# A tricky trick of the light...July 6, 2010 1:51 PM   Subscribe

Help me solve an optical puzzle. How might I make this fictional camera obscura work, given the parameters of my fictional location?

I am writing a story that requires an underground chamber, buried beneath the side of a sloping valley, to have an optical lens within the chamber that projects as wide a view as possible of the valley (above-ground.)

I was thinking a giant lens of some sort, suspended over the floor of the chamber, would help disperse the light and magnify the image. But I don’t understand the optical mechanics necessary to help make this work. I have a only a rudimentary understanding of how camera obscuras work (despite having been IN one myself,) so I suppose my question is in three parts:

1. Where exactly would the mirror need to be placed for a panoramic view of the valley above to be projected into the underground chamber?

2. What would physically need to be above ground (on the sloping valley itself, above the underground chamber) in order to make this happen?

3. How large an opening for light would be needed, and would that opening need to be directly over the lens?

One last note: for story-reasons, the camera can’t be at the very top of the valley walls, but it can be somewhat close to the top.
Any ideas or suggestions would be much appreciated! Thanks.
posted by np312 to Science & Nature (12 answers total) 1 user marked this as a favorite

Okay. I don't know the answer myself but you could do worse than ask a question in the forums over here: mamutphoto.com.

What you have in mind is, essentially, an ultra large format camera.
posted by run"monty at 2:32 PM on July 6, 2010

Try again. mamutphoto.com
posted by run"monty at 2:34 PM on July 6, 2010

If the light in the valley is bright enough, or if the viewer or imaging apparatus in the room is sensitive enough to light, you don't actually need a lens at all. Just make a tiny hole in the wall and you've got a pinhole camera, projecting a full view of the valley against the far wall, and as an added bonus the entire image is in perfect focus. Just very dim.
posted by squidlarkin at 4:28 PM on July 6, 2010

I assume the mirror you mention is to reflect the projection downwards, so that a horizontal view of the valley gets projected down on the floor. If so, it doesn't really matter where the mirror is - the lens could be in the ceiling and the mirror reflecting the valley into it, or the lense could look out over the valley and the mirror reflect the light coming out of it downwards , but this will put the top and bottom of the image slightly out of focus, so there is an advantage to putting the mirror before the lens.

(You could also use a curved mirror to do both reflection and focusing, and so do away with the lens altogether)

I think you should just find a lens and play with it. Buy a two dollar broken telescope or magnifying glass or whatever from a thrift store. Cut a hole in a cardboard box.
A convex lens will focus light, and the far side of the focal point, the image will be inverted, but an image. The further away from the focal point the projection surface is, the larger (and thus dimmer) the image.
posted by -harlequin- at 5:14 PM on July 6, 2010

Light travels in straight lines, so if the only way light can enter a chamber is through a small hole, a doubly-reversed image of what's outside will be cast on the wall opposite the hole. The smaller the hole, the sharper and dimmer the image.

If you had a darkened chamber under a hill, with a small hole in the roof, and a mirror no bigger than the hole angled at 45º like a half-open lid over it, then you should get an image on the floor, oriented with its bottom closest to the real scene outside:
``` O
/|\                           \ <--mirror
|                     ,------ ------,
/ \        ,----------'               '---------
----------'     |                           |
|                           |
|                           |
|           >->o            |
+---------------------------+
```
Then if you hung a second oblique mirror in the middle of the room, to reflect the image back towards the wall, it should be right-side-up — but still reversed left-to-right:
``` O
/|\                           \ <--mirror
|                     ,------ ------,
/ \        ,----------'               '---------
----------'     |                           |
|  o                        |
| /|\         / <--mirror   |
| / \                       |
+---------------------------+
```
The optimal size of the hole is given by a formula based on the distance from the hole to the wall; if your hole and wall are 100 feet apart (say 20 feet to the second mirror and 80 to the wall) we get about 4.5".
posted by nicwolff at 5:25 PM on July 6, 2010

Oh, not at the top of the hill? OK:
``` O
/|\                           \ <--mirror      ,----------
|                     ,------ ---------------'
/ \        ,----------'                     |
----------'     |                           |
|  o                        |
| /|\         / <--mirror   |
| / \                       |
+---------------------------+```

posted by nicwolff at 5:28 PM on July 6, 2010

Oh, with talk of holes and lenses, in case it's not clear, a lens allows a brighter (and thus larger) image. The amount of light you're painting the image with is the surface area of the cross-section of the hole/lens. So a hole is a much simple method, but dim because it doesn't let much light through. A lens is more difficult, but the greater the size of the lens, the more light is being scoped up and focused into the image, so the brighter it will be. (Or the larger it can be if you want to spread the light more thinly over a wider area)

If the distance between hole/lens and projection surface (floor) is fixed, a lens allows you to determine the size of the image, by making it more or less convex, ie changing the focal distance, whereas as explained by nicwolff, a hole haws more rigid geometry.

Also consider that for best focus, the projection surface should be the same distance from the hole/lens. So if the ceiling is low, the floor should be bowl-shaped.
posted by -harlequin- at 6:09 PM on July 6, 2010

er... "the projection surface should be the same distance from the hole/lens" = "all points on the projection surface should be the same distance from..."
posted by -harlequin- at 6:10 PM on July 6, 2010

So if a small hole was placed halfway up a steep (say 45 degree) slope, and inside the underground chamber, if the line of light coming in through the hole bounced off an angled mirror the same size of the hole, onto a large lens suspended below it-- would it conceivably project a large, reasonably bright picture (of the landscape viewed through the small hole) onto the floor below?

Assuming the angles are lined up and all.
posted by np312 at 6:54 PM on July 6, 2010

The first mirror has to be right by the hole, so that all light entering from the scene is reflected to the wall. And I'm not sure this would really work well; there are diffraction issues. But for fictional purposes, yes.
posted by nicwolff at 8:06 PM on July 6, 2010

Using a hole (eg an inch or less), the image will be very dim. If the image is large, it will be dimmer still. This might be a useful plot point - I would imagine that the image would initially be completely invisible, and only becomes visible once your eyes have had five minutes to adjust to darkness. To get a large and bright image, you would use a lens.

This is also something you can try at home with blankets over a doorway to a dark room :)
posted by -harlequin- at 10:35 AM on July 7, 2010

A camera obscura or pinhole camera could even occur spontaneously in nature.
A cave with a flat wall facing a tiny opening might be witness to an ever changing, upside down, projected image of the world outside.
But the hole would be too small for anyone to ever enter and see the image..

Also - best 3-month-exposure-of-a-graveyard- from-inside-a-beercan photo you will ever see:
http://www.pinholephotography.org/SSGB%20Page.htm

And - A good camera obscura how-to from the same clever Bristolian chap:
http://www.pinholephotography.org/camera%20obscurer.htm

Wishing you the best of luck with the story development.
posted by driftingclouds at 5:35 PM on July 7, 2010

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