I've only been in LF for about five years so I'm still learning every day. Today my lesson was about the ground glass and plane (pain?) of focus.
I just received a beautiful new 4x5 ground glass from Steve Hopf (email@example.com). I ordered it with no marks - no grid or crop lines, so I can see a clean, bright image without distractions. Installation should be a piece of cake, right? Just swap the new ground glass for the old.
I selected one of the four Graflok backs for my Toyo cameras (45AII & 45G) and removed the six screws which hold in place the two retaining clamp strips. To my surprise, what came out was not just the old ground glass but a sandwich of Toyo factory ground glass plus a Fresnel screen on the film side of the glass. No wonder my image was hard to focus! It was bright, but confused by Fresnel rings (a standard side effect of Fresnel lenses).
Some extended searching on the Web and a little reflection on Newtonian optics lead me to believe that the Fresnel lens has a focal length of its own, effectively modifying the plane of focus as it falls on my ground glass. General advice is that you mount the Fresnel on the side of the glass away from the lens, and that furthermore you really ought to match the Fresnel to the specific lens focal length. Toyo probably understands this and accounted for the change of focus in their construction of this ground glass-Fresnel sandwich. No matter; I donít want a Fresnel so I can ignore all that.
I inserted my new ground glass and screwed things back together. Then I started to think. (Isnít that backwards? I frequently forget to think before I act, instead of after). Will the film-facing side of my new ground glass be at the proper plane of focus? Oh, boy, now Iíve messed up. Iíd already disassembled all four of my Graflok backs to see what kind of glass each contained. How do I re-establish a standard?
Now that I was thinking, that didnít seem so hard. Recently Iíve moved on from Readyloads to sheet film. I bought a bunch of film holders on eBay and then got hold of a very nice set of Grafmatics holders, which are all I use at the moment (and theyíre great.) So, to determine the designed focus plane of the film, I loaded up some discard film sheets, cocked the dark slide, and started measuring. My digital micrometer soon gave the answer. Or answers: 4.75mm, 4,79mm, 4,85mm, 4.91mm, Ö from the contact face of the Grafmatics to the actual emulsion face of the film.
Back to lab methods. I ran six test sheets through the Grafmatics a couple of times, measuring each sheet twice at all four corners. The best I can determine is that this particular Grafmatics is aimed at about 4.8mm depth from contact face to film emulsion. Then I repeated the test on a couple of random Elite film holders, which are much less precise since they are flexible plastic compared to the stiff metal of the Grafmatics. Nonetheless, the measurements indicate about 4.5mm-4.9mm.
With this target in mind, I thought it would be easy to check the position of the ground glass in my Toyo back. I selected one of my discarded glasses and clamped it carefully into the Toyo back, resting on the ground metal edges provided for the glass. Inverting the back to expose theside facing the lens, I again applied my digital micrometer to measure from the ground contact surfaces on the camera back to the lens-facing side of the ground glass.
Wow! 5.05mm, 5.10, 5.08mm, 5.15mm. Hmm. I better check another Toyo back. 4.65, 4.73, 4.69, 4,80. After measuring all four backs twice, carefully remounting the ground glass each time, I discovered that each is different in both its average and its consistency from corner to corner. When I take the trouble to focus carefully with my 4x loupe after swings and tilts, a .5mm change of focus is significant. But whatís the point if the film is not going to be on the same plane as that to which I focused?
So I mounted the Hopf glass into the camera back which offered the most consistent measurements just under 5.00mm. I marked another back as usable and the remaining two as suspect for fine focus. Now Iíll order another ground glass from Steve Hopf. And Iíd love to hear your comments. Is all this unnecessary? Did I do something stupid or miss something obvious? Or is it worth the trouble I took?