Wide angle focus questions - front focusing -Schneider vs. Nikon

[schafphoto]

2017 Bosscreen Groundglass Update: It has become evident that all three of the Calumet/Cambo/Boss-screen wax-filled ground glasses (groundglassi?) that I have tested have caused front focusing on every lens – most noticeably on the wider angles. All my Bosscreens had the “original shiny black shim tape" on the two contact edges but this was not enough of an offset to put the plane of focus in the right place. I have added a heavy paper shim IN ADDITION to this tape (134# Cover Stock - Crane Crest) from a paper sample book I had.

Since this was my finding with all three of my groundglasses, in multiple rotating and non-rotating frames, I thought I would post my findings for anyone having focus shifts with Calumet or Cambo 4x5 backs. This includes two Bosscreen Stabilitech Zoetermeer Holland BCB4x5Z Cambo 4x5 grid screens and one CAMBO branded C-143 Bosscreen Groundglass with blackprint grid.

-Schaf

[Jim Noel]

The first thing to check is to make sure the ground side of the glass is toward the lens.

[schafphoto]

The first thing to check is to make sure the ground side of the glass is toward the lens.

But Jim, the wax looks “ground" on both sides! ;-) In the case of Bosscreens the text facing the user is also a good way to indicate correct attachment.

[consummate_fritterer]

Stephen,

You describe a classic ground-glass positioning problem. It may have to do with the fact that you have after-market Bosscreens installed. If, for example, your Cambo originally came with a Fresnel/ground-glass sandwich with the Fresnel on the lens side of the assembly, then your Bosscreen will be too close to the lens (closer than the film) by 1/3 the thickness of the original Fresnel (this latter fact might give you a starting point for determining shim thickness). The fact that you see the error more with shorter-focal-length lenses is that the shorter the focal length, the less Depth of Focus you have at the film plane (as opposed to Depth of Field in front of the lens, which is greater with shorter-focal-length lenses). Lens design plays a role here too I would expect. The point here is that if you position your ground glass correctly using the most sensitive lens, i.e., your 72mm, it will be right on for the rest.
<snip>

This is why I had the technician, recommended by Bill Maxwell, to install his Maxwell focusing screen on my gen1 Chamonix 4x5... the first Chamonix model which had the spacing issue caused by the factory Fresnel lens.

[David Karp]

This is all interesting. My first two LF cameras were Cambo/Calument. The stock camera came with a GG only. No Fresnel. I purchased a Cambo branded Bosscreen from the old Calumet store in Los Angeles. They just swapped the screens. I used both cameras that way until I sold them. There was never a problem with focus before I purchased the Bosscreen or after. The first camera had a revolving back. The second a newer repositionable back. I wonder what the difference might have been.

I really liked that Bosscreen. It was really easy to focus. Super sharp, as Cambo called their screen (it also said Bosscreen on it). I was sorry it could not be easily used on another camera without shimming (although Bruce's technique would have made that process easier, had I known of it then).