A big one. Like they used to have in coastal resorts in N. America and UK, probably others.
One survives at Land’s End in San Francisco- take Geary west until you run out of street - ignore the trourist-trap restaurant, walk down the exterior stairs, and pay the $1, enter the tacky looking “camera”.
Wait until eyes adjust to the darkness (there is a reason it’s shaped like a camera).
Be awe-struck Giant Camera, San Francisco, California fro a quick idea…
Can’t find an entire enalger with the mirror, and a full set (usually 5) of condensors.
Now the problem: what is the curvature of the viewing screen? Can it be worked out trial-and-error? Since I have no idea of the specs of the condensors, it gets a bit tricky.
(the fact that i have no idea what the specs of a lens mean makes it a tad more difficult)
Camera obscura calculations are in one of the old Edmund experimental optics booklets. They available online (pdf) here: http://www.anchoroptics.com/documents/
The curvature of the viewing screen must match the curvature of the lens’ focal ‘plane’. You’ll have to determine that experimentally. Just project onto a flat surface, and measure lens/screen distance for focusing something in the center of the light cone, vs the distance for focusing on something at the edge of the light cone. That’ll give you two points on the curve. Approximating the curve with a spherical section should be good enough for your purpose.
There is, or at least was, a company in Bristol PA whose specialty is making giant plastic lenses. I don’t mean Fresnel lenses, with the angled grooves embossed in them. I mean, a big heavy thick chunk of acrylic, with curved faces. I saw a picture of one they made that had to be two feet thick on the center, with angles as steep as 45 degrees. They use thick slabs of plastic that are made as windows, and they laminate these as needed to build up thickness. Then they turn them with a diamond tool lathe. They get used in things like simulators for training pilots, captains, and railroad engineers.
It’d be cooler than two speckled puppies in a red wagon to see some of these giant things in a camera obscura.
Yes, I really have no idea now to resolve any of the formulae in CalMeacham’s thoughtfull post.
Am I evan close to assuming I would want identical condensors, or should I just get a full set of the old Laritcos and play? All I know is that, in their original purpose, the different sizes were used to enlarge different sizes of negatives - from 35mm (24mm x 36mm) to 5" x 7".
The Night Gallery episode "Camera Obscura featured a post-Wild Wild West Ross Martin as the purveyor of a mystical Camera Obscura, dueling with Rene Auberjonois back in 1971 (who the hell knew about Auberjonois in 1971?)
http://us.imdb.com/title/tt0660796/
It was the first time I ever heard of a Camera Obscura that large, or built in the manner the OP suggests, with an overhead tower projecting downwards onto a horizontal screen (and if you ignore the magical aspects of the device in this episode)
Here are some more pictures and diagrams of the overhead Camera Obscura, and reference to the Basil Copper story the episode is based on:
Last year, another science teacher at my school made a camera obscura in her classroom. She just covered the window up with heavy black paper, and cut a small hole in the paper. Then she hung her overhead projector screen close to the window. Adjusting the screen distance and the size of the hole, she got it focused pretty well. It was really cool–with the room darkened you could watch the screen and see people walking around outside and cars driving by on the road. And, of course, it was all upside down.
She is teaching elsewhere now, so I may “steal” the idea and make one in my classroom this year.
