What are those surface characteristics?
So a 0.2mm bulge on the centre seems to be somewhat accepted. One adjusts the body length to 0.05mm while there is 4 times the variation in film height! Fortunately there is some depth of focus, which goes up with f-number and magnification.
@Light Capture using a target on a piece of film to collimate lenses is a good idea. I once tried it but the results were a bit different... so I dismissed the method.
Now how can one make a piece of glass with 0.2mm recesses...
Mmill down the edges of a plastic focus screen?Now how can one make a piece of glass with 0.2mm recesses...
Coincidentally (or perhaps not?), Harold Merklinger discusses the consequence of a 0.2mm shift in the film plane in his book The Ins and Outs of Focus. It varies with focal length of the lens. Since the book is freely available on the internet, I guess I may quote it here:
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Well, let’s suppose we are photographing a person indoors. He is seated in a high backed rocker about 10 feet from our camera. About three feet behind him is a window. Through the window we can see part of the house next door and beyond that the mountains. We use a variety of lenses from 300 mm to 20 mm in focal length. Unknown to us a bit of film is stuck on the film pressure plate of the camera and so the film is actually .2 mm closer to the lens than it is supposed to be. What objects will be in focus? Using the basic lens formula, Equation (1), we obtain the answers shown in the table below. f is the focal length of our lens, Dis the distance from the lens to the object actually in focus. The subject’s eyes are at a distance about 3.000 meters (about 10 feet) from our lens.
View attachment 367847
From this table we find a number of interesting results. With the 300 mm lens, we focused on the bridge of the subject’s nose, but his eyes are in perfect focus. With the 200 mm lens we focused on his eyes, but his ears are perfect. With the 100 mm lens we can see every grain in the wooden back of his rocker. With the 50 mm lens the window is perfect. With the 28, the house next door is just beautiful. With the 25, those mountains aren’t bad at all. With the 20, nothing, absolutely nothing is in focus. We have a soft focus effect throughout!
What we see here is that long lenses are actually quite tolerant of errors at the film plane. Short focal length lenses, on the other hand, require absolute precision!
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Merklinger is talking about 35mm here, but earlier he discusses the effect of format, concluding that although the geometry of larger formats means they are more tolerant of errors at the film plane, they have a larger image area for film curvature to manifest, and this will have the greater impact. Hence cut film, vaccuum film holders, or even glass plates for critical work; also very small apertures.
In my opinion this discussion is flawed. It neglects the depth of field in the subject space at the actual taking aperture in use, which is greater for wide angle lenses at a given f-number. The depth of focus at the film plane, to first order, depends on the f-number in use, and not on the focal length. Because the f-number defines the taper of the cone of light toward/away from the point of best focus at the ideal film plane. If you have a 0.2mm focus offset, but an acceptable circle of confusion of 0.03mm, then the f-number that allows depth of focus to cover the error is 0.2/0.03 = f/6.7. This is true whether you are using a 200mm lens (where the focus offset at the subject is small, but the depth-of-field is small), or a 20mm lens (where the focus offset is past infinity, but the depth-of-field is large in the subject space).
Pictorially, the effects of a film plane offset might still be worse for wide angle lenses because one is trying to get the entire picture sharp from near to far, as opposed to tele lenses where one accepts or wants some blur of the subject away from the focus point. But this text gives a misleading impression.
I agree with Dan that the proof is empirical, at the small-offset level one has to test the system on film before knowing whether to get agitated or modify the camera over a nominal 0.2mm offset.
I don't believe 35mm cameras suffer much from this, though if the film remains in the film gate for an extended time, them maybe it would be an issue.
So a 0.2mm bulge on the centre seems to be somewhat accepted. One adjusts the body length to 0.05mm while there is 4 times the variation in film height! Fortunately there is some depth of focus, which goes up with f-number and magnification.
@Light Capture using a target on a piece of film to collimate lenses is a good idea. I once tried it but the results were a bit different... so I dismissed the method.
Now how can one make a piece of glass with 0.2mm recesses...
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