Installing Bosscreen in my Horseman VHR

[Emil Ems]

I recently acquired a Bosscreen for my Horseman VHR on ebay. The screen is clearly designated for that Horseman, as specified in the package.

However, the instruction sheet does not go into detail as concerns the installation. The VHR has a groundglass with the Fresnel on the lens side and, in consequence of that, has most probably a shimming installed that places the the focusing plane somewhat on the inside of the groundglass package, seen from the lens side.

Has anyone in this large group of experts ever installed a Bosscreen in his Horseman VHR? If so, would the shimming for the factory groundglass still be accurate for the Bosscreen or would I have to reshim upon installing the latter.

I would be most grateful for advice from someone who has gone down that route before me.

[robc]

I have not installed a boss screen on this camera but I have installed one on a technikardan.

A suggestion for you.

take your camera as is with its old screen including fresnel and focus on a subject at infinity. Make an image. Then remove fresnel and make another image with just the groundglass and then install the bosscreen without adjusting anything and focus on same subject at infinity and make another image. Compare the the negs for sharpness. Even print and compare sharpness. If the bosscreen image is not as sharp or sharper than the other two images then you need to adjust.

Don't fiddle with depth adjustment unless you really need to.

N.B. use focus at infinity for test because at infinty you will have narrowest depth of focus which will show up any error the most.

With my bosscreen I can measure, using a micrometer, the total thickness of the screen and the thickness of the outer glass. Therefore I can easily calculate thickness of inner glass+wax layer.

Thickness of innerglass + wax layer should be subtracted from standard filmholder depth to film( 0.187 inches to film, but check your film holders ) to give depth of film back to inner glass surface.

Alternatively take it to an expert to have it set for you.

www.largeformatphotography.info/holders.html

[Brian Ellis]

I've used BosScreens on three or four different cameras, mostly Technikas. I was sufficiently uncertain of exactly how to install them and how the shims in my cameras affected the installation that I sprung for about $50 and had them installed by S.K. Grimes. I recently bought a Maxwell screen for my Master Technika. It comes with detailed instructions so I figured I could do it myself. I was wrong and ruined about 40 negatives made on a photograpy trip to the Smokies. After I saw those photographs I could tell what I did wrong and fixed it but I can't replace the photographs I lost.

For the relatively small cost I think it makes sense from a peace of mind standpoint if nothing else to have someone who knows what they're doing install these things if you're at all uncertain, though I'm sure many people have successfully done it themselves and maybe someone here can tell you exactly how to do it. I just won't try it again myself after my Maxwell experience.

[Emil Ems]

Thanks to you all for your kind suggestions. In the mean-time I found out that I was wrong in my initial assessment of the VHR groundglass. It appears that the Fresnel is not placed before the groundglass (seen from the lens) but rather behind it. Thus it really will be a problem to install the Bosscreen as far as I can see. Since the wax in the Bosscreen is placed a smitch behind the first glass surface I will most probably not be able to use it without some milling down of the frame where the screen should sit. I would hesitate to do this.

[Oren Grad]

Emil, before giving up you should try contacting Schneider Optics, the current US distributor for Horseman. I don't know if he's still there, but in the past, Steve Inglima at Schneider has given me very knowledgeable answers about Horseman equipment - actually, everyone I've ever spoken with about anything at Schneider has been helpful in getting me the information I need.

[Michael S. Briggs]

On cameras with normal ground glasses (or with a ground glass plus Fresnel on the photographer's side), the Bosscreen is designed to install without modifying the camera. It's been a while since I tried one, but it should come with directions and with a set of shims. The shims compensate for the thickness of the glass in front of the wax. The focus plane is that of the wax, but you have to partially compensate for the thickness of the glass in front of the glass. Because the index of refraction of glass is higher than air, it alters the converging beam from the lens to shift the focus farther from the lens. As I remember, the inner glass is smaller so that it can sit inside of the gate and forward of the normal position. The shims should position the wax behind the position of the ground surface of the ground glass. It's been a while, so I don't remember exactly how to install the screen, but you definitely do not need to mill the frame of you camera. If you are missing the shims, perhaps contact the ebay seller to see if they have still them, or the manufacturer or US distributer (Bromwell?) for replacements. Or have S. K. Grimes, Inc. take care of the work for you.

I liked the focusing image from the Bosscreen, but mine quickly delaminated from normal summer temperatures.

[Michael S. Briggs]

The shims may come pre-glued to the outside piece of glass. So look at the screen and see if they are there. If so, it should just install on your camera without adjustments, since you say the Fresnel lens of your current screen is on the photographer's side. But of course make some tests of the focus accuracy.

The phrase "behind the position of the ground surface of the ground glass" should be "behind the position of the ground surface of the original ground glass".

[robc]

my bosscreen came with tape and not shims. (for technikardan)

older versions may have had shims.

www.stabitech.nl/Bosscreen.htm#top

before you change anything measure the depth of your current setup with a depth micometer so that in the event of reverting to your old setup, you know what it should be set to.

the wax layer should be in the same plane as the ground side of your old ground glass with a possible difference of the thickness of the tape. The ground side should have been on the lens side of the glass unles your setup was unusual.

[Michael S. Briggs]

Rob, perhaps you will regard this as a wording difference, but the purpose of the black tape is, via the thickness of the tape, to set the position of the screen. I therefore call it a shim.

Because there is glass in front of the wax layer, the wax layer should not be at the position of the ground surface of the original ground glass. The inner glass shifts the focus backwards. The purpose of the tape/shim is to account for this effect. Assuming an index of refraction of 1.5 for the glass, the focus shift is 1/3 of the thickness of the glass. (The equation is shift = thickness * (1 - 1/n), where n is the index of refraction.) The link that Rob provided shows a diagram of the Boss screen with dimensions: the inner glass has thickness 1.0 mm and the black tape (aka shim) 0.3 mm, agreeing with this calculation.

[robc]

I have a question.

Is the focus shift the same for all angles of incidence of light rays on the glass. Obviously for perpendicluar light rays there is no focus shift.