Members: 75,267   Posts: 1,660,672   Online: 978

1. Building a Camera Obscura

Hello all,

I am building a camera obscura that will be used as a screening room as opposed to making large images with. My brightness needs to be superior because the imagery being captured in the camera will be a live performance that viewers observe from inside the camera. The obscura is going to have an 11 foot focal depth. I am curious to see if anyone has any insight on what kind of lens might help with this. I have contacted an artist who is doing something similar but he is taking some time to get back to me. He seems to be using something that exceeds 1000mm. Any ideas would be greatly appreciated!
Thanks.

2. A photo group I'm part of did a camera obscura a couple of years ago. We ordered a simple positive meniscus lens with the help of an opthamologist member of the group. If you're saying that the wall of the obscura will be 11 feet (3353mm) from the lens and we estimate the performers are also 11 feet from the lens--you'd need a focal length of 1676mm. That's a diopter of +0.60. The largest blanks we could order were 75mm, so that would be f/22--acceptable for a darkened camera obscura (as long as the performers are brightly lit). The diopters are available in increments of 0.12, so you'd round off to whatever multiple of 0.12D is closest to the diopter you need.

3. You can cheaply make very long focal lengths by combining plus and minus dioptre spectacle lens with the same numerical power but with opposite signs (of course). When the lenses are in contact they cancel and the focal length is infinite. When the lenses are separated slightly a positive power appears. Gullstrand's Equation explains the underlying calculations.

4. Maybe ask Reinhold to make you a massive fast meniscus lens?

5. They make these things called projectors :-p I think they call the room you're talking about a "moving picture theater"

Pay no attention to me... Carry on...

6. All of these ideas have helped greatly. I have a good understanding now of what I might need. Had a few questions via research discoveries:

would I be able to make something useful out of an darkroom enlarger condenser lens?
what resources are available for information on lenses/custom lenses?
Where might I find spectacle lenses?

thank you all so much for your help

7. also, what about fresnel lenses? do they create large or small image circle diameter?

8. Originally Posted by jasonclements
Where might I find spectacle lenses?
I order mine from my local optometrist. I got 75mm +2 dioptre and a 75mm -2 dioptre spectacle lenses for \$20 each and now can make any focal length from infinity to 500mm by arrangingly them appropriately. I assured the optometrist that the lenses were for experimental purposes and not for vision correction or medical purposes.

 APUG PARTNERS EQUALLY FUNDING OUR COMMUNITY: