Room sized camera

[Paul Sorensen]

We want to try something at the school I manage and I could use some help with it. We are considering turning our studio into a camera obscura either with a very large old lens (read cheap) or with a pinhole in the wall.

We have already started playing around with the pinhole and it is clear that we will need to make a relatively large hole in the wall and insert a pinhole plate in it. One issue is, we don't know what size the hole should be. We would like to be able to project something on a board or screen at least 5 or 10 feet from the wall, and something very large. Any ideas about how large the hole should be and how precise it needs to be?

Also, we have considered just installing a giant old lens in the wall to project a more focused image. Is there such a beast that would not cost an arm and a leg? I keep seeing old barrel lenses in the range of 12" or so for really low prices on eBay, but I am not seeing anything bigger. I am guessing that they are out there, but I have no idea. One other idea, could we use a projection lens?

In either case, image quality is a secondary consideration, although we may try to make some images on paper negatives, mostly this would be to demonstrate basic optics in a really fun way to our students.

Thanks!

Paul.

 

[TheFlyingCamera]

Check out the La Jolla Museum of Modern Art. They had an installation of a room-sized Camera Obscura some years ago (I don't know if they still have it). The hole was pretty decent-sized, and it could project an image many feet across on a wall the other side of the room. You can contact them through their website - http://www.mcasd.org/information/index.asp .

 

[David H. Bebbington]

One issue is, we don't know what size the hole should be. We would like to be able to project something on a board or screen at least 5 or 10 feet from the wall, and something very large. Any ideas about how large the hole should be and how precise it needs to be?

I think as a general rule pinholes for cameras are made to have an f number of around 160 to 200. I am sure this would be a good basis for experiment, i.e. if you want an image at 20 feet from the hole take the figure of f200 and try a hole 1/200 of 20 feet or 1.2 inches. The problem with a lens is you would need a focal length of 20 feet, and you will not find one (at least I can't think of one). The only thing that strikes me is if you have any large junk lenses, you might find one element removed from one of these might work. An old telescope could work, too, although the size of the lenses might mean any projected image was rather dim.

 

[rogueish]

There's a guy who turned a delivery truck into a moving camera. He bought a submarine periscope lens out of an army surplus stores. His images looked pretty good.

 

[Paul Sorensen]

There's a guy who turned a delivery truck into a moving camera. He bought a submarine periscope lens out of an army surplus stores. His images looked pretty good.

Cool idea. I might try to find some sort of lens element to play with, but at this point it seems that the pinhole might be a better route, at least to start. Thanks for the info on the size of the pinhole. I guess when you are using a large focal length, the hole sizes become large enough to manage with a household drill. That's a benefit!

Paul.

 

[David A. Goldfarb]

You could experiment with a lens barrel or a LF shutter with the lens cells removed, and just use the diaphragm to see what size hole makes a good image for you.

 

[Mongo]

For a simple lens (like a positive meniscus), you can find some pretty long lenses out there. SurplusShed has a 5025mm focal length lens (about 16.5 feet); it's only about 1/2" diameter, though, so you'll need a pretty dark room. (For $4, it's probably worth a try.) Search the old surplus optical companies...you'll probably come across something that you could use.

(David's idea about using the iris from an LF shutter as a variable pinhole is an excellent idea.)

 

[Loose Gravel]

Check out this camera obscura in San Francisco

http://brightbytes.com/cosite/sanfran.html

 

[Donald Qualls]

The longer the focal length, the larger you need to make a pinhole, but the dimmer the resulting image gets (the pinhole size increases as the square root of the focal distance).

What you need is a lens with a focal length of around 120 inches, the bigger the better. You might consider shopping around the Edmund optical catalog for a positive and negative with suitable focal lengths to create the composite focal length you need -- you'll want the negative to have 1/3 diopter less power than the positive, about, with diopter power being the inverse of the focal length in meters. Combine the two and you'll get a doublet lens with about 1/3 diopter total power, or about 10 feet focal length. Get a lens four inches in diameter, and you'll be close to f/32, which should project a bright enough image from a sunlit exterior to be easy to see with other light sources turned off. You could also do the same with two positive lenses, allowing the first to form an inverted real image and then using the second to project that real image, now upright, onto your screen -- and in this case, you can much more easily adjust the focal length and image size by varying the lens spacing. That, in turn, lets you teach a little optics as you go -- show the (small) inverted image from the first lens, and the similarly (small) inverted image from the second, and then demonstrate how they combine to form a system that will project a large, upright image -- including using the lens equations to predict what image size you'll get with different spacings.

 

[Paul Sorensen]

SurplusShed has a 5025mm focal length lens (about 16.5 feet); it's only about 1/2" diameter, though, so you'll need a pretty dark room. (For $4, it's probably worth a try.)

They also have a 4000mm that is 42mm in diameter, so it should let in quite a bit more light that the 1/2" one or a 1" pinhole. At least I should hope so. It's also $4, so I think I will get one to see what I get.

Thanks again to everyone for their ideas. I would also love to hear more!

Paul.

 

[claytume]

Paul

I did something similar for the local school my kids go to, these are 9 and 10 year olds. I took some sheets of cardboard, tape and a pocket full of flat steel washers. Once the window was closed off with the cardboard and a 4x4" hole cut in the centre it was a simple matter of changing the washers (on another piece of board) until we got something that worked. The process took maybe 5 minutes and we used about a 20mm hole projected onto a wall about 3 metres away.

The image was bright enough to see after standing in the room for about a minute, this was on a dull overcast day. The most interesting thing about it was the super wide view. It covered the side walls as well as the one opposite the hole.

Clayton

 

[removed account4]

abe morell does this sort of thing in hotel rooms..

http://www.abelardomorell.net/publications2.html

and then there is the portable camera obscura ... like this one :

Dead Link Removed

- john