Mechanical and optics help needed for camera tinker project


Some time ago I acquired a beautiful old bellows plate camera, quite closely resembling the top one on this page. Mostly I try to use old cameras with the film they're designed for, but plate photography is going a bit to far for my tastes. Therefore I set out to convert the camera to a digital one.

Something like the SVP FS1700 seems perfect for turning into an improvised digital back, but I think I'll need something to look through and focus as well. I've made a quick sketch of what I think would be a good construction, it can be seen at http://www.incarnations.nl/temp/camera.jpg. (1) and (2) are mirrors, projecting the image on the ground glass plate in the camera to the viewfinder on top. When the camera has been aimed and focussed, mirror (1) can be flipped down, allowing the scanner (3) to take the picture.

I'm a kinda decent tinkerer, but I've got no experience with lenses, mirrors and the like. A couple of questions I'd like to have answered:

1) Does this have any chance of actually working?
2) What would be the best way to determine the proper placing and angle of the mirrors?
3) The distance between the scanner and the glass plate might cause some focus problems. Could this be worked around with lenses or an extra ground glass plate, or will this cause more trouble than it's worth?
4) As an added bonus I'd like to add split-images focus to the viewfinder, but I have no clue how this works. It can't be as easy as just buying the prism/glass and inserting it into the viewfinder, right?

Any help or advice anyone can give me would be greatly appreciated!

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Prfesser5 years ago
I'm not terribly knowledgeable about cameras.  Iceng probably has the best advice if you just want the thing to work.  OTOH if you want it to work much as it did before...

What you've drawn is essentially the optical train for a single lens reflex camera (SLR). You may want to google that topic to see how it's done. So, to address your questions:

1) Absolutely. It's going to be a fair bit of work though.

2) You can sketch the setup, and use a protractor to determine the light path, then adjust the setup and try again.. Just remember that the angle that the light makes striking a mirror (angle of incidence) is exactly the same as the angle the light makes leaving the mirror (angle of reflection).

3) I assume you can't adjust the distance from lens to scanner, so you may want to see if you can adjust the lens-to- 'viewer ground glass' distance.  In the simplest arrangement those two distances are made to be the same.

4) Google it; I'm sure there's a sketch and description of how that works.  Somewhere.  I dunno.

Advice:  (A) Use first-surface mirrors if possible (aluminized on the front, not the back); it avoids unwanted ghost reflections.  (B) The viewing image will be upside down (I think).  May want to check this.  An SLR uses a penta prism to deal with this.

Good luck!
Fransvb (author)  Prfesser5 years ago
Thanks for the advice. I think I'll go and put the screwdriver to an old Practica SLR camera, see if I can figure out how the split-focus works. Maybe I can even salvage it or just strip most of the camera and use the body for this project.

I also noticed the ground glass plate takes up a lot of the light, maybe I'm going to have to replace it somehow..
iceng Fransvb5 years ago
My SLR drops a mirror into the light path for eye ball work and then raises it
when a picture is recorded to film. This means no image degrade action.

BobS5 years ago
The huge area of the plate cannot be matched by digital sensors. So even a full frame sensor (high end digital reflex cameras) would see this as a tele lens.

It would be interesting to look at the image quality, both at the center and the periphery of the plate area. Probably surprisingly good, but can only obtained with the small aperture available.

Nowadays, much more types( very different) of optical glass, coatings methods of fabrication, quality control and computing power (for designing) are available. That's why more modern lenses, with only a few stops down, can yield even better images, but at a MUCH larger aperture.

Just compare the digital images from the optics of the antique plate camera, a garage sale 135 mm tele lens and a similar modern tele- zoom lens. (zoom takes a huge toll on quality and contrast, so a 1980s fixed tele lens might even out perform the contemporary 28- 200 mm zoom objective lens.
iceng5 years ago
If making a digital camera why use a split screen ?
Just show the real image on an LCD screen and avoid image degrading.

Fransvb (author)  iceng5 years ago
The improvised digital back I linked to (a standalone 35mm slide scanner which would require some adjustments) has a fairly crappy LCD screen, I don't think I'd be able to properly focus using that one. That's why I guess I need the added viewfinder.

If you mean the split-images focus mentioned in question 4, that's just to help focussing. It would be applied between (1) and (2) in the image (attached below), or just in the opening to the top left of (2). When recording the picture, the split-images screen wouldn't be between the camera and the scanner and would not degrade the image. Still, I have no clue how such a screen works (and haven't been able to find information on it) so this entire 4th question may just be impossible.

While I think it would be easier to just skip the entire middle part and just attach the scanner to the back of the camera, I seriously doubt I'd get a usable result.