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BUILDING A DIRECTOR'S VIEWFINDER


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#1 Mike Medavoy

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Posted 16 May 2008 - 01:13 PM

Hi!

I need some help in building a director's viewfinder that can take 35mm BNCR lenses. Like the one from Visual Products (pic attached), which costs $1,800! Obviously I want to do it for much cheaper.

In the last few months, I bought the major pieces that I need, all second-hand, eBay etc. So far I got: Arri 35BL viewfinder extender, Arri 35BL eye piece, a BNCR mount from an Arri III camera and an Arri 1.85 ground-glass (although I will try to get a Super 35 instead, if I can find a cheap, second-hand one). So that's my student Frankenstein project (pics of the separate pieces also attached).

The last thing now, I need to build an intermediate piece to join the Arri III BNCR mount in front of it to the viewfinder extender behind, which will also obviously hold inside the ground glass carefully placed at a distance of 61.468 mm from the BNCR lens mounting surface (the BNCR flange distance).

I need some help, though. When I manually project an image onto the ground-glass and try to look at it from behind through the viewfinder extender (trying to mimic how would I see it through the finished finder), I cannot get it in focus. If I try to move back, everything is fuzzy. If I try to get as close as possible to the gg, I can see just a very small portion in focus (sort of as in macro mode). Whatever I try, I cannot get the complete image in focus and fitting nicely in the viewing area of the extender. That's perhaps because the extender currently has a big prism system inside, at the opposite end from where the eye piece mounts... Should I remove that? I don't know.

Can somebody with strong optical knowledge help me out? How can I attach the viewfinder extander behind the groung-glass in order to see the image clearly in focus, edge to edge? Getting back to the Visual Products image and a standard BNCR finder, is there a special prism/viewing system behind the gg slot and inside the viewing tube?

I would really appreciate some pointers! Thank you all in advance!

Best regards,
Mike

Attached Images

  • Visual_products_1.jpg
  • Pieces_small.jpg
  • BNCR_mount_1.jpg

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#2 Mike Medavoy

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Posted 19 May 2008 - 03:47 PM

Can anyone help with a suggestion?
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#3 Mitch Gross

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Posted 19 May 2008 - 08:15 PM

The extender is the wrong device. from the ground glass to the eyepiece there needs to be a certain optical distance (I cannot recall) to allow the viewfinder to coorectly focus with the proper field of view. The extension is designed to go between the standard mount point of the eyepiece and the eyepiece itself, with the inside optics compensating for the now-added distance. So forget the extension finder -- just remove it from your design. Now float the eyepiece back and forth until you can find the correct distance and then use this as a guide for making some sort of tube to go between them.

Good luck, you really need someone with a machine shop to do this properly.
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#4 Mike Medavoy

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Posted 20 May 2008 - 10:16 AM

Thanks Mitch!

I understand... I will try it later today. Since I already have the viewfinder extender, I can use that as the tube (since the eyepiece already can very easily screw at one end), and cut it at the other end to the right length.

I will post results...

Thanks again!
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#5 Mike Medavoy

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Posted 23 May 2008 - 02:37 PM

No luck.

I'm sure there's gotta be some optical component in the viewing tube because I can't see any image no matter where I move the eyepiece by itself.

I screwed back the eyepiece on the viewfinder extender and removed the big prism system at the other end, and now indeed I get a very sharp (and properly oriented) image in the eyepiece - but I can see only a small part of the imagefrom the ground-glass (about half of it). No matter what I do, I can't get the entire image from edge to edge...
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#6 Mitch Gross

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Posted 23 May 2008 - 04:07 PM

So you have an optic for 16mm instead of one for 35mm. You'll need some form of interim optics to un-magnify (is that a word?) the field of view by 2x. Edmund Scientific might have something, but you will likely need an optical tech to have it cut to the proper diameter to fit in your tube and a mechanical tech (perhaps the same guy) to mount it there.
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#7 Henri Titchen

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Posted 23 May 2008 - 05:33 PM

Great project. Keep us informed how it works out.

A very inexpensive supplier of lenses and associated items is called "Surplus Shed". http://www.surplusshed.com/

They stock binocular parts including eyepieces and other bits and pieces that you may find useful.

Henry.
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#8 Patrick Neary

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Posted 23 May 2008 - 06:54 PM

Hi-

Couldn't you just adapt a high quality loupe (like those for checking 35mm and medium format slides)?

You just need to see/magnify the image on the ground glass, right? Seems like messing with the camera eyepiece is making it more complicated than it needs to be.
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#9 Mike Medavoy

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Posted 23 May 2008 - 08:07 PM

The viewfinder is from an Arri 35BL, so for 35mm cameras... I did try it on an actual Arri 35BL and could see the image nicely.

Yes, I think I need something to "un-magnify" the image about 2x, for it to be seen in its entirety.

Will check those sources - thank you! And will post results, if any.
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#10 David Auner aac

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Posted 24 May 2008 - 03:39 AM

So you have an optic for 16mm instead of one for 35mm. You'll need some form of interim optics to un-magnify (is that a word?) the field of view by 2x.


Well, magnification can be used both ways. Anything over 1x yields a larger image, anything lower than 1x yields a smaller image. So here we need 0.5x magnification to see the whole picture...

Cheers, Dave
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#11 Mike Medavoy

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Posted 24 May 2008 - 12:10 PM

Well, magnification can be used both ways. Anything over 1x yields a larger image, anything lower than 1x yields a smaller image. So here we need 0.5x magnification to see the whole picture...

Cheers, Dave


Hi David,

How exactly should I achieve this? I suspect I need some kind of lens/optical element right behind the ground-glass.

Thanks!
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