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Old August 15th, 2005, 03:46 PM   #1
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Anyone tried making a focusing screen?

Just wondering if anyone has tried this? Is it worth it?
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Old August 15th, 2005, 03:49 PM   #2
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When you say 'focusing screen', do you mean something different to ground glass? I mean, people have been making there own ground glass and wax screens, sure. So could you just explain what you mean?

thanks,
Wayne.
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Old August 15th, 2005, 04:32 PM   #3
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More like the Nikon D focusing screens - the pattern on those seem like that eminate from the center (like a circular pattern) and go outward, very different than the GG with wax or fine grit sand paper.
And there are also ones that have condensers built right in, stuff like that.

Not regular GG, sorry about the confusion.
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Old August 15th, 2005, 05:09 PM   #4
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Mandy,
Are these screens, like the nikon D screen Ben is using in his thead, are they made of plastic do you know? Do you have any idea as to the process thats involved in producing them?

Wayne.
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Old August 15th, 2005, 07:19 PM   #5
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I believe they are made of plastic - the process is what I am trying to find out, maybe it is a machined impossibility by hand sort of thing, not sure, but I know that focusing screens are really good in the GG category, so I thought I would put the question out there.
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Old August 15th, 2005, 08:27 PM   #6
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Metal and glass...metal and glass.
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Old August 17th, 2005, 10:58 AM   #7
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the focusing screens, like the nikon D are fresnel lenses... but really high quality... basically look at it this way. beattie and nikon are brands that sell screens... but one costs more and could result in a better image. you can also by a fresnel from like a drug store at home from what i hear... but the quality will suck. the beattie and nikon are much finer. crap what do i know...
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Old August 20th, 2005, 09:38 PM   #8
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Though I don't think its the way that those are done, I'm pretty sure you could get a fresnel laser eteched and that would be technically a focusing screen I guess. Probably be way more expensive than just buying a beattie screen though.
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Old August 21st, 2005, 07:21 PM   #9
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As far as I know, a Fresnel is a clever way of making a condenser lens flat. Because we have some space between all lenses and GG, I shouldn't focus on the Fresnel and just use a good condenser.
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Old August 21st, 2005, 10:10 PM   #10
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What would the point be in making your own fresnel? You can get fresnel companies to make a one-off fresnel (well, a Beattie is a fresnel plus a diopter) to your focal length. The time and setup to even attempt to machine or laser-cut your own fresnel would be so much greater than the cost of just having someone who is already proficient at it, do it for you.

And yes, fresnels have focal lengths!
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Old August 25th, 2005, 04:40 AM   #11
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An ideal focus plane material

I'm not sure a fresnel lens would be a suitable image forming surface as it is intended to be an approimation to a much fatter single lens (could be concave or convex).

What you need at the focusing plane is something that disperses the incident light evenly in all the _useful_ directions, whilst not absorbing any nor dispersing it in unuseful directions.

Dispersing in unhelpful directions is the main problem with both gg and wax. They tend to send some light back out the objective lense and some bouncing against the inside of the barrel. Both 'wrong directions' reduce the light available for the second image (at the 3CCD) making the system darker aka slower. The light bouncing off the barrel is the worst as it causes artefacts and diffuse greying of the image (loss of contrast).

The ideal focus plane for the 35mm image would be either purely reflective or purely transmissive. (Most of the adapters I've read about have a transmissive 35mm focal plane.)

Such a material might be covered with very small fisheye lenses. (For a refelctive plane these would be silvered like an astronomer's parabolic reflector.) The curvature of the lenses would be such that they did not disperse the incident light in either of the unwanted directions but,
only-&-evenly onto the next lens. By using two different diameters of such microscopic lenses they could be packed so as to account for >90% of the surface of the plane. (The remaining 10% non-ideal areas between the lenses could be blackened to block mis-directed light. More complex prismatic lense arrays might resemble the lens of a car headlight.)

Does such an array of lenses already exist? Well there is the surface of CMOS and CCD chips. A less expensive approximation for our purposes might be an old glass master for CD stamping.

Is this what was intended by the initiator of this thread?

However, ultimately all such adaptors are ways to fit an undersized camera, to the right choice of lenses. The best technical solution will always be to put the retina of the camera (CCD/CMOS/whatever-comes-next) at the first image plane, and make it the right size for the image.

You cannot escape the geometry which dictates the relationship between depth of field and image size, and thus the size of the camera.

For the moment adapters are an affordable method. Much better will be 35mm high fps CMOS chips which are gradually becoming more common and more affordable. vis the HD threads on this forum.
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