Problems with Boss screen installation

[peter_chipman]

Hi, everyone.

 

Ok, after reading through threads on this forum, I worked with a

friend on installing a bosscreen on my 4X5 (a Bender).

 

Initially, I thought we were going to line up the black plastic

strips on the Boss screen with the plane of the film, but we instead

made the surface of the larger piece of glass line up with the plane

of the film in the film holder.

 

So, I'm assuming that we now need to do the following: Shim the four

ground glass holders so that the two plastic strips (which I've read

correct for the light bending as it passes through the smaller sheet

of glass)line up in the plane of the film in the film holder.

 

Am I right and we need to shim, or do we have it right the way we did

it and we don't need to shim?

 

Thanks,

 

Pete.

[brian_ellis3]

Bosscreens are camera brand specific (at least the three I've owned were) so you hopefully have one made for the Bender (or know how to compensate if you don't). I also understand, possibly incorrectly, that what needs to line up with the film plane is the wax center piece, not either of the two piceces of glass. FWIW, I've had two of the three Bosscreens I've owned installed by Steve Grimes and the third by Marflex. For me, the $50 or so cost was worth the freedom from worry that I did something wrong but then you're probably more mechanically inclined than I am. If you don't get definitive answers to your question from someone more knowledgeable than me, I'd at least ask Steve Grimes even if you choose not to have him install it, he's always been very helpful to me in answering questions.

[peter_chipman]

Sorry, forgot to mention that I'm familiar with Boss Screens being "brand specific," but I did not see that as a problem, as I'm not adverse to sanding/shimming my ground glass holders to put the wax layer into the film plane.

 

I'm just double-checking to make sure that the two plastic strips, representing the wax layer, are what need to be in the film plane, and therefore, I need to shim my holders since I've incorrectly sanded them down to put the larger glass plate in the film plane.

 

Another thing that I found in a previous thread, but that doesn't seem to come up in more recent threads, is that the black plastic strips DON'T represent the wax layer, but are actually correcting for the slight bending of light that occurs as incoming light passes through the smaller layer of glass and hits the surface of the wax. The black plastic strips add a little distance to the wax surface to compensate for this. If you read the Boss Screen "Instructions," it shows the wax layer as being 0.12mm, and the black plastic strips as being 0.3mm. This leads me to believe that the black plastic strips are NOT representative of the width of the wax layer, but as posted before, are correcting for light bending plus the width of the wax paper by placing the wax layer slightly further back. However, the end result is the same, as you should have the surface of these plastic strips in the same plane as the film.

 

Am I correct, or am I confusing the issue?

 

Thanks,

 

Pete.

[arne_croell]

You are correct I think. A glass plate with a refractive index of 1.5 (about average) increases the light path length by 1/3rd of its thickness compared to air(just multiply the geometric length by the refractive index). The smaller glass facing the lens measures about 1mm on my Bosscreen, so that is your 0.3mm shift right there.

[arne_croell]

One correction of my previous post: To get the focus displacement d caused by a plate of glass the formula is: d=t·(1-1/n), with t=plate thickness. The numbers were correct: for n=1.5, d=t·0.333