HELP on building Optical Viewfinder for a 35mm adapter

[Forrest Schultz]

the thing is, i want to incorporate an optical viewfinder to view the image off of the gg. and to do this, i can stick a peice of clear glass at a 45 degree angle, standing up between the gg and camcorder. and facing off to the left. so the image then gets flipped sideways and shot off to the left. it will then be met by a mirror facing up at a 45. that way it shoots the image straight up and makes it appear rightside up. but the thing is, you still will be viewing the image very small. and i need to know how i can get the image to apper very big when i put my eye to the eyepeice. so i need to magnify the image off of the mirror.

Does anyone know how to do this. basically, it cant be much different from cinema motion picture camera viewfinders. if someone knows how film camera viewfinders work. then this shouldnt be to hard of a thing to copy. thank you for any help. i included a picture to show what im talking about.

http://i11.photobucket.com/albums/a1...deradatper.jpg

PS: i am doing this because i dont think ill be able to mointer the image for my new video recorder at full live speed. long story, but thats why i need to build a viewfinder. thanks

[Bob Hart]

That piece of clear glass may give you some problems with internal reflections, = parallel ghost images if it is going to be continuously in the path unless it is a proper optical material.

You might investigate Edmund Scientific and their magic pieces of glass which can be had in various percentage ratios of transmissiveness and reflectiveness. In my ignorance, I cannot now recall what the stuff is named. The process is called image splitting.

Any splitting off of enough of the image (30%??) to be of use in a viewfinder is going to reduce the amount of light available to videocamera itself which is already to get a short ration due to the groundglass device.

[Bill Porter]

Sounds like you need a pellicle mirror rather than the clear glass.

[Forrest Schultz]

thank you. i have an edmund optics magazine, and i have checked out the split mirror prisms and such. they would work also...but i would lose 30% by sending it to the viewfinder. with the clear peice of glass, i still see about 40% of the brightness of the image, and through the glass, i lose perhaps no more than 2%. thats why im using clear glass. it works just like a teleprompter in a broadcast staion. the cameras are hidden behind angled peices of glass that a tv moniter below reflects onto which the reporter can see. ive already tested the idea, the only thing i need to solve is what types of lenses do i need in order to magnify the image back into my eye. thank you, if anyone knows how this works, even how motion picture viewfinders work. i would be very grateful. thank you guys for the help!

[Valeriu Campan]

Forrest,
Try to google info about Bolex 16mm cameras viewfinders. They are using a pellicle mirror/prism with a transmissive /reflective factor of ~80/20. Also, Canon has some stills cameras in the EOS RT range that use a pellicle mirror. You could canibalise one of them or find the spare part.

[Bob Hart]

Edmund also make or stock a thin glass panel which does a similar sort of thing as a split prism.

A like method is used in the CP16R and (RA) motion picture cameras. I investigated it a little as I had ambitions of using a security camera + 25mm c-mount lens as a video assist and found the available image from the split a little dark.

In the CP16 however the image split is not between the film and the viewfinder. When the shutter is running, the mirror image becomes intermittant. The image split occurs where an outlet for a video assist would be installed and also serves as an image mixer.

In the CP16R the image passes from the lens to a mirror on the rotating shutter. The mirror is at a 45degree angle. The viewfinder image travels in a vertical direction to a partially transparent piece of glass. It reflects off this glass horizontally to the left into the viewfinder path.

This piece of glass, as well as bouncing the image, also allows the LED displays which are set in the right side of the camera, to pass horizontally through to the left into the viewfinder path, to be visible into the viewfinder. It allows the mixing of images from two sources into a common outlet.

It also allows a small ration of the viewfnder image to pass vertically up into the video assist camera.

Don't take my word as correct on this as it is now over ten years since I had the viewfinder apart. I have mostly forgotten the details and had to ransack my feeble brain for the memory.

Whitehouse AV in California might be able to explain it a little better or another CP16 operator might be able to do better than I here. It was Whitehouse AV who advised me to look up Edmund.

[Forrest Schultz]

Thanks Bob, that's exactly what i was talking about. Would you happen to know how they magnify the image, like when you put your eye up there, the image appears closer than it really is, do you know what kind of lenses are inside the tube to do that. i want to make my viewfinder like on of those long ones you see on motion picture cameras. they are called viewfinder extenders, and thats what i wish to do with mine. thanks

[Bob Hart]

Forrest.


What I forgot to mention was that at the same film plane distance from the taking lens via the 45degree shutter mirror is a special groundglass screen which is made up of glass fibres bonded together, cut off and polished into a thin panel. Safe area marks are printed onto it.

It is the image from this which passes via the splitting system. The eyepiece lenses I was not able to get at.

I think it may be a straightforward magnifying system. When you tilt the movable eyepiece upward, the viewed image rotates clockwise (I think???) and you end up moving around to the side of the camera when looking straight down.

The path as far as I can determine is a four-reflector erecting system which uses four mirrors or a mix of mirrors and prisms. Lens >> Mirror 1 - the 45degree shutter, >> groundglass, >> Mirror 2 - the glass splitter panel >> Mirrors (or prisms?) 3 and 4 - the two 90 degee elbows in the short extension outside of the camera body which carries the eyepiece and eyecup.

At a guess, and a really wild one at that, the centreline distance from the eyecap via reflectors or prisms or whatever they use would seem to be about 8 inches. When you look at the path of a Bolex, the distance overall seems about the same. Given the same 16mm frame size and the fair average optical specification of the human eyeball, there is probably some optical rule which comes into play. - I'm way out of my league here.

The CP16 is made from a lightweight magnesium alloy cast and machined. Beautiful work with compound curves and was probably a nightmare to nut out.

The patternmaker or diemaker work represents the best of what US engineering then had to offer before CAD systems took the toolset out of the hands of the craftsman and automated them via the computer keyboard.

The magnesium alloy has one flaw. It corrodes. The junction where the eyepiece fits into the case is now welded together by a white powdery bloom and attempts to separate the parts just for the sake of looking at them do not justify the risk of destructing them. So that is where my knowing ends.

[Forrest Schultz]

Thank you very much Bob, your posts are always very informative. I think i got the idea for building viewfinder into the adatper. and ill get to that soon. thanks!

[Régine Weinberg]

D'ont do that, you allready loose light by you 35mm adapter, and keep in mind no not 35mm or 16mm camera uses an optical viefinder, reason for that a CCD is so lousy on light compared to real film that all camera today is not using an optical viewfinder anyway. And kep in mind that like in old days if you eye is not pressed on, light will go in there if there is no way as on all classical ones to close it. Too much work anyway, if you canibalize a Minolta A2 you will have one of the best electronic viewfinders.

The stupid guy whois no guy anymore but a girl

[Ben Winter]

Wow...an optical viewfinder is pretty gutsy on a 35mm adapter. If you do it with good results I'll be impressed.

[Bob Hart]

Light fogging the film from the viefinder path - I had forgotten that, yet it was a basic rule.

The CP16 reflex finder has a small iris controlled by a knurled ring around the eyepiece near the cup. The Bolex had a flap operated by a little silver lever in the top section back to the eyepiece.

If you were setting up, then locking the camera off and shooting without using the finder, such as in a landscape, especially if shooting very early or late upsun, or shooting with wide aperture into a shaded subject on a verander, such as a one man band interview where the camera operator steps out from behind the camera and talks to the interviewee, then light could get in and spoil the shot.

Video has made me lazy but I do take an old towel or paper cup to toss over the eyepiece so the sun doesn't get in and barbecue the little LCD inside.

[Régine Weinberg]

HERE WE ARE
there ias so much mechannics involved why not look for a Bolex in your local area hanging around in a shop for years and canibalize it. You will have a perfect viewfinder. you can even get some 16mm scrap or a russian camera think of it.

[Bob Hart]

16mm or 2/3" video optics likely won't work to satisfaction in the proposed application. The motion picture format is 35mm and the frame size varying from 24mm wide to 36mm wide, depending on the choice of the adaptor builder.

The best choice would be to study the design of a 35mm motion picture camera viewfinder extension and replicate that.

The only other route and it would be a limiting one, would be to look for any older 35mm motion picture lenses fashioned after the style of 16mm zoom lenses like the Pan Cinor Som Berthiot or Angenieux which had the reflex viewfinder split and viewfinder extension incorporated in the lens itself.

Both had detachable eyepieces. Angenieux had a choice or short or long extensions. I don't know if 35mm lenses were made in this style. If there were, then likely they were zooms.

A search under "Angeniuex 25mm-250mm"+"Michell" might yield something but I am really only guessing here.

If such a lens existed, then it might be more convenient overall to make the adaptor around the lens. It would not be feasable to dismantle such lenses for the viewfinder optics as the tiny split-field focussing prisms and the image splitters themselves were integrated into the body of both lens types.

This enquiry really needs better brains and knowledge than mine.

[Bob Hart]

Correction to above. - "Michell" should read "Mitchell".