is. Measuring/testing to find the focus is in order. Notice that when the frosted surface of the ground glass is finally correctly positioned so that what is sharp on it is rendered sharp on the film it will not be in the same place it was before adding the Fresnel lens and not be in the same position as the surface of the film.

Lots of cameras were designed with OEM Fresnel screens between lens and ground glass. My Graphic View II screen is like that as are my Fresnel screens on my Wista cameras. The manufacturers have tested and made sure the focus is correct when doing this, or at least they should. Chamonix cameras had an issue with this early on with their Fresnel screens (which has since been corrected IIRC).

If you are adding an after-market Fresnel screen to a camera and simply slap it on between the lens and ground glass without making any modifications, you will not have correct focus. Even if you find a way to attach the Fresnel screen so that the ground glass position remains unchanged from its original position, it will still be wrong since the Fresnel screen moves the focus away from that position just by being there. You'd have to test (e.g., the old ruler-at-an-oblique-angle test) and adjust the position of the ground glass/Fresnel sandwich until you had correct focus.

When you add a Fresnel screen between the eye and the ground glass, this problem does not exist. No modifications or repositioning of the ground glass is needed, which makes it the easiest way to add a Fresnel to a camera that didn't come with one from the factory.

Best,

Doremus

shutterfinger said:

IF you place a Fresnel between the lens and ground glass on a camera that never had a Fresnel THEN if you move the position of the ground glass to mount the Fresnel then the focus at the film plane will be off unless you modify the back to put the ground glass back to the optically correct position, BUT if you can mount the Fresnel between the lens and ground glass without moving the ground glass position then no adjustment to the ground glass is needed. ...

Click to expand...

You would still need to reposition the ground glass to compensate for the focus shift introduced by the Fresnel lens itself, which is about 1/3 the thickness of the Fresnel screen. The "optically correct" position for the ground glass in this case is not the same distance from the lens as the film.

beemermark said:

Guess I don't it. If the Fresnel moves the plane of focus back 1/32" then by my way of thinking moving the adjustable bellows on the view camera to move the lens board back 1/32" would maintain focus. And as long as focus in sharp on the GG then the negative will be sharp since the film sets behind the GG. And no matter where the Fresnel lens is you are always focusing on the GG. For what it's worth the directions for my Toyo 45AII has the Fresnel between the GG and the lens board.

Click to expand...

Keep in mind that focusing on the ground glass and focusing on the film are two independent things. Only if the ground glass image is in the same optically correct position as the the plane of the film will the ground glass be useful as a focusing tool. Cameras are carefully engineered so that those two planes align. Messing with that alignment messes with focus. Sure, you can move the ground glass back 1/32 of an inch and then refocus on it by using the bellows. The problem is, you haven't moved the film back by 1/32 of an inch so it is closer to the lens than the plane you focused on by that amount and focus will be off. It's no different than focusing correctly with a working system, inserting the film holder and then focusing closer by 1/32 of an inch before making the exposure.

Best,

Doremus