Homemade Boss Screen
David L. Glos
All this talk about the Boss Screen piqued my interest and I decided to try
making my own. Its perhaps not as difficult as might be thought........with
the proper materials. I'm the university research lab rat type, and as such,
have access to some things not found in your typical cupboard. On of those
items is an interesting waxy film called Parafilm "M". Their quotes, not mine.
It is stretched over the top of beakers and petri dishes for
preservation purposes; a bit like Saran wrap but with better sealing
properties.
To be brief, I found some old Kodak 3.25"x4" lantern slide glass, and cut a
piece of the Parafilm to about the same side. Placed the film between two of
the glass sheets, put the stack in a 120 deg Celsius lab oven and weighted it
down with a 2 lb lab weight. Let the whole affair 'cook' for the better part
of 3 hours, then turned the oven off and let it return to room temperature. I
did not touch the stack until it had cooled. The finished sample looked like a
sheet of cloudy glass. There were some small bubbles at the periphery, that I
think can be cured by a more even clamping scheme. Otherwise, it worked very
nicely as a focusing screen. Don't think it had quite as much contrast as the
best ground glass, but the complete absence of grain made it very easy to
achieve critical focus with a magnifier. Will toss the sample in a -30 degree
lab freezer to see how it handles the cold.
BTW, you can buy the Parafilm "M" through any decent lab supply house, like
Fisher Scientific. It really is not expensive.
Let me know of anyone finds this interesting.
David Glos
David Glos
Univ. of Cincinnati
513.558.6930
Beakman
David L. Glos (glo...@email.uc.edu) wrote:
: All this talk about the Boss Screen piqued my interest and I decided to try
[snip]
: BTW, you can buy the Parafilm "M" through any decent lab supply house, like
: Fisher Scientific. It really is not expensive.
: Let me know of anyone finds this interesting.
: David Glos
Just be sure that the plane of focus with the new screen corresponds with
the film plane. The "outermost" sheet of glass in the Bosscreen is larger
than the inner layer so that when it is installed it is this outer layer
which sits where the old ground glass sat. They also provide two precise
shims which go between this glass and groundglass frame to get the focal
distance exact.
David
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David L. Glos
In article <beakmanE...@netcom.com>, bea...@netcom.com (Beakman) wrote:
>David L. Glos (glo...@email.uc.edu) wrote:
>: All this talk about the Boss Screen piqued my interest and I decided to try
>: making my own. Its perhaps not as difficult as might be thought........with
>
>[snip]
>
>: BTW, you can buy the Parafilm "M" through any decent lab supply house, like
>: Fisher Scientific. It really is not expensive.
>
>: Let me know of anyone finds this interesting.
>
>: David Glos
>
>
>Just be sure that the plane of focus with the new screen corresponds with
>the film plane. The "outermost" sheet of glass in the Bosscreen is larger
>than the inner layer so that when it is installed it is this outer layer
>which sits where the old ground glass sat. They also provide two precise
>shims which go between this glass and groundglass frame to get the focal
>distance exact.
>
>David
Yep, thought of that already. If I have to have someone else cut my 4x5 glass,
I may have a total of $10 in the whole project, so I'm not that worried. I
also have a whole selection of brass shim stock, from 0.0005-0.014" thick.
Will probably start with the following technique for shimming. First, I will
measure the thickness of the pair of glass panels before placement of the wax
film. For sake of discussion, lets say it turns out to be 0.100" (it was, I
just checked). Then I will bond my panels with the Parafilm/oven cycle
process, and remeasure the thickness (happens to be 0.107" for my experimental
panels). I don't really know where in wax the image is formed, but lets start
by shimming to get the midpoint of the was layer at the film plane. Based on
the previous numbers, the wax layer is 0.007" thick, and I would have to shim
the screen OUT by 0.0035". Not too much rocket science involved.
Note: I'm not trying to do this as any kind of commercial venture. The whole
discussion just raised an eyebrow and made me think that it may not be that
tough.
DLG
Steve Alley
David,
Will you have to make changes in the way you focus,
since the focusing surface is now in the middle of two sheets
of glass rather than on the surface of the GG?
Steve
David L. Glos wrote in message <6e62ul$hlp$2...@news.ececs.uc.edu>...
>All this talk about the Boss Screen piqued my interest and I decided to try
>making my own. Its perhaps not as difficult as might be thought........with
>BTW, you can buy the Parafilm "M" through any decent lab supply house, like
>Fisher Scientific. It really is not expensive.
>
>Let me know of anyone finds this interesting.
>
>David Glos
>
John Sparks
David L. Glos (glo...@email.uc.edu) wrote:
>In article <beakmanE...@netcom.com>, bea...@netcom.com (Beakman) wrote:
>>Just be sure that the plane of focus with the new screen corresponds with
>>the film plane. The "outermost" sheet of glass in the Bosscreen is larger
>>than the inner layer so that when it is installed it is this outer layer
>>which sits where the old ground glass sat. They also provide two precise
>>shims which go between this glass and groundglass frame to get the focal
>>distance exact.
>>
>>David
>Yep, thought of that already. If I have to have someone else cut my 4x5 glass,
>I may have a total of $10 in the whole project, so I'm not that worried. I
>also have a whole selection of brass shim stock, from 0.0005-0.014" thick.
>Will probably start with the following technique for shimming. First, I will
>measure the thickness of the pair of glass panels before placement of the wax
>film. For sake of discussion, lets say it turns out to be 0.100" (it was, I
>just checked). Then I will bond my panels with the Parafilm/oven cycle
>process, and remeasure the thickness (happens to be 0.107" for my experimental
>panels). I don't really know where in wax the image is formed, but lets start
>by shimming to get the midpoint of the was layer at the film plane. Based on
>the previous numbers, the wax layer is 0.007" thick, and I would have to shim
>the screen OUT by 0.0035". Not too much rocket science involved.
The layer of glass between the lens and image forming plane probably
affects the focus position. Probably why the real Boss screen comes
with some shims. Otherwise the smaller front glass should put the wax
layer in the same position as the ground side of the ground glass
(assuming you are using a camera designed to have no fresnel between the
ground glass and the lens, some cameras like most speed graphics were
designed to always have a fresnel mounted in front of the GG). I've
heard that the layer of glass moves the focus point back by about 1/3
the thickness of the glass if I'm remembering right. I'm sure it
depends on the index of refraction of the glass, but this probably
doesn't vary much unless you are using some exotic glass type. I would
check the correct placement of the ground glass with a photographic
test.
John Sparks
David L. Glos
Tis true that there will likely be a focus shift with that extra piece of
glass in the optical path. Perhaps an easy way to check would involve cutting
a piece of glass, smaller than the already installed ground glass, place it in
the optical path in front of said ground glass, and recheck focus. Since the
original position of the ground glass would not be changed, the focus shift
could wholly be attributed to the extra sheet of clear glass. Of course, I
could just critically focus with the ground glass, lock everything down good
and tight, and then install the new panel with proper shimming to achieve the
same level of critical focus.
In article <6e73gp$t42$2...@nonews.col.hp.com>, spa...@col.hp.com (John Sparks)
wrote:
>The layer of glass between the lens and image forming plane probably
>affects the focus position. Probably why the real Boss screen comes
>with some shims. Otherwise the smaller front glass should put the wax
>layer in the same position as the ground side of the ground glass
>(assuming you are using a camera designed to have no fresnel between the
>ground glass and the lens, some cameras like most speed graphics were
>designed to always have a fresnel mounted in front of the GG). I've
>heard that the layer of glass moves the focus point back by about 1/3
>the thickness of the glass if I'm remembering right. I'm sure it
>depends on the index of refraction of the glass, but this probably
>doesn't vary much unless you are using some exotic glass type. I would
>check the correct placement of the ground glass with a photographic
>test.
>
>John Sparks
David Glos