Argh, broken ground glass

[Tim Shawcross]

First day on holiday in NZ and I managed to crack my ground glass. Luckily I bought a couple of spares.

I'm using a horseman monorail camera, and it has frensel placed in front of the GG (on the lens side), so I'm a bit concered about how I have placed the new ground glass in. I took the old assembly appart and made note of how it went together:


The ground glass and frensel are secured together by two small brackets that run the long sides of the ground glass. The frensel/GG are laid out like this:
frensel (smooth side facing lens, rough side facing photographer), followed by GG (rough side facing lens and smooth side facing photographer)

Once these two are is assembled together, this assembly sits on two little ledges (both of which have two small shims on them). The GG actually sits on the ledges, whilst the frensel sits/hangs below the ledges. There are then two brackets that screw down to secure the whole assembly to the back.

I think this means that the rough side of the GG is where the film plane is, whilst the frensel is actually in front of the film plane (closer to the lens).

When i took the old broken ground glass away from the frensel, I noticed no shims between them. So I think it's simply a matter of placing the ground glass on top of the frensel (with the rough side facing the lens), then securing the frensel and GG together using the small brackets. Then this assembly can be secured back on to the back. Is this all there is to it?

I usually use a camera without a frensel, so replacing GG is easy as it's just a matter of putting a new one where the old one was. I'm concerned as I'm not sure what the frensel is doing to the location of where the image is focused, and by the fact that I thought most frensels are placed between the photographer and the GG, not the lens and the

Tim

[Turner Reich]

Maybe Dave Parker of Satin Snow will know.

tr

[stompyq]

Tim I am in the same boat as you. My wista has a fresnal b/w the gg and the lens. I tried to replace the ground glass with a satin snow version but noticed the film plane shift (I am not sure about this). I am totally confused about the posts that explain this phenomenon. I'll be watching this post carefully for answers. Good luck in New zealand!!!

[Jim Noel]

I do not have a Horseman among my several LF cameras.

However, if the original fcusing screen has a fresnel with smooth side toward the lens, and the ground glass surface against the rough side of the fresnel, placing a Satin Snow, or any other ground glass so that there is nothing between the focusing surface and the lens will cause the focusing surface to be the thickness of the Fresnel closer to tthe lens. The result will be poor focus on the film.

[Dave Parker]

Most cameras that have a fresnel installed from the factory, have the fresnel first and then the ground glass and yes the ground side of the screen is where the actual film plane is located, the fresnel provides magnification a small amount due to it actually being an optical lens. There are some models of camera that were designed with the fresnel behind the glass. Unless you have shims and such then there should not be any adjustment needed once the combinations are replaced and snugged down, be sure not to tighten the holding screws to much as you can crack the screen.

Stompy, could you post the reason you think their is a film plane shift? if you took it apart and reinstalled in the exact same order, there should be no film plane shift, there are very few cameras that are particular about thickness, due to the fact that actual image in produced on the ground side of the screen and as long as this is maintained in the same spot as the original, there should be no focus shift, you could in theory put a 1 inch thick screen on the back of the camera and as long as the ground side of the glass is in the same place, the focus should remain the same. With few exceptions the focus screen is in fact one of the simplest items on a LF camera.

Also, if you remove your fresnel and just put the ground screen in as Jim said, there will be a focus shift, the only way to get rid of the fresnel is to do testing to find out how much the glass would need to be shimmed to compensate for removing the fresnel, a good starting point is about 2/3 the thickness of the fresnel, but this can vary as different fresnels will have different magnification factors.

Dave

[Tim Shawcross]

Thanks for the replys. I think it should be ok, as far as I can see the ground side of the new the GG is in the same position as the old one so fingers crossed all the film should come out correctly in focus. I have no way of getting anything developed till I get back, so I'll have my fingers crossed. I'm shooting mostly landscapes, at around f22-32, so hopefully if there is any small focus shift, it should'nt be significant enough to have detrimental effects.

Tim

[Robert A. Zeichner]

I owned and used a Horseman 45FA for a number of years and the order of assembly sounds identical to what I had. As long as you make certain the Fresnel "hangs" in front of the gg and that it doesn't get sandwiched between the gg and the mounting pads, you should be fine. I always test gg/film plane coincidence whenever I make a change like this and I would highly recommend you do the same. You'll want to test using some fine print or engraved material like currency (dollar bills, here in the states) at about a 30" or 7.5cm. Shoot wide open (adjust lights to correctly expose at the widest aperture). If you can stagger the subject matter in 1" or 25mm increments that will be better yet. Shoot straight on and focus on the middle subject. Observe very carefully, the condition of focus on the gg and make an exposure. Without moving the set up, go soup the film and when dry, compare the result with what you still see on the gg. Compare the condition of focus on the subject you focused upon with the staggered subjects. If it looks the same, you're golden.

[stompyq]

Most cameras that have a fresnel installed from the factory, have the fresnel first and then the ground glass and yes the ground side of the screen is where the actual film plane is located, the fresnel provides magnification a small amount due to it actually being an optical lens. There are some models of camera that were designed with the fresnel behind the glass. Unless you have shims and such then there should not be any adjustment needed once the combinations are replaced and snugged down, be sure not to tighten the holding screws to much as you can crack the screen.

Stompy, could you post the reason you think their is a film plane shift? if you took it apart and reinstalled in the exact same order, there should be no film plane shift, there are very few cameras that are particular about thickness, due to the fact that actual image in produced on the ground side of the screen and as long as this is maintained in the same spot as the original, there should be no focus shift, you could in theory put a 1 inch thick screen on the back of the camera and as long as the ground side of the glass is in the same place, the focus should remain the same. With few exceptions the focus screen is in fact one of the simplest items on a LF camera.

Also, if you remove your fresnel and just put the ground screen in as Jim said, there will be a focus shift, the only way to get rid of the fresnel is to do testing to find out how much the glass would need to be shimmed to compensate for removing the fresnel, a good starting point is about 2/3 the thickness of the fresnel, but this can vary as different fresnels will have different magnification factors.

Dave

Mine was only a observation dave. Heres what i did. I first focused the camera against a magazine article i tacked to the wall. everything was lined up and the camera was on a sturdy tripod. Rechecked focus making sure the plane of fucus was such that the lettering was in focus. I replaced the wista's GG with the satin snow GG. there was a obvious change in the focus plane which needed to be adjusted. I'll be happy if somebody points out an error in what i did. By the way one thing i did notice was that the SS gg was a bit thinker than the plane sheet of glass that the wista uses.

My apolagies to the original poster. I am not trying to hijack your thread

[Dave Parker]

Stompy,

Have you taken any shots with the new glass? I would suggest taking a couple of test shots to double check, but as I said, if the ground side of the glass is in the exact same position as the original, you will be good to go, the thickness is not as important as the position of the ground portion of the glass. One other thing? Does your Wista, use a clear piece of glass over a fresenl? if so, then it is in fact focusing on the fresnel? and hence the ground glass you use for replacement would have to go in the spot the fresnel was originally to maintain the same focus plain. I have seen some cameras that use the fresnel as focus screen with a protective clear glass over it.

Dave