We all want perfect or near perfect focus but 5x4 film holders and ground glass focusing each have their own tolerances so does anyone know at what point smaller apertures cancel out these variations.
Steve
We all want perfect or near perfect focus but 5x4 film holders and ground glass focusing each have their own tolerances so does anyone know at what point smaller apertures cancel out these variations.
Steve
I don't know that one could quantify that point so easily. Having experimented with this quite a bit, I would suggest that the only thing aperture (Depth of Field) might be counted upon to compensate for is our own error in focusing. If GG coincidence is off and the film plane is off in the direction that compounds the error, you'll never find an aperture small enough to compensate. 4x5 film holders were built to a nominal depth of .197" +/- .007". If the GG is aligned to .197" +/- as little as possible (maybe .001" is possible in a wooden camera, less in something like a Linhof or Sinar?) then, if the holders are within range of specified tolerance, your results should be fine. It's best to get the GG in the proper place to start with. The tolerance specified for film holders is to compensate for the fact they are made of plastic and subject to wear.
You can easily quantify this by calculating the depth-of-focus (note: this is distinct from depth of field) at the aperture of interest. As long as the depth-of-focus is larger than your GG/camera tolerances, you can say that your aperture is compensating for your camera tolerances.
Science is what we understand well enough to explain to a computer. Art is everything else we do.
--A=B by Petkovšek et. al.
As said already, that kind of calculation can be done but it seems like an academic issue to me. Or, it would be specific to a particular camera and film holder. Even shooting at open aperture, which I do frequently, I have never suspected (meaning, "seen evidence") that manufacturing/assembly tolerances have been an issue.
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When I replaced the GG in my Super Graphic I made a lot of measurements, and no, it isn't academic. It is the difference between sharp and soft images. I had to shim my GG a bit, and it made a huge difference. The easiest place to find stable shim material is something most of us use a lot: film. Sheet film is 0.007-in, and 35mm is 0.003-in.
Aperture doesn't. Most people focus wide open and then stop down so unless your lens shifts focus (like an Imagon) you had best be dead on wide open. With a lens that shifts focus with aperture you have to focus at shooting aperture which can sure make bracketing a challange if you are changing aperture and have little time to re-focus.
Linhof has used a zeroed groundglass system for decades so there should be no reason why others can't be just as accurate, if desired. For many years Linhof actually put a serial number on their 8x10 camera backs which had to be supplied when buying an 810 to 45 reducing back to guarantee the same film plane for both 45 and 810 on the same camera! The reason for this was so 45 Polaroids and 45 test shots would have the same focus as 810 film. Since people stopped wanting that feature Linhof stopped doing this about 20 years ago.
f# = tolerance / circle of confusion
Proof: similar triangles formed by tolerance/coc and aperture/focal length.
For a coc of 0.1 mm (very forgiving by most people's standards) and a film holder tolerance of 0.2 mm (8 thou, but who's counting) apertures of f2 and smaller will cover the slop.
Summary: most photographers don't need to worry about it.
At f16 your focus spread is about a millimeter, so that will cover a condition of combined intolerance of plus-or-minus 0.5 millimeters. That should be good even for home made cameras and film holders
What will kill you, though, is when the film shifts during an exposure. That won't be minimized by a small aperture. Only a rapid shutter duration will aleviate that one.
Guys, I just replaced the GG on my 5x8 Chamonix and notice that the new "boro" GG is just a fraction thicker than the "stock" Chamonix GG. To my feeble mind it should not matter because focus is made on the surface of the GG that is closest to the lens. However I am probably wrong so please advise.
Thanks
You can always test for gross errors by shooting an angled-away yardstick at close range...focus on the 1.5 foot mark at wide open aperture and then when you develop your film see if the 1.5 foot mark is in focus. To check the whole image area, you can do a similar test with a resolution chart. This will tell you if your camera back is on wrong, at least.
Science is what we understand well enough to explain to a computer. Art is everything else we do.
--A=B by Petkovšek et. al.
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