Fresnel, GG and focussing questions

[jeannicolas]

Some context first; I bought an Omega View 45F camera that came with the following arrangement: Lens -> Fresnel (ribbed side facing GG) -> GG (frosted side facing lens) -> Eye

After cleaning it, I accidentally put it back differently, like this: Lens -> GG (frosted side facing lens) -> Fresnel (ribbed side facing GG) -> Eye

The first thing that I noticed is that it was much brighter, and much easier to focus when arranged that way, so I was momentarily very happy with this discovery, until I started having doubts about shifting the focus plane, even if only by a tiny amount. After doing some research on the forum, it appears my doubts were justified, and that it might result in out of focus pictures. Even removing the fresnel from my camera and using GG only actually improves my ability to view the image and focus it (partly because the fresnel is very old, and yellowed by age), but again, unless I use shims, I am shifting the focus plane since the fresnel usually sits between the GG and the lens.

With that in mind, some questions I had:

- Does anyone know for a fact that the arrangement the camera came with (Lens -> GG -> Fresnel -> Eye) *is* the correct arrangement for Omega/Toyo View 45F? I found some indications that it might be the case, but no solid info.

- If I mostly do landscape images at f32+ is it safe to assume that the focus plane shifting might actually not really matter?

I guess I could always try the 2 arrangements and take some wide open shots, focussing on something close to see how big of an impact the shift has? But curious if anyone has answers to these questions offhand!

Any info appreciated!

[Bob Salomon]

The focus plane is critical. If you like the gg and fresnel positioned that way have a service center adjust them so the ground side of your gg is properly positioned.

[Doremus Scudder]

Since you don't know if the Fresnel/ground-glass sandwich you have is 1) OEM, 2) in the original (correct) orientation and order, I would think some testing would be in order.

Set up a long ruler/yardstick at an oblique angle to your camera, use a longish lens, wide open and work rather close up to the subject to minimize depth of field.

Focus carefully on a particular mark on the ruler using a good 6x-10x loupe; you should see it very sharp on the ground glass with marks on either side of the focus point becoming quickly out-of-focus.

Then, make a negative, rearrange your Fresnel/ground-glass sandwich and make another negative. Repeat this for every possible combination of Fresnel and ground glass.

Keep good notes (maybe include a post-it with the shot info in the scene so you can identify which negative goes with which combination).

Develop your negatives and examine them closely with your loupe to see which combination gives you the best focus on your chosen focus point.

Then, reassemble the Fresnel/ground glass to correspond to that.

Bob's your uncle.

FWIW, I do this as a check with all my cameras from time to time when I'm printing and have a tray of print developer sitting in the darkroom. Set up the shot, make the neg, toss it in the print developer for five or six minutes and then stop and fix and check the wet neg with the loupe.

Best,

Doremus

[Bernice Loui]

The Omega 45E/F user manual:
https://www.butkus.org/chinon/omega/...ga_44e_45f.htm

Omega 45E/F ground glass / fresnel install from 2012:
https://www.largeformatphotography.i...l-lens-removed


Do read this current discussion on ground glass, fresnel lenses and related:
https://www.largeformatphotography.i...recommendation

Adding a fresnel to focus the light/image from the ground glass into a smaller viewing area might be helpful in some situations, but NOT all ground glass viewing situations are improved with using a fresnel. Adding a fresnel as a viewing aid is a mixed baggie at best. In many ways, better to learn and adapt to see the GG image as is with a good quality GG plain no fresnel or similar viewing aid.


Bernice

[maltfalc]

if the fresnel and gg are the same thickness, you can mount the gg in front as long as the frosted side is on the back. the fresnel can be installed either way or left out if you want.

[jeannicolas]

Thank you all for the replies! It sounds like I should just go ahead and make some tests to find out since I'm not sure if the initial fresnel/gg "sandwich" is even the correct arrangement!

[Doremus Scudder]

if the fresnel and gg are the same thickness, you can mount the gg in front as long as the frosted side is on the back. the fresnel can be installed either way or left out if you want.

...but you have to take focus shift into account! A Fresnel screen placed between lens and ground glass displaces the focus by approx. 1/3 of the thickness of the screen. If that distance is not compensated for in the design and positioning of the screen, focus will be off. E.g., if you just place an aftermarket Fresnel between ground glass and lens even making sure the frosted side of the ground glass is in the same place, focus will NOT be the same on the ground glass as on the film.

Placing an aftermarket Fresnel screen between ground glass and eye does not have this problem. Many cameras were designed with a Fresnel between ground glass and lens, but these have the frosted side of the ground glass properly positioned so that the focus is the same on both ground glass and film. If you remove this sandwich and replace it with a plain ground glass, focus will be off.

Testing in case of uncertainty is the best way to find out if your Fresnel/ground glass sandwich is installed correctly.

Doremus

[jeannicolas]

For anyone curious, here's what I ended up doing; my main problem was that the fresnel I had was in really bad shape, and using it made the image on the GG harder to see, and harder to focus on. I found a cheap replacement fresnel that appeared to be of the same thickness but was in great shape, so I went ahead and tried the following "sandwich": Lens -> (new) fresnel -> GG -> Eye.

The difference was striking... much brighter image, and much easier to focus... BUT how about focussing, did I mess up the focus plane by doing this? So I took a test shot to find out, trying to use a setup similar to what Doremus suggested. Here's the result; I was focussing on the "29" mark of the ruler. My test was probably not ideal (I only have a 135mm and a 90mm lens so I used the 135mm wide open at f4.7), but I am satisfied with the result. It looks like the focus on the resulting image is right where I wanted it, and even if it's not exactly perfect, it's well within what I consider acceptable for my use-case! In other words; Thanks everyone for all the info, I have a sandwich that I am happy with! The only bad news is that while doing all this testing, I ended up breaking my ground glass. While it still holds in place and "works" there's a big crack right in the middle, so I have to find a new one

[maltfalc]

...but you have to take focus shift into account! A Fresnel screen placed between lens and ground glass displaces the focus by approx. 1/3 of the thickness of the screen. If that distance is not compensated for in the design and positioning of the screen, focus will be off. E.g., if you just place an aftermarket Fresnel between ground glass and lens even making sure the frosted side of the ground glass is in the same place, focus will NOT be the same on the ground glass as on the film.

Placing an aftermarket Fresnel screen between ground glass and eye does not have this problem. Many cameras were designed with a Fresnel between ground glass and lens, but these have the frosted side of the ground glass properly positioned so that the focus is the same on both ground glass and film. If you remove this sandwich and replace it with a plain ground glass, focus will be off.

Testing in case of uncertainty is the best way to find out if your Fresnel/ground glass sandwich is installed correctly.

Doremus

i hate having to repeat myself. i AM taking focus shift into account! this is the same arrangement as a speed graphic. the original position of the fresnel was between the gg and lens, with the plain side of the fresnel facing the camera's lens and the lens side of the fresnel in direct contact with the frosted side of the gg. that means the fresnel won't shift the focus any more or less than a plain sheet of acrylic. if the gg and fresnel are the same thickness, you can place the ground glass with the plain side facing the camera lens and the frosted side against either side of the fresnel or without the fresnel at all. the glass of the gg will shift the focal plane the same amount as the acrylic of the fresnel did, or at least close enough to not be noticeable. the focal plane is the same, both physically and optically. there's no uncertainty here. and aftermarket fresnels have absolutely nothing to do with anything i said.

[maltfalc]

For anyone curious, here's what I ended up doing; my main problem was that the fresnel I had was in really bad shape, and using it made the image on the GG harder to see, and harder to focus on. I found a cheap replacement fresnel that appeared to be of the same thickness but was in great shape, so I went ahead and tried the following "sandwich": Lens -> (new) fresnel -> GG -> Eye.

The difference was striking... much brighter image, and much easier to focus... BUT how about focussing, did I mess up the focus plane by doing this? So I took a test shot to find out, trying to use a setup similar to what Doremus suggested. Here's the result; I was focussing on the "29" mark of the ruler. My test was probably not ideal (I only have a 135mm and a 90mm lens so I used the 135mm wide open at f4.7), but I am satisfied with the result. It looks like the focus on the resulting image is right where I wanted it, and even if it's not exactly perfect, it's well within what I consider acceptable for my use-case! In other words; Thanks everyone for all the info, I have a sandwich that I am happy with! The only bad news is that while doing all this testing, I ended up breaking my ground glass. While it still holds in place and "works" there's a big crack right in the middle, so I have to find a new one

dropped mine less than two feet and it snapped in half. replaced it with a thin sheet of frosted plastic from a broken lcd monitor sandwiched between the fresnel and a sheet of plain acrylic. gives a really clear image and it's a lot more impact resistant now.