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Thread: camera obscura

  1. #11

    camera obscura

    Chris --

    Thanks for your response -- that's very helpful info about the diopeter lenses.

    Your tent project is very cool! (And I've long-admired your benders...)

    Annie
    (cordelia)

  2. #12

    camera obscura

    I’ve been experimenting with diopter lenses for a tent camera obscura that I’ve been working on and would advise that cheap diopter lenses or cheap close-up don’t give a completely sharp image (the image is still very cool—just not as sharp). Flaws in a lens are amplified the longer the focal length is. So keep that in mind when picking your optics.

  3. #13

    camera obscura

    Hei, they want to do it Sept 28 through Oct-31, 2009 this year, and gave me the title 'curator'.

    I'm not sure I've ever cured anything!

    Sorry I have been vapor-Murray, nearly non-existent.

    Someone explained it already...diopter is a measure of lens power...the first number on an eyeglass prescription is basically a diopter number.

    What I used was a +0.25 diopter (4m or 4000 mm f.l.), coated glass lens, about 40 mm diameter, from Surplus Shed (surplusshed.com).

    I think the improved brightness was appealing...couldn't tell you if aberrant ...the photos above are of the lensed C.O.

    M

  4. #14

    camera obscura

    I would love to see some obscura video! That's something I don't think has ever been done.

  5. #15

    camera obscura

    It's been a while, but I'm actually doing the project I described at the beginning of this thread next weekend (and it is being funded by a grant and supported by my local arts council). Exciting!

    So, I have a few more questions about diopter lenses for you guys, if you're still poking around.

    1. Are they "additive"? In other words, can I stick 2 or more lenses together thereby increasing the focal length?

    2. Is a "diopter" lens different from other kinds of lenses in some way, or is a + diopter lens simply a positive lens being measured in opticians' units?

    3. Does one just stick the lens over the correctly sized pinhole, thus only using a tiny bit of lens, or does one make a hole big enough for the whole lens, and in that case, how do you figure the correct diameter for the lens?

    I ask these questions for a couple reasons. Firstly, the "room" in which I'm making my camera obscura is quite large. It's round, with a diameter of 45 feet. Because there is, actually, a support column in the middle of the room, I may set up my screen (onto which the image will be projected) only halfway across the room, but that's still 22.5 feet from the pinhole. This is going to lead to quite a dim image, if my math is correct, so I'm thinking that having some lenses up my sleeve to intensify things is a good idea. I have few "diopter" lenses with weakish powers (+.3, +.5), and I also have some positive lenses from Surplus Shed or the like (left over from Alan Greene's lensbuilding workshop at f295 this year). I haven't measured the focal lengths of the surplus lenses, but there might be something stronger amongst them.

    According to the formula I found online, (1 diopter = 1/focal length in meters) putting the screen halfway across the room (22.5 foot focal length = 6.85 meter focal length), I should have a a +.145 diopter lens. If I put the screen all the way across the room (45 feet = 13.7 m), I should have a +7.3 diopter lens. Is that even right? Holy exponential increases!

    Any thoughts much appreciated -- especially since I did exceptionally badly at math at school.

    Oh, and, if you're anywhere near Boston next weekend, definitely come see the camera! http://www.powderhousecameraproject.com

    Thanks!

  6. #16

    camera obscura

    I think I've just answered my questions 2 and 3. Apparently, stacking lenses is ok. And yes, they should be "stopped down".

    Would still love your thoughts!

  7. #17

    camera obscura

    Stacking diopter lenses decreases the focal length. The final diopter number will be about equal to the sum of the individual lenses.

    You could use several weak diopter lenses and screens to cover 360 degrees in segments.

    A plano-convex lens of 22.5 or 45 foot focal length has a quite shallow curved surface. Such a lens can be ground and polished by many amateur telescope makers. Large UV filters might be a good source of glass for such a project. Used ones can be quite inexpensive. Google for lens grinding or telescope making for supplies and information on this subject.

  8. #18

    camera obscura

    I did poorly in high school math too. What I was told is that a +1 diopter will focus at 1 meter and a +2 diopter will focus at 1/2 a meter. I also know that combining two diopters together adds their power. So a +1 and a +2 will equal a +3 which will focus light at 1/3 a meter. I was also told that a +.5 diopter will focus at 2 meters. So perhaps if you can find a +.25 diopter maybe you can set your screen up at 4 meters. That +.145 diopter might do the trick and get you close to 8 meters of space. I wouldn't worry about a pinhole at this focal length. On large camera obscuras I use a hole around the size of a small coin. With the lens, just make some sort of mount with a coin size hole that the lens will fit over. The large the hole is the more light will shine on the screen and the quicker it will be for the audiences eyes to adjust.

  9. #19

    camera obscura

    Ah, very interesting. It seems like I'm going to be adjusting where I put the screen more than anything. I'll bring all the diopter lenses I have scrounged together and see what combination does the trick.

    I played with this concept in my home camera obscura today (the back hall can be easily converted with a bit of tape and plastic, though it's a tiny space) and was pleased to see how much it sharpened the image.

    (Hey Chris -- I met you at f295 -- we went to a sports bar. I'm the one with the crazy-lady red curly hair and black plastic glasses. Kinda quiet. One of your cardboard pinhole things is sitting not two feet away from me -- I should send you some pics i took with it!)

  10. #20

    camera obscura

    I'd love to see what you've done with the tube camera. Good luck with the camera obscura.

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