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Camera Obscura II

Sarah Fox , Nov 11, 2011; 10:38 p.m.

Hi all,

The recent camera obscura thread got me to thinking about a conventionally constructed home that we will be retrofitting with numerous green energy technologies. Without belaboring the details, one aspect of the home's design will involve using styrofoam-core shutters to block heat leakage through the windows of unused rooms, which of course will render those rooms completely dark.

Now what better way would there be to provide a minimum of lighting to a room than to introduce a camera obscura in each room, which would project views of the river behind the house? Pretty fitting for a photographer's home, eh?

The problem is that the light from a tiny hole (even maybe a 1 cm dia hole) is going to be quite dim. The obvious way to make this work would be with a lens, but the lens would have to be of quite long a focal length, generally about 14 or 17 ft, depending on the room. It would also need to be of a fairly large diameter.

I've looked around for simple meniscus lenses, fresnel lenses, and/or parabolic mirrors that might have this sort of focal length, but the best I can find is on the order of 1m focal length (about a quarter of what I need). I suppose I could combine converging and diverging lenses, where the diverging lens is very slightly weaker than the converging one, but I'm still stumped as to where to find such beasties. Image quality isn't of paramount importance. I just want a cool projection to amuse me and those who visit our home.

Any ideas? Any thoughts where I could find such lenses?

Thanks! :-)


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Sarah Fox , Nov 11, 2011; 10:57 p.m.

BTW, if I understand the diopter correctly, 1 diopter is a 1m focal length, and 0.25 diopters would be a 4m focal length (about right). So would there be a source for a fairly large 0.25 diopter lens, or would getting a (smaller) corrective vision lens from an optician be my only bet?

jim jones , Nov 11, 2011; 11:00 p.m.

I've seen 1/2 and 1/4 diopter positive lenses for macro work. The 1/4 diopter lens would focus distant objects at about 13 feet. Some combination of eyeglasses might do the same. Second hand stores may have a variety to play with. Avoid those that also correct for astigmatism.

Evan Goulet , Nov 12, 2011; 07:40 a.m.

Have you tried Edmond Optics or Thor Labs?

Mike Gammill , Nov 12, 2011; 09:32 a.m.

Try Edmund Scientific or Surplus Shed. They may still have some long focal length meniscus lenses. At long focal lengths no color correction is needed. If you can't find one with a long enough focal length you could combine it with a negative lens of sufficient strength to get what you want. This might be your best bet since surplus lenses usually are only listed with approximate focal lengths. By combining a weak positive focal length with an even weaker negative focal length, you can vary the distance between the two lenses to change the effective focal length to what's right for your needs. If you get good results, please post some pictures of it. If you have specific questions you are more than welcome to send me an email.

Matthew Rusbarsky , Nov 12, 2011; 10:36 a.m.

If you are turning your living room into a giant camera, you might as well have a ground glass. I'm envisioning a freestanding screen of translucent material between you and the window. You could then focus the camera by moving the screen. This eliminates the need for really long lenses of specific focal lengths and white walls.

JDM von Weinberg , Nov 12, 2011; 12:19 p.m.

Very interesting concept.

Wish I could help, but I hope you can do it and will share the results with us here. :)

Tom Mann , Nov 12, 2011; 12:22 p.m.

I can't add anything technical to what the previous posters have said, but I will add: *VERY COOL*

Great idea!

Tom M

Tom Mann , Nov 12, 2011; 01:27 p.m.

Curiosity got the best of me, so I started poking around. You may have already found some of these links, but in case you haven't:

- - They are an outlet for Surplus Shed. They currently are selling several large diameter, long FL lenses, e.g., a 150MM DIAMETER F/27 PCX LENS 4000MM FL for $25.

- - Gullstrand's Formula to synthesize a very long FL lens from the combination of a positive and negative lens of the same power.

- - Suggestion to use quarter diopter lens blanks which apparently are readily available from eyeglass labs

- - A review of quite a few long FL lenses (all over 13 cm diameter) that could be used as the objective lens on refractor telescopes or your application. My guess is that finding any of these will be a hit-or-miss proposition and potentially be expensive.

- - Cute ideas on how to make a camera obscura without permanent modifications to the structure of the room + other interesting tidbits.

- - A large compendium of links to camera obscura topics. I didn't attempt to look at any of these, so quality = ?.

Finally, if you are into DIY projects, one would be to hand grind and polish a sheet of transparent plexiglass, much like folks making home-made mirrors and lenses for astronomical telescopes. Since you need such a long FL not much material would need to be removed, and a plastic blank would grind much faster than a glass blank. Plexi tends to fog when ground, but there are ways around this. Conceivably, you could make a huge diameter (albeit crude) lens this way.


Tom M

paul wheatland , Nov 12, 2011; 02:36 p.m.

try projection lenses from various format projectors?

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