Hi everyone! Planning on building a few camera obscuras for my students to play around with. Do you know where I can find cheap plastic elements. Pinhole projects too dark of an image to work with. I've thought about the element from a cheap magnifiying glass...any help would be appreciated. I'm looking for an element that would throw a decent image circle with focal length of 12 inches or more. Thanks!...
Don't get a cheap plastic magnifying glass; I bought one at a 99-cent store for the same reason you're pursuing, and the image it threw was barely recognizable. It doesn't even work as a magnifying glass; the lens surface has waves in it. The crap that comes out of China these days... (Take that, all you Shen-Hao users!)
"I love my Verito lens, but I always have to sharpen everything in Photoshop..."
Mark,
Check your e-mail. I have been trying to contact you for the last two days.
Surplusshed.com or Edmund Scientific's websites are good places to start. Both have educational optics are reasonable (or sometimes cheap) prices.
Get a plastic Fresnel lens, the type that they stick on the rear window of buses and such. Poor quality, long focal length.
Or find a cheap close-up filter, the strength is given in dioptres, which is the inverse of the focal length in meters: +2 = 1/2 m focal length, +4 = 1/4 m.
How big of a camera obscura are you making?
You can get a set of diopter filters really cheap. +1 = 1000mm focal length. +2=500mm +3=333mm.
I paid under $10 for a set of three that were 67mm in diameter.
If you need really bright, get a lens from a condenser assembly.
I've projected images with a page-size (about 8x10) plastic Fresnel that were, at least, recognizable. These lenses are cheap, large, and have long enough focal length to produce a big image. You can get them in office supply stores (or used to get them in dime stores, if you can still find one -- try a Dollar Store or Dollar Tree, maybe Big Lots?). Tape one to a window, cover the rest with foil or black plastic sheet, and use a piece of white matt board, posterboard, or styrofoam sheet for a screen to accept the image and you're good to go -- if it's sunny outside, you only have to dim the room as much as you would for an overhead projector.
If a contact print at arm's length is too small to see, you need a bigger camera. :D
Anderw,
Some history might be of help. A regular magnifying glass will have a limited region of sharpness, due to its sharp focal zone being a parabola and not a flat plane. In 1812, Wollaston determined that a meniscus lens with a stop in front of it on the concave side would yield a reasonably flat field for the camera obscura. The diopter filters Darin mentioned would be one way to obtain such a lens. (Perhaps front element from Toy Binocular, Telescope, reading glases ??)
Mount as such: Object Side ------> : ) Image
The next improvment to the camera obscura was the advent of the Achromatic landscape lens of Chevalier in 1821 (note these dates are pre photography). Acromat lenses are available at the previous mentioned sites. These would be mounted flat side toward Object side with a stop placed in front of the lens. Try starting with a 1 in diameter stop at various distances in front of these lenses (coma is affected by the placement of the stop).
Mount as such: Object Side-----> : [) Image
I hope this helps... Mike
Don't get a cheap plastic magnifying glass; I bought one at a 99-cent store for the same reason you're pursuing, and the image it threw was barely recognizable. It doesn't even work as a magnifying glass; the lens surface has waves in it. The crap that comes out of China these days... (Take that, all you Shen-Hao users!)
--Mark Sawyer, 2005-04-30 10:35:16
Is there realy any need of that kind of ignorance on this forum?
Very satisfied owner of Shen-Hao,
Dan Jolicoeur
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