Optic Freaks - Let's Build A Camera Obscura

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…

OK - as far as I can tell, this is a first-surface mirror (google), a pair of PCX condensers, and a screen.
Since the advent of digital photo, the 1st three are dirt cheap on ebay - Durst made some very nice enlargers using first surface mirrors and condensors… Ebay photo/darkroom search for “latico” for the condensors.
Like this one:
http://cgi.ebay.com/DURST-200-Laborator-LATICO-138-Condenser-Head-Lens_W0QQitemZ200269316890QQcmdZViewItemQQptZLH_DefaultDomain_0?hash=item200269316890&_trksid=p3286.c0.m14&_trkparms=72%3A1234|66%3A2|65%3A12|39%3A1|240%3A1318|301%3A1|293%3A1|294%3A50

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.

Squink writes:

Well, it’s the Petzval Surface, and , while it might be easier to do this empirically, you can always calculate it:

Or you could simply use a pinhole, resign yourself to losing a lot of might, and use a perfectly flat plane.

After you measure the index of refraction of your mystery lens. :wink:

It’s not my fault you don’t know the index of your lens.

OK, here’s a source for nifty optics:
http://www.surplusshed.com/

Anybody find anything ready-to-use as a screen?

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".

Thanks again!

There is one at the top of the bluff above Santa Monica (CA) beach. I have never been inside, and don’t know what the deal is on it.

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.