Optical design for a tent camera obscura

[bam707]

Hello all,

I've been trying to come up with an optical design to basically form images onto a sheet positioned near the ground, in an old camera-obscura fashion. Hopefully this image helps to visualize.

http://www.pinholeblender.com/COimages/mirror.gif

I know that I will require a 45 degree mirror as well as a converging lens to focus the light onto the screen. I got stuck in some of the calculations.

Important points / parameters:

1. Imaging things very far away (>100m)
2. Distance from lens to screen should be roughly 1.5m
3. Magnification needs to produce a final image of roughly 50cm squared (larger is okay)
4. Lens diameter should be equal to or less than 60mm, any focal length is okay
5. Planning on using an achromatic doublet for wavelength refractive index correction

Here is my inspiration: http://www.abelardomorell.net/posts/tent-camera/

The image in this case is projected onto the ground. I plan on doing the same.

Alternate questions:
>> Is a pinhole required to essentially make the incoming light from a point-source? I want to maximize the number of photons actually on the ground.
>> Because the things I am imaging are a far distance, the light would be entering the lens in a parallel fashion, correct?
>> I was thinking of maybe using two lenses, almost as a projector. Off basis?


I appreciate you all, and thank you for your input.

 

[Andy Resnick]

A colleague of mine in the Art department and I set up a camera obscura last year as a Intro Physics demo, it was really amazing:

http://imagizer.imageshack.us/v2/800x600q90/11/fq9y.jpg

Do not use a lens, that's the whole point of a camera obscura. The size of the pinhole is a critical consideration; too small and the image is too dim, too large and the image is too blurry. What we did was to have 'flaps' with differently-sized holes that allowed us to experiment and find an optimal size (about 1/4", as I recall).

There's not much to it, just make sure the room is as dark as possible and have patience until your eyes adjust.