Looking for a lens...

JBrunner

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With a ten foot focal length, for a camera obscura. I went to Walmart, and they were out. Any ideas?
 

Steve Smith

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That's about a third of a Dioptre. See an optician or perhaps get some weak reading glasses. Walmart may have some!


Steve.
 

Murray@uptowngallery

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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.

Murray
 
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JBrunner

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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)
 
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Murray@uptowngallery

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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.
 
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JBrunner

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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.
 

Chazzy

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I never cease to be amazed at some of the things that people are doing. Good luck with this!
 

Murray@uptowngallery

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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
http://hyperphysics.phy-astr.gsu.edu/Hbase/geoopt/gullcal.html#c1
 
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Mike Kennedy

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I really don't know if this will help but I have 4 broken lenses.Glass is complete but they are beyond repair.You're welcome to any one of them.

Mike
 
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JBrunner

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Mike Kennedy said:
I really don't know if this will help but I have 4 broken lenses.Glass is complete but they are beyond repair.You're welcome to any one of them.

Mike
Click to expand...

I love APUG. The spirt of kindness and help here ia simply amazing.

Mike, most camera lens elements are pretty short, on the order of a couple of inches. Placed together they usually create even shorter lengths. I'm tring to focud an image at 10 feet. I have played with busted lens elements, and although they have served to demonstrate the principle quite well, I haven't found a combination that comes even close to working for the actual CO.
 
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JBrunner

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Ok,

I have e-mailed a local optician with the request for a lens. Hopefully they will realize I'm not a crank.....
 
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JBrunner

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Murray@uptowngallery said:
So, JB, is this a 'traditional' C.O. that illuminates a table of sorts, that constrains you to a fixed length of 10 feet?
Click to expand...

This one will come strait in my bedroom window, and give me the morning upside down and backwards on the wall at the foot of my bed. The distance from the window to the wall is ten feet.
 

removed account4

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jason

i think in abelardo morell's book he says how to set a room up
like a camera obscura ... i don't have it, but i know he does this sort of thing
and i remember he has diagrams or instructions &C ...
maybe the book has lens info ? or maybe he just uses a just a hole, and no glass ...

good luck!

john
 

jimgalli

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Jason, we've got 117" f8 mirrors out here where I work that will resolve 40 lppm. On half frame 35mm I can only get about half the moon in a frame. Talk about cheeze. They would look damn funny sticking out of a bedroom wall though.
 

Steve Smith

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JBrunner said:
This one will come strait in my bedroom window, and give me the morning upside down and backwards on the wall at the foot of my bed. The distance from the window to the wall is ten feet.
Click to expand...

I have a friend whose parents used to own a large victorian era house which was built for a portrait photographer. There was a downstairs toilet which had a lens mounted in the wall which projected an image of the garden on the wall opposite so you could watch it whilst sitting (did I spell that correctly?).

The distance in this case was about 6'


Steve.
 

Murray@uptowngallery

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Holy Krap...a pair of those for a 1.3 meter Rapid Rectilinear kinda lens, albeit not made with achromats...

No Ball Bearing shutters that big but I do have a box of those large lazy susan bearings...
 

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TracyStorer said:
Does the sun pass through that view?
A well focused image of the sun can start a fire.
Click to expand...

i read last year of a photographer whose lens was open, and while he
was preparing his photo stuffs, the sun focused through his lens onto his bellows and
they started to burn ...
i think i have the story right ...
wish i could remember who it happened to ...
 

TracyStorer

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It actually happened to me with the 20x24 on an outdoor shoot with Wegman up in Maine in the '90s. The fresnel on the ground glass focused the sun on the bellows and burned a neat hole....a little black tape and we were good to go-lesson learned "keep the viewing door on the 20x24 closed when not in use."

Best,
TS
 

Robert Hall

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There was the photographer who was in a somewhat recent View Camera or Camera Arts who does hours long daylight exposures. I saw one of his negatives at Photo LA, the sun had burned right through it. Can't say much for the "art", but it was interesting.
 

Murray@uptowngallery

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Happened to alot of people I guess, enough to be mentioned in numerous books...some (fSU?) cameras have metal focal plane shutters that someone usually declares immune to the solar burning 'problem'.
 
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