Camera Obscura II

<p>Hi all,</p>

<p>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.</p>

<p>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?</p>

<p>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.</p>

<p>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.</p>

<p>Any ideas? Any thoughts where I could find such lenses?</p>

<p>Thanks! :-)</p>

<p>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?</p>

<p>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.</p>

<p>Have you tried Edmond Optics or Thor Labs?</p>

<p>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.</p>

<p>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.</p>

<p>Very interesting concept.</p>

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

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

<p>Great idea!</p>

<p>Tom M</p>

<p>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:</p>

<p>http://www.astrophoto.co.uk/surplusobjectives.php<br>

- - 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.</p>

<p>http://www.controlbooth.com/wiki/Gullstrands+Equation<br>

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

<p>http://www.natscience.com/Uwe/Forum.aspx/optics/3046/Help-finding-a-converging-lens-with-focal-length-of-several-feet<br>

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

<p>http://alag3.mfa.kfki.hu/astro/giantlenses/200mm.htm<br>

- - 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.</p>

<p>http://www.funsci.com/fun3_en/sky/sky.htm<br>

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

<p>http://brightbytes.com/cosite/links.html<br>

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

<p>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.</p>

<p>HTH,</p>

<p>Tom M</p>

<p> </p>

<p>try projection lenses from various format projectors?</p>

<p>Paul, unfortunately, the FL from most projectors are much smaller (eg, 70 mm to 10 or 15 inches) than needed for a camera obscura (several meters).</p>

<p>To quickly get an idea of what FL projector lenses are available, just look on ebay:<br>

http://www.ebay.com/sch/Projection-Lenses-/83874/i.html?_catref=1&_fln=1&_trksid=p3286.c0.m282</p>

<p>Tom M</p>

<p>Wow! In less than 24 hr, I think y'all have got this thing almost solved for me! Thanks, Tom, for the great links -- especially the source for the 150mm dia, 4000mm meniscus lenses!</p>

<p>There's this one spot in the living room where there would be approx a 17 ft projection distance. That would put the focus of the lens at about 57 ft. That's pretty close. Just in front of that wall is a very short wall at 14' distance from the wall (focal distance of 208 ft). </p>

<p>Then there's another room with a less optimal view with a 12.67' width. That's so close to the 13.12' focal length that I should get infinite focus.</p>

<p>Mike, I'm intrigued by the idea that I can get variable focus by combining the weak positive focal length lens with the weaker negative focal length lens and varyin the distance between them. I THINK I understand how this might work, but I'm not certain. I'm going to play around with some eyeglasses tonight! :-) I might be emailing you later.</p>

<p>Thanks, guys! I'll definitely be back with this. BTW, we haven't bought the house yet, but we have a verbal acceptance on the contract. This project will be a long way down the road, BUT I will absolutely, positively post a picture of the results when I get there! :-)</p>

<p>Jim, I had no idea that close-up diopters came that weak. I did actually find a few, but they were unspeakably pricey!</p>

<p>Evan, I did look at Edmund Optics and didn't find what I needed. They've gotten incredibly expensive, BTW! I haven't tried Thor Labs. That said, I don't think I could find much better than the price/lens that Tom found. :-)</p>

<p>Mike, wow, I like Surplus Shed! They have the 4000 mm, 150mm dia lenses too -- also for $25.</p>

<p>Matthew, I like the ground glass screen idea. Unfortunately it would block the view whent he window shutter is opened. There's no practical way to implement it. However, I don't mind the projected image falling all over the room and its contents. In fact I sort of prefer it that way.</p>

<p>Tom and Mike, I've played around with some eyeglasses and closeup diopters and have (re)discovered the working principles of zoom lenses, or so I think. Anyway, I can see how the variable focus works. I look forward to learning how to design the right lens combo to make this project work. :-)</p>

<p>Playing around with the Gullstrand equation, I see I can combine a +3.00 and -3.00 diopter lens very nicely. With a 21mm distance between the two, I can get that 17 ft focal length. I'm looking into obtaining eyeglass lens blanks. Does anyone know how large these blanks can get? Perhaps someone knows who makes Elton John's glasses?</p>

<p>And then looking at <a href="http://www.surplusshed.com">www.surplusshed.com</a>, I see I can order a 66.5mm diameter 308mm fl lens for $4 and a 158mm diameter -310mm fl lens for $25. These lenses, spaced at about 16mm, would give me the same 17 ft focal length. I wish I could find a larger lens combination this closely matched in focal length.</p>

<p>I feel I'm on my way! :-)</p>

<p>Sarah, I cannot say a single useful thing to help you out technically. Just that i think it's a cool idea!<br>

Do you know the work of <a href="http://www.abelardomorell.net/photography/cameraobsc_01/cameraobsc_01.html">Abelardo Morell</a>? In my localised National Geographic, there was an article on his Camera Obscura work - inspirational, to say the least.</p>

<p>I knew this was ringing a bell. <a href="http://www.abelardomorell.net/photography/cameraobsc_01/cameraobsc_01.html">Abelardo Morell's site. </a> If you dig round there you can also find a PDG of a NationalG article about his work. I'm trying to wrap my head around the reason for some of the images not being inverted. His tent camera certainly has a mirror or prism behind the aperture.</p>

<p>Hi Sarah - I'm glad some of the links were useful. One comment -- I've not done the calculation, but I suggest you roughly estimate the f-stop of whichever lenses you are considering, and then have a look at the scene the lens will be viewing to see if there are any important components of the scene that won't be at infinity, and hence might be out of the depth of field range of the lens. Just like with a ordinary photo, you may be forced to make a trade-off between light gathering and DoF.</p>

<p>Cheers,</p>

<p>Tom M</p>

<p>Wouter and Matthew, Yes, I've seen Morell's work -- at least a little bit of it. It's fascinating. I wish I had thought of the concept! I won't be replicating his photographic style/concept. My camera obscura living room will simply be a living, non-photographic work of art. (Well, maybe I'll take a few Morell-like phots to show off the house -- and to post here. The view is due west, BTW -- sunsets over the water. :)</p>

<p>Tom, I've done some preliminary computations on depth of field, and the limitations are sobering, to say the least. The setup will be the ultimate ultra-large format "camera," and we all know that DoF shallows as format gets larger.</p>

<p>Fortunately the main aspects of the view are the islands across the river. My hyperfocal distance will be about 1500 ft, for a near-field object distance of about 750 ft. That will make the object blur at 750 ft about 33mm, using the lens pair I found. That should be quite sharp. Then there are various trees in the yard, the patio railing, etc. All of those should be close to the maximum object blur of 66mm. The railing will likely disappear altogether. Trunks and branches of trees will be recognizable. Leaves will be blurry blobs. </p>

<p>Here's the thing, though: I can always stop down a faster lens, but I can't "stop up" a slower lens. I'm going to make this thing with Waterhouse stops, so that I can expand my DoF if I so desire. It will also be re-focusable, if I decide the trees in the yard are an important part of the view. If I decide to get whimsical, I could do a Morell-like self-portrait, with a close-focused projection of myself sitting at a patio table, enjoying the blurry sunset! What bokeh! :-)</p>

<p>Sarah,</p>

<p>Did you do it yet?</p>

<p>I want to see.</p>

<p>Not yet, Richard! I'm still trying to source lenses from opticians, but I have a few other items on my desk before that. I'll definitely post results here, but it could be quite a long while.</p>

<p><em>"...If I decide to get whimsical, I could do a Morell-like self-portrait, with a close-focused projection of myself sitting at a patio table, enjoying the blurry sunset!..."</em></p>

<p>With so many different ways to play with a setup like that, you can't help but have fun with it. For other entertainment, I could imagine taking ordinary DSLR photos of the external scene, and then using a few LCD projectors to randomly flash the sharp digital photos right-side-up and upside down around the room at about the same brightness and location as the corresponding parts of the "live" scene.</p>

<p>:-)</p>

<p>Tom M</p>

<p>Does this window get the afternoon or morning sun? If so why not make the lens mount for the window interchangeable so you can substitute a pinhole for the lens should a solar eclipse be visible. You could safely view the phases of the eclipse from the comfort of your unused room. Or a sufficiently small waterhouse stop would have almost the same effect.</p>

<p>A few LCD projectors?! Yikes! :-O But yes, I think I'll have a lot of fun with that location!</p>

<p>Mike, the window faces due west. I'm going to make a waterhouse stop holder for the lens so that I can do exactly that sort of thing. However, the magnification won't be all the great. </p>

<p>A better way to get the high magnification of an eclipse is to mount an automotive inspection mirror (somewhat like a dental mirror, except larger) on a tripod, place it as far away from your window as possible, and direct it through a slit in your curtain. Then slip a piece of paper over the mirror with a small hole in it. You'll get a very large projection out of it; however, you'll have to readjust the mirror rather frequently. I did that for my kids when they were very young. I have no idea whether they remember it.</p>

<p>Still another rig I made was a mirror that redirected light into a 135mm lens, which created a virtual image at the focal plane. In line with that lens was a 50mm lens that projected the light diverging from the virtual focal plane to a large projection screen about 150 ft away in the darkened hallway of an elementary school. It made a very nice, bright, sharp image, and the teachers would take turns having their classes come and view the eclipse in all its glory. As I recall, the image of the sun was about 4 ft in diameter! :-) </p>