Looking for a lens...
With a ten foot focal length, for a camera obscura. I went to Walmart, and they were out. Any ideas?
Have you seen that book on primitive photography, the one about making wax paper negatives and salt prints? I have it at home and so I don't have the title/author handy, but he talks in there about getting lens elements from scientific supply places. That's probably your best bet.
That's about a third of a Dioptre. See an optician or perhaps get some weak reading glasses. Walmart may have some!
I have two solutions.
Plan B is a lens I call an Abominatar.
I used a website called hyperphysics and the Gulstrand's equation calculator.
I took a +0.25 diopter lens from Surplus Shed and an 'eyeglass blank' from Surplus shed described as +6 diopter on one side and -6.25 on the other. Both were glass, one was about $4 (coated, 42 or so mm diameter) and the other was $0.75, 70 mm diameter and uncoated.
I assumed it would be approximately -0.25 diopter net.
Now, with no spacing, I would expect it to be approximately 0 diopter or infinite f.l.
With the Gullstrand's Eqn. calculation, one can see the effect of spacing the elements.
I think I selected spacing for 8 m, but don't really have any idea where the node or nodes would be, and it didn't matter anyway.
I have a couple mailing tubes telescoped together, about 6' long, straightened best I could with a tape measure, glued with wood glue then painted flat black inside. I used Goop or clear Liquid nails to glue the lenses to the plastic mailing tube caps, then painted the other parts of the caps flat black. To paint the tube interior, I put one cap on, poured a few ounces of paint inside, put the 2nd cap on and rocked and spun the tube until I couldn't see any unpainted area with a flashlight. I painted another by bending a paint roller in a vice until it was straight
The group that wants me to do the Camera Obscura hasn't followed thru with any timing yet, so I have only tested it in my own gallery at night, because I didn't want to take the time to black out all the windows.
I projected a 50W LV halogen lamp onto a wall. It's kind of hard to aim, handheld (6 feet long, theoretically 8m f.l. +/- 2m assuming the nodes are between the elements, but I don't know). The difficulty in aiming, and the physical length relative to the diameter made me suspect it may not have a very large circle of illumination...but it may be better in a darkened room.
If it doesn't work well enough, plan C is to use one of the 0.25 diopter meniscus lenses (4m f.l., about f/100) alone.
Plan D is a 13 m Zone Plate care of someone who made one for me.
Plan A is a variable iris (4-45 mm) for a 'normal' Camera Obscura.
I found the diopter formula, and it is as simple as I remember.
D=1/FL (fl in meters)
As the Steve said, turns out it's exactly +0.33 diopter. Sound like reading glasses might do it.... (hehehe I said "do it"... hehehe)
Last edited by JBrunner; 01-15-2008 at 05:00 PM. Click to view previous post history.
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I haven't seen reading glasses that weak...almost 'attitude glasses'.
I just made three lenses from $1 store reading glasses. I kept the frames on two of them because they were so bizarre looking (funky hand-painted design).
JB, I'd offer you one of the 4 m meniscus lenses but it would be contingent upon me LOCATING the @#*& things.
I bought a 10-pack, but I seriously lose things in my basement and garage. I need different colored cardboard boxes. They all blend together when they're stacked 6 feet high and 10 feet deep.
I'll look, but no promises. And they are 3' too long.
I don't know what kind of DOF one gets with a lens Camera Obscura, so I was planning on using a portable wall they have so it can be adjusted for focus. The ZP and the pinhole (relative pinhole) will cover better, I imagine.
Weakest reading glasses I can find are 1.00 which works out to 1 meter. Perhaps I can find some old glasses from someone with astigmatism. I think that condition requires a divergent lens, that I could combine with a 1 power diopter. So simple, yet so difficult.
Your offer is most generous, Murray (if they turn up) but I don't know what I would do with the extra distance.
I never cease to be amazed at some of the things that people are doing. Good luck with this!
Getting a little cabin fever?
One of the concerns I had was determining the exact f.l. to focus onto a wall, but the C.O.'s I've seen tend to be a 3D display covering corners, ceiling , floor, etc.
I didn't think I could control the f.l. from a window to a flat wall precisely enough to match the lenses depth of focus, which I thought would be more analogous to film plane flatness, and not like DOF which can be very wide with a small enough aperture. That's why the reliance on a moving wall is anticipated.
I'll tell you what though, a 4 meter zone plate is a sight...I think it's 39 mm diameter or something like that...big, anyway, compared to a sane camera size.
Unless you need optical funk, I would think astigmatism correction for someone's deformed eye (I've got one myself, no offense aimed at any astigmatic folks) would give you a deformed image...not terribly critical I guess, for a wall.
Goodwill Stores have an Optical Dept. now, but I bet it's not to offer (incredible) bargains (like a surplus place)...they're probably in it for the money (like everyone else) to raise funds for their operations.
One mo thing...
A place that has an on-site lab may have 70 mm 'blanks' or standard diopters.
A helpful optician told me he could get them for $20 a pair, which was no bargain.
It may be worth your trouble to go check out that hyperphysics-Gullstrand's Equation 'Calculations' page & experiment with spacing and standard diopters.
Another option might be +1 and -1 and spacing. The spacing alters the cancellation of +1 + -1 = 0. I ended up with the rediculous spacing of 2m because that's what I conveniently had at negligible cost.
I also ponder whether a given situation results in a mor 'difficult' focal stability....extremely close spacing or very long spacing...I suspect the long may be worse...centration etc. Very close approximates a thick lens which is a more 'natural' occurrence, eh?
The variation in spacing gives you far more variation than the few extant standard or surplus options...
Here is the link
Last edited by Murray@uptowngallery; 01-15-2008 at 09:59 PM. Click to view previous post history.