THE VINTAGE CAMERA ONLINE FORUM > CLASSIC PHOTO AND FILM TALK
Large format camera using photo paper.
finniii:
I'm building a large format camera using an old lens from a protector, which is 35mm across with no aperture. So I have a few questions about this design.
[list=]1. How sensitive is b&w paper? I heard i was about 6-25 ISO, but how do I find out exactly?
2. What size aperture should I make and does it have to perfectly round?
3. Does an aperture still work if it's in front of the lens?
4. What sort of exposure times should I expect?[/list]
It would be great if you could give me some advice!
Thanks!
PapaD:
The best way to determine the "speed" of the photographic paper (it will vary from emulsion batch to batch and from tpe to type) is to test it with different test exposures after the fact, with a constant development time as per paper/developer data sheets.
The shape of the aperture and the placement in front of the lens as opposed to behind it WILL make a difference, however, the difference will depend upon the focal length of the projector lens used. a round shape as near as possible, is most predictable as to image quality. Some older cameras used drop shutters in front of the glass. In fact, these drop shutters may be available from vintage camera auctions and other sources.. Good luck. This will work, but will require a lot of trial and error.
Peter:
http://photo.net/large-format-photography-forum/00DYWN.
I experimented a long time ago with photo paper in a wooden 13x18 camera with a Goerz lens to see what the lens did - believe it was several seconds exposure (broad day light) with aperture full open (about 25-30mm opening). And fiddling in the dark room to load the cassettes.
Michael Cleveland:
For a variety of reasons, the projection lens is less than ideal for what you are trying to do. You should be able to find a barrel lens with diaphragm pretty reasonably. They aren't all expensive, and you can find an appropriate focal length easily as well. You won't need anything more than a lens cap shutter for the speeds you'll be working with, though a manual drop shutter is probably controllable with better precision.
You can reverse the f/16 rule to get an idea of paper speed, but it will take a series of controlled tests, as noted. Bright sunlight, lens at f/16 (aperture diameter 1/16 the focal length), standard development. When you get a good exposure, the reciprocal of exposure time will give you the equivalent ASA/ISO. I expect you'll find very low numbers, possibly even fractional ISO values, but it's apt to be different for different papers, so be prepared to do a lot of testing.
You do not need to use a perfectly round aperture. Some inexpensive cameras have used square apertures, or other shapes, though you may find geometric artifacts of the aperture shape in effects like flare and out-of-focus points of light. The primary concern is that it be as close as possible to unilaterally symetrical (i.e., square, as opposed to rectangular). Circular or roughly circular apertures are best because they have that symmetry in all directions. An aperture in front or behind the lens will cause distortion (usually a degree of barrel or pincushion distortion) in varying degrees depending on focal length and the exact location and size of the aperture. Ideally, in a compound lens, the aperture should be at optical center, between the front and rear element sets.
Projection lenses are designed to project a flat field from a flat field, so are not ideal for camera work, though you may not notice the effects in all situations or subjects.
justineoaker:
Wow. This forum is great! I'm a newbie in photography but as I scan your replies, I could definitely learn a lot in no time! Thanks a lot!
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