How can you revert an upside-down image in a camera obscura?

Sure. Put a second aperture behind the first one. But by then, you would not have enough light left to appreciate the phenomenon. Why bother?

hmmm, well if that’s the only way, then I don’t think we will. that sucks :(

How odd. We were just discussing this here.

Therefor, the only way to reverse the image is to build the camera as a super large single lens reflex, complete with mirror housing and three angle prism. You may be able to find a super large mirror, but good luck on finding such a large prism. And then of course, you’d have to make the mirror swing up during exposure, unless you used a pelical 50/50 design.

Quite an undertaking.

Were talking about viewing, or actually photographing?

If viewing, then hang yourself upside down. Or put a 45 degree mirror in the ceiling and look up. But that would only flip the image, it wouldn’t invert it.

If photographing it, well… just flip the fricking picture over.

yeah, not worried about the photographing part. We just wanted to show both ways to visitors viewing our project

Check out the Edmunds Optical Catalog Do a search for invert. There are prisms that will do this for you. The best setup is a Pechan Prism pair but that will cost you.

Since you don’t need to photograph, then @worriedguy is on the right track, but I don’t think you need to buy a stand alone prism. I don’t know if this would work, but it would be worth an interesting experiment, and I see no reason why it wouldn’t work. It runs in line with @worriedguy‘s suggestion.

Instead of buying a stand alone prism, just purchase an old film SLR camera from eBay. Any one will do, and they can be had for $20. all day long. Get one with a standard 50mm lens.

Mount that camera in the camera obscura pinhole, facing the subject. If the subject is extremely well lit, and the projection screen is in a very very dark room the light path from the SLR may very well be enough to project the light out the back of the SLR eye piece and onto your screen. Obviously, the higher quality SLR camera with a fast f1.4 or f1.2 lens would provide greater light transfer.

The lens/prism combo on the old SLR would act to reverse and invert the image properly on the projection screen. However, if the participants are behind the screen, the image would be reversed again. Have them stand in front of the screen. This might help if you use a highly reflective surface like a projection screen.

Wouldn’t a second lens do the job?