Re: Camera Obscura formula

Hi Barry

It's going to depend on what you want to do with it ... but the basics of focal length / dimensions etc. will follow the same lines as with any other camera ... so if you were to look at large format cameras, lens recommendations, etc. you'd find useful information there.

This is one I built for my degree show at art college last year; one you could sit inside. My intention is to use it to combine a photographic image (VERY soft) with images drawn using a light brush. In the degree show I simply put a supply of drawing paper and pencils inside it, and marked a cross at the right distance for someone to stand and be in focus, and invited people to use it. Friend A would stand on the marker while Friend B would go inside and sketch them. Lots of people loved it and during the week the show was on I pinned up sketches on the outside as people made them. Some of the sketches were really good ... people like the way it frees them up from worrying about where the lines should go, and they can concentrate more on how they make the marks.

My aim was to image a standing figure, a modest distance from the camera (c. 3m) onto a 12 x 16 sheet, and to have the image form on a surface a convenient height for someone sitting down on a normal chair. So that's how the dimensions came about. The focal length of around 600 needs to be achieved with a suitable lens: my first mistake was buying a couple of long lenses on ebay at around the right focal length, only to then discover that because they were telephoto designs ... basically air survey lenses ... the back focus was much shorter than I wanted. I ended up using a large old condensor lens from a (presumably) 10x8 enlarger which someone had scrapped in the college years before ... I was just lucky that the lenses were still lying around and I got wind of someone wanting to chuck them too. The aperture with my setup is about f 4, I guess, and though I devised a set of slide-in stops to sit below the lens ... I can use them like a shutter from inside the camera ... the image doesn't actually sharpen up much as you stop down. Some of that is down to the mirror, which, if it were front-silvered, would perform better. Actually, if I rebuild the mirror / lens assembly, which I probably will, I'd look for a front surface mirror with about the right focal length and go with that.

http://www.harrysmart.net/fine/studio_04/obscura.html

I have one or two images made with it both as a camera, and as a tool for drawing with light, but they're all a bit crap, so they're not online yet, and I still haven't really worked out how I want to combine the two. But it's been fun so far.

I had a couple of other simple things sitting beside the big one in the show ... very rough and ready camera obscuras made out of cardboard boxes with one side cut out and replaced with tracing paper. Opposite the tracing paper I had lenses either taped over a hole in the box, or, in one case, a simply 'symmetrical' lens made by taping up two condensor lenses ... c. 3 in. diameter ... in a roll of corrugated cardboard, the whole assembly sliding in and out of a hole in the cardboard box made by cuttign a star-shape and pushing the flaps in where they helped hold the tube. Very rough, as I say, but focusable by sliding the lens tube in and out. Another was made using the fresnel lens salvaged from an overhead projector, with a crude cardboard flap that could be flipped in front of it to stop down the aperture. There are cheap lenses to be salvaged / scrounged all over the place, and measuring their focal length is very easy ... just find a white wall facing a large window and move the lens by hand till you can image the scene through the window ... when a distant chimney or something similar is sharp, just measure the distance from the lens to the wall. Finding lenses and measuring their focal length could be a fun part of the project ... me doing that kind of stuff in the studio in college certainly provided hours of harmless fun for the other folk in the studio. I never got on to filling spheres and cylinders with water, but that would have worked too, and you could look up the early lens designs that followed that principle, or make microscopes with beads of water.

Another thing I had in the show was a piece of mirrored acrylic ... about 3 foot by 1 foot ... held in a curve on a simple board that I screwed to the wall ... it gave a 'panoramic' view, which I also used to take photographs ... just using a digital camera to photograph the reflected image ... you can play about with the curvature very easily, and the digital images can be stretched or unstretched in Photoshop to restore natural proportions. If you look into some of the stuff online where people have made reflective panoramas using hubcaps or silvered packaging materials as the lens, you should get lots of ideas for inexpensive educational play.

I found, actualy, that I turned from being the photography geek who sees images everywhere, to being the photography geek who sees cameras everywhere.

If you want more details of the big camera obscura, just message me off-list.

Harry

  ----- Original Message -----
  From: Barry Kleider
  To: alt-photo-process-l@skyway.usask.ca
  Sent: Friday, August 19, 2005 4:29 PM
  Subject: Camera Obscura formula

  Does anyone know the formula for figuring out the optimal dimensions, aperture and other details for building a camera obscura?

  By the way, Thanks to all who have been helping me think about teaching optics.

  Barry
Received on Fri Aug 19 10:29:59 2005