Hi I’m looking for some thoughts/help with an issue that has recently crept into my LF negatives. (Cambo 4x5) I photograph a lot of buildings, and I've noticed that some of my photos are focused in front of the selected focus plane chosen on the GG. For instance, I take a photo of a building across a parking lot that is just shy of infinity (80’) away. No, swings, tilts, shifts. GG aligned and leveled w/ lens board. Look through GG, choose focus point wide open, stop down to f22 (my standard go-to Fstop), expose 3 sheets of film at around 1/30th second (sometimes with polarizer). I get the film back and the building is in focus... but barely... just at the edge of focus (or the edge of the circle of confusion that makes it seem in focus based on DOF) The parking lot is razor sharp half way to the building at 40’ away.
It seems that If my calculations are right my GG in the springback is too close to the film back inner plane where the film back slides in. The GG probably needs to be shimmed backwards, away from the lens. I can do this through trial and error and film tests.
Here’s the thing I need help with. I use very wide lenses when I see this problem. It's MUCH more pronounced with my Schneider 72XL than my Rodenstock 55mm, or my Nikon SW 90mm f8, or my Nikon 120 SW. I know these lenses need tiny movements to go from in-focus to out-of-focus. So a millimeter is a BIG move and I will need to make smaller adjustments. I am also using the unusual wax-infused super-bright BOSSCREEN GG (which has the stock black tape shims in place and is custom made specifically for CAMBO), I have two Bossscreens, both have same focus plane if I swap them out. I am not using a Fresnel. While I have your attention, I want to confirm my understanding that depth of field moves away from the point-of-focus 1/3 closer and 2/3 farther away, which is what a hyperlocal focus scale seems to confirm. I’m not seeing this in practice, in fact I’m seeing no real depth increase behind the focus point. Maybe it is the design of ultra wide lenses? Maybe my GG is way off.
I will start a test of my lenses with a 1mm shim to see if the focused image on the negative jumps behind the spot chosen on the GG... Then I can fine tune +/- from there. Let me know if I’m on the right track of if I have it all backwards and save me some film. Thanks!
-Stephen Schafer
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