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Old February 25th, 2008, 02:29 AM   #1
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Where do you focus on the ground glass?

That is a general question, I was adjusting back focus on Mini 35 and it seems, it has two areas to focus on, one,
1 Small sharper particles looking like a dust on the surface of ground glass
2 Ground glass itself, seems to be just a bit deeper with thicker grain pattern.

Does it make sense or I am going nuts...

CHeers!
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Old February 25th, 2008, 08:01 AM   #2
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Oleg.


I have found that a slightly sharper result can be had by doing as you suggest, looking for the focal plane within the thickness of the groundglass layer, not the most visually apparent surface texture as is the recommended practice.

I have found that when using camcorder autofocus on the Sony Z1, the camcorder also chases the focal plane within the groundglass layer when there is a sharp image upon it. It sometimes finds a different place when presented with the groundglass texture alone without any image upon it.

If a groundglass finish is created by dressing the glass with a five micron grit, one might expect that the deepest pits and highest peaks of the texture will be up to 5 microns apart, therefore the groundglass layer be up to 5 microns thick. Just where within that layer the sharpest image yield occurs I could not even guess. I would expect that it is not at the bottom of the pits or at the peaks.

While the pits might be expected to be fairly sharply defined at the bottom, the peaks might be partially polished and there might be a tendency for rough microlenses of random focal length to be formed. If the camcorder focus is upon this area, then the result might be softer than if the focus falls upon an area within the groundglass thickness where straight bevels in the sides of the pits exist.

This totally unfounded and unproven theory on my part is based on observations when operating my own home-made device. The point of sharpest image relay may be caused by my practice of setting the SLR lens backfocus by finding the sharpest image when the lens witness marks are set to a given distance. The SLR lens backfocus on my device is adjusted by moving the disk motor mount with three adjustment screws. The relay focus is then set.

The P+S Technik is fixed and relies on build quality and precision. I understand the SLR lens backfocus or collimation is achieved by a normal precision camera build and maintenance practice of projection from the image plane (groundglass) onto a screen. My guess is that this method finds the ideal point within the thickness of the groundglass layer and the SLR lens backfocus is set to it but it is just that a guess and uninformed at that.

P+S recommend using a Putora focus chart. I have been using a local Lemac chart and set backfocus using the resolution blocks as well as the Siemens patterns.

This is purely subjective on my part but I think going after the image within the thickness of the groundglass layer is good for up to another 5% of sharpness in the image.

Last edited by Bob Hart; February 25th, 2008 at 08:32 AM. Reason: error
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Old February 25th, 2008, 11:52 AM   #3
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Bob!

Thank you very much for your detailed answer, I will go for a deeper ground glass pattern! That makes a lot of sense, appreciate it.
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Old February 25th, 2008, 07:11 PM   #4
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Oleg.


If you own a Minii35, please do not change anything in the Mini35. I am not an engineer. Treat with suspicion anything I say.

P+S Technik have made their groundglass to be absolutely the finest grade of texture they can go to without getting pass-through of aerial image. You cannot make it to work any better than it does.

If you are making your own groundglass adaptor, then there is no problem.

I made one groundglass which was absolutely the best for my own AGUS35 home build device. I was too much hurry to finish the disk. There was a pattern of deeper pits overlaid by the 5 micron grind.

My groundglass disks were made from cut blanks of optical glass which I first had to figure and polish down, then dress with 5 micron aluminium oxide to achieve the groundglass finish. The disk was broken at an airshow and I have not been able to make another just like it since.

The P+S Technik groundglass? My guess is that it might be engineered something like that or a layer of finer grade translucent material infused into the coarser groundglass texture.

One or two of us who post here have tried combinations of microcrystalline wax and beeswax between clear glass panels or laying the wax onto a previously dressed groundglass.
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Old February 29th, 2008, 01:49 AM   #5
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Bob, that is a great very informative info, Thank you!
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