Focus screen foulup - I think

[Bob McCarthy]

I live in the Dallas /Ft Worth area, is there anyone local to me who is capable of evaluation/repair of the problem?

From what I've gathered the boss is temp sensitive. Texas is pretty hot in summer!!

I'm going to disassemble the unit today. Maybe it's mounted incorrectly. The question then comes down to, does it require a glass or not? It does have a frosted surface, it has a bright surface (micro fresnel surface). It does not appear to conform to the assembly instructions on the vendor page.

Bob

[robc]

You need to get it checked by someone who knows what they are doing!

At each end of the gg there is a thin metal strip each of which is held on by two screws. Removing those screws will release the GG (or beattie screen in your case).
When you remove the screen you will find four little metal strips, one at each corner, which the GG should rest on. Those little strips each have a hole in the centre. There is a very fine screw on the opposite side of the back which protudes through the back and onto which the little strips are placed with the screw end in the hole.
The strips will probably fall off when you take the GG off. The strips are not shims. They are rest plates. Height is adjusted by turning the screws from the other side.
If the screws have never been adjusted then you will probably not be able to see the slot of the screw from the other side because when the back is new it will have been set and black paint will cover the end of the screws. If you can see the slot clearly with a loupe then someone has probably adjusted them at some time. You will need a fine watch makers screwdriver to adjust them if you take the plunge. If they have not been adjusted then leave them alone.

The four little rest plates are not absolutely flat on my back and all slightly bent so in theory they should go back in original positions and orientation. But since someone has been playing with your back I doubt that was done because until you have disassembled the unit you don't know that. In other words, after reassembly you need to check position of GG and adjust if necessary, by means of turning the screws.

The frosted side of the glass rests on those four plates and the plane of the frosted glass is where your film plane should be. i.e. where a filmholder WITH FILM IN IT should be. Depth to GG frosted surface should be the same as depth to film in film holder.

There is a standard for film depth but what is more important is making sure that film depth is the same as GG depth regardless of what the standard is. Therefore, first work out the average depth of your film in your film holders and second set the GG to the same depth. If that conforms to the standard, fine, if it doesn't, don't worry about it. Having all filmholders of same make and vintage helps greatly with this.

N.B. because of the way the GG fits the camera, any make of GG which fits should not need height adjustment if screws have not been altered and rest plates are fitted in exact same position as original settings. But until you check depth you won't be sure about that...

Linhof way of using a fresnel is to use a normal GG and then place a fresnel on outside of GG under two clips.

Good luck.

[Bob Salomon]

'The two holding clips in the center are for attaching accesories such as a viewing bellows. "

No, those are the clips to hold the Linhof fresnel screen, nothing else is mounted there.

[Michael S. Briggs]

......

The question then comes down to, does it require a glass or not? It does have a frosted surface, it has a bright surface (micro fresnel surface).

..........
Bob

I answered this in my second answer, of timestamp 03:00.

Bowed Linhof SuperScreens are a common but not universal problem. Ken mentioned his experience above. Other discussions that mention problems: http://www.largeformatphotography.in...ead.php?t=3515, http://www.largeformatphotography.in...ead.php?t=8400,
http://www.largeformatphotography.in...ead.php?t=9715
and http://www.photo.net/bboard/q-and-a-...?msg_id=003AeW. There are assertions that the problem is related to an early version, or leaving off the cover sheet, or if leaving off extra shims along the long side, or from hot temperatures.

Yours is wrapped, for whatever reason. I'd replace it, so why worry about details of how it is installed? The only detail to check is whether the frosted surface is facing the lens and resting on the metail strips that RobC and I have described. If so, you can probably simply lay a replacement screen onto the strips, with its frosted surface on the strips, facing the lens. If you can't find a local repair place and don't want to ship the camera, try a photographic test of the focus accuracy, as described in some of the previous links that I gave.

The principal is that you can switch between screens if the frosted surface stays in the same position with nothing in front of it. So if the focus is currently correct on your camera and the frosted surface of the screen faces the lens and rests on the metal strips, you can replace the screen with a glass ground glass. It's that simple. If the current screen doesnt have the frosted surface this way or someone has misadjusted the focus, then the focus would need to be adjusted.

[Bob McCarthy]

I did take the screen off the camera and found all as you discribed. The adjusting screws were still painted over. The plastic screen has taken a warped set, thought it appeared to be a somewhat different amount (lessor) off the camera.

The screen itself when laid back into position seemed to be somewhat undersized upon examination. The long dimension did not fully cover the metal feet when the screen was centered into position. Maybe 1/3 (or a little more) of the silver feet was exposed.

Since the screen has taken a set, this one is trashed. I had a suggestion to apply heat and will try that as a temporary fix.

The camera is definately misfocusing using the picket fence/polaroid test, but that may be a good sign as I would expect it to, if the screen was bowed.

Maxwell, Boss, or Linhof?

Bob

[robc]

I think I would be tempted to get a satin snow and then if that was not bright enough I would put a fresnel on top of it...

[KenM]

If you have the time and access to a drill press and a very flat surface, you can make one of these: http://www.acanadianplace.com/images/depth-gauge.jpg. Took me about an hour to make it. I used it to make a minor adjustment to my back on my MT. I also had the chance to use this tool on a brand-new just-out-of-the-box MT 2000, and I found that my back was closer to spec - ok, by 0.001", but still!

It also works to check your film holders. It's handy tool, and not that expensive tomake either. The dial indicator is about $20 or so, and I already had the plex. Even so, you could get a big enough piece of plex for about $10-$20 (or even less) to make the jig.

The only thing I need to add to it is a couple of small screw hooks to the left and right of the dial indicator, so I can stretch an elastic over it to hold the dial indicator down. While it was nice and snug to begin with, it's loosened up over time, so it needs to be held down.

Regarding construction, once you've glued everything toegether, lap the bottom of the plex on a flat surface to ensure that it's flat. I had a very heavy piece of granite that I use for sharpening my chisels that worked very well for this.

You can also buy a depth micrometer, but these are quite expensive. I have one, but it only has a 4" base, so I can only use it to measure the corners.

[Bob McCarthy]

I want to thank all who responded to my original post. I'm appreciative to all for the sharing of information and experience to this rookie in LF. I have much to learn, but am energized by the opportunity. I hope I can reciprocate at some point.

BTW, I am going to start with a new Linhof ground glass, and go from there. Should get it this weekend.

Bob

[robc]

Just took delivery of a couple of MIDI SHEET MAGNIFIER's from

http://www.magnifyingglasses.co.uk/a...nifiers_8.html

focal length seems to be about 100/150mm and boy do they make a big difference on my Linhof GG with reflex viewer. And at only £3.00 each I saved myself £92 on the Linhof item.

Sure they are cheap plastic but there is no focus shift when I put the fresnel on and its so much brighter. They are about 1mm thick. Dimensions were only 95mm on the short side but the long side needed to be trimmed at boths ends by about 8mm to fit.

Recommended!