Camera Obscura growing pains

[dcpetersen]

I'm not a photographer at all and I don't know the jargon so I appologize in advance for not using the correct terminology.

I recently built a very rough camera obscura on wheels and I've been pushing it around Brooklyn and Manhattan for the past few months. I've been using a pinhole rather than a lense which has been fine for my purposes but now I want to upgrade. So these are the specs I'm working with, my current camera with the pinhole has a distance of about seven feet from the pinhole to the film. It produces an image about 5'x6'6". I would like to build a new camera of roughly the same dimensions, but I need a lense that can focus an image from seven feet of roughly the same dimensions. Any recommendations or even information on what specs to search for would be appreciated.

[Eric Dinger]

If you want to focus at infinity you'll need a lens around 213 cm, over 2 meters! I havn't the foggyiest on where to get a lens that long.

Can I ask, what do you use for film? I havn't heard of any thats 5x6.5 feet!

[Roc Chan]

Try a low power reading eye glass +1.25 or +1.5 range.

[Walter Calahan]

Try a Zone Plate.

[dcpetersen]

To Eric:

I said film in the posting to keep it simple but actually I've been doing drawings on paper from inside the camera.

[Terence McDonagh]

Strangely enough I have a huge 30" lens labeled "Camera Obscura", and a 1.4 meter lens as well. I'm in NYC, so you'd be welcome to borrow either/both if you want to try them out.

The 30" lens has plenty of dings and a few scratches, but should be fine for what you're doing. You'd have to stop down the lens to get the coverage though.

The 1.4 meter lens has no adjustable iris, and has a little separation around the edges, but should also be fine.

PM me if you're interested.

[James E Galvin]

You might try a .5 diopter spectacle lens - 2 meter focal length. I looked at Edmund's online, didn't see any achromats that long except for expensive telescope objectives. You could use (from Edmundoptics.com) a plano convex paired with a plano concave of 2-3 inches diameter, focal lengths chosen so the combination gets you the 2+ meters you want. This would make a simple lens, not corrected for aberations, but slow enough so aberations might not be too bad.

[Donald Qualls]

Telescope objectives are commonly made in this focal length range, though they're probably bigger than you want. To get a focal length a little over 2 meters, you need a lens a bit weaker than 0.5 dioper -- a +0.5 spectacle lens is close, but if you can get a +0.75, and mount a -0.25 some distance behind it, you can adjust the final focus by changing the spacing (like the front two elements of a Cooke triplet, more or less); the further apart they are, the shorter the focus will be, with minimum being the +0.5 diopter (exactly 2 meters) difference in the lens powers when they're almost in contact.

A friendly optician can probably get you full size lens blanks in those powers for $10 to $20 each. :)