Really confused now, need urgent advice please.

[Ed Bray]

I had an issue with the back on my 8x10 Plaubel Monorail see thread here (http://www.largeformatphotography.info/forum/showthread.php?96745-Help-with-issue-with-8x10-camera-Please&p=953043#post953043). One of the suggestions was to check the distance between the rear of the back and the ground glass and then check the distance from the rear of the back to a sheet of film inside the film holder when correctly fitted into the back. The implication was that they should be the same or if not then the ground glass would be slightly further from the rear face and this difference would be the amount that the film holder would have to be after it had compressed the foam light seal when in position.

Well I took the measurements using a straight edge across the rear face of the back and used my vernier caliper's depth gauge to find the distance between the rear face and the inner face of the ground glass (which is a Boss Screen) and it was 14.72mm give or take 0.15mm to various points across the inner face of the ground glass and corners. I then inserted a film holder with a sheet of exposed film and took the same measurements, and this time the measurements across the film were 16.64mm again give or take 0.14mm, but this is the wrong way around as I was expecting the film to be closer to the rear face of the back than the ground glass. The only thing that makes some sense is if I take into account the thickness of the ground glass which is circa 2mm.

The thing I am finding troubling is that, when I have focused correctly the image is sharp on the outer face of the ground glass and turns out sharp on the negative. I can't believe that the depth of focus would be great enough to cover the discrepancy of circa 2mm so it just doesn't make any sense to me????

Hoping for some help here please.

[Mark Sawyer]

Very odd...

I don't know much about the Boss Screens, but I think they have the focusing plane between two pieces of glass, which moves it slightly back, (conventional ground glass has the focus plane on the inside surface). Perhaps someone removed the felt in an attempt to make up for it? Just a wild guess...

If you need to shoot test shots wide open at some point to confirm registration, use paper negatives to save time and money.

[Ed Bray]

Very odd...

I don't know much about the Boss Screens, but I think they have the focusing plane between two pieces of glass, which moves it slightly back. Perhaps someone removed the felt in an attempt to make up for it? Just a wild guess...

If you need to shoot test shots wide open at some point to confirm registration, use paper negatives to save time and money.

That makes some sense then, both bits of glass are approximately 2mm each, so if the screen material was between them that would account for the difference.

Paper negatives are out at the moment, but if I can work out a way of adding a light seal without comprimising the distance all may be well.

[Mark Sawyer]

Actually, thinking about it, the felt would move back both the frame that holds the gg and the filmholder equally, so whatever thickness is there wouldn't make any difference...

And if (compared to the film plane location), the Boss Screen is 2mm closer to the lens, but the focus plane is 2mm behind the inside surface, it would still be in registration with the film plane...

This is making my head hurt...

It sounds like regiistration isn't an issue. The original problem could still be either processing or light leaks though.

[Leigh]

...when I have focused correctly the image is sharp on the outer face of the ground glass...
I don't understand this statement.

The GG only presents one image, that being on its etched surface.

- Leigh

[Mark Sawyer]

Leigh, the Boss Screen doesn't have an etched surface. It has a layer of something else (paraffin wax?) between two sheets of glass. The image is visible on that inside layer rather than on the etched side of a single piece of glass.

[Leigh]

Leigh, the Boss Screen doesn't have an etched surface. It has a layer of something else (paraffin wax?) between two sheets of glass. The image is visible on that inside layer rather than on the etched side of a single piece of glass.

It sounds like that's the problem.
If the focus plane is shifted relative to the mounting surface, as compared with the OEM GG, you'll get a focus error.

- Leigh

[Mark Sawyer]

Yes, but it sounds like that problem was corrected by a previous owner when the Boss Screen was installed. If it's ever replaced by a conventional gg, that will have to be shimmed back the thickness of the inside piece of glass on the Boss Screen.

So it's back to the probelm of the streaks; processing error or light leak? (I suspect processing, but really can't say from the scans.) A light leak could also have happened away from the camera.

[Kuzano]

How can such a small difference make such a distance in focus... It's relative...

Think of it as the per cent difference of the amount, relative to the focus point in your lens to the ground glass/film plane difference. Then apply that to the total distance of the focus point of the subject to the focus point in your lens. A small distance inside your camera could represent a much larger distance out of focus over the total distance outside the camera. A millimeter difference in the inside camera distance, could translate into a many feet difference in focus over the total distance outside the camera. ???

Any problem with that consideration? I submit that the perfect solution is a focus point in lens to a film plane distance that is EXACTLY the same WITH NO SLOP, NO PLUS/MINUS from the focus point in the lens to the focus (etched surface) surface of the GG. In fact you cannot even calibrate a rangefinder until you exactly match those distance, if you plan to add a rangefinder to the equations. NO SLOP... EXACT MATCH.

It is, very simply a matter of exacting measurement...and know where those GG focus surface and film planes are located.

[Ed Bray]

Thanks all for your comments.

The focus seems correct as it is currently mounted. I think I will install some open cell foam and see what difference if any it makes, if it makes a difference I will remove it again, if not I will leave it in situ.

[Ed Bray]

How can such a small difference make such a distance in focus... It's relative...

Think of it as the per cent difference of the amount, relative to the focus point in your lens to the ground glass/film plane difference. Then apply that to the total distance of the focus point of the subject to the focus point in your lens. A small distance inside your camera could represent a much larger distance out of focus over the total distance outside the camera. A millimeter difference in the inside camera distance, could translate into a many feet difference in focus over the total distance outside the camera. ???

Any problem with that consideration? I submit that the perfect solution is a focus point in lens to a film plane distance that is EXACTLY the same WITH NO SLOP, NO PLUS/MINUS from the focus point in the lens to the focus (etched surface) surface of the GG. In fact you cannot even calibrate a rangefinder until you exactly match those distance, if you plan to add a rangefinder to the equations. NO SLOP... EXACT MATCH.

It is, very simply a matter of exacting measurement...and know where those GG focus surface and film planes are located.

As it currently stands the difference is 2.08mm, as the inside glass of the Boss screen is about 2mm I can only assume that the actual difference is circa 0.08mm which is very small (circa 0.00315 inches).

[ic-racer]

As it currently stands the difference is 2.08mm, as the inside glass of the Boss screen is about 2mm I can only assume that the actual difference is circa 0.08mm which is very small (circa 0.00315 inches).

You should be fine with that. At f5.6 you have a little over 0.08mm of leeway at the film plane.

[Jim Jones]

. . . And if (compared to the film plane location), the Boss Screen is 2mm closer to the lens, but the focus plane is 2mm behind the inside surface, it would still be in registration with the film plane. . . .

Not quite. The index of refraction of the inner glass must be considered. Optically, the focusing plane of the Boss Screen should appear to be about 1.3mm behind the surface of the glass, depending on its index of refraction.

[Jeffegg2]

Not quite. The index of refraction of the inner glass must be considered. Optically, the focusing plane of the Boss Screen should appear to be about 1.3mm behind the surface of the glass, depending on its index of refraction.

The image is focused on the waxed portion of the glass. Where is "Appears" to be has nothing to do with it, the image is "Acutally" focused on the frosted part of the focusing screen. This distance will be the same place as where the film is placed.

[Arne Croell]

The image is focused on the waxed portion of the glass. Where is "Appears" to be has nothing to do with it, the image is "Acutally" focused on the frosted part of the focusing screen. This distance will be the same place as where the film is placed.

Yes the image appears on the wax/paraffin layer, but what Jim meant is that the optical path length is different in the glass plate in front of that layer than if the same distance is traveled in air, which is the case when a film holder is inserted. The difference d can be calculated through the index of refraction: d=n2·t2 - n1·t1, where n2 is the refractive index of the optically denser material (here: glass) and t2 its thickness, and n1 is the index of refraction of the optically less dense material (here: air) and t1 is its thickness. In the case assumed here, t2=t1=2mm. Assuming that the Bosscreen uses regular floatglass with an index of n2=1.52, and using n1=1.00 for air, results in an additional path length of 1.04mm, i.e. the wax layer has to be 1.04mm further away from the back frame than the film to produce a focused image. Since the wax layer also has a certain thickness, the real value might differ slightly in reality. The Bosscreen has a black paper-like material glued to the top and bottom rim of the inside of the main glass, where it rests on the frame of the camera back, to accomodate for this.