# 6' x 10' camera obscura

Discussion in 'Extreme, Retro, Instant and More' started by al_wildey, Jun 22, 2005.

1. ### al_wildey

Hello all!

I am constructing a 6' x 10' camera obscura out of an enclosed cargo
trailer and I need a little help. Essentially it is a big pinhole
camera and I need assistance determining the optimum aperture,
figuring out focal length, and how to optimize the plane of focus so
that it falls on the back wall instead of in front of it or behind
it. While I wouldn't mind playing around with some lens element
construction, my primary aim is to use it as a pinhole. I may also
put a second hole in the side so there are two distances that images
project--one that goes ten feet from aperture to back and one that
is six feet from aperture to the back. Any and all advice and
assistance gratefully appreciated!

2. ### silent1

The simple approximation of optimum pinhole size is to take the square root of the focal distance (in mm) and divide by 25 to give the hole diameter (also in mm). For 10 feet, that would give a 2.2 mm hole, while 6 feet would give 1.7 mm.

The distance isn't terribly critical on these; you'll lose some sharpness if you're several inches off on the distance, but that's all -- and since no feature in the projected image can be smaller than the hole anyway, it's going to look pretty blurry to your eye. Not to mention time -- you'll have the equivalent of about f/1400 on the longer distance. Even in sunshine, that's going to be a pretty dim image; you'll have to let your eyes adjust a bit to even see it (and you'll have to pay close attention to light leaks, which could easily overwhelm the dim image).

3. ### kelly_flanigan|1

A simple weak reading glass lens of +1 diopter has a focal length of 1 meter; one that is +0.5 diopter has a focal length of 2 meters. A +0.75 diopter has a focal length of 1.33 meters. Adding a lens will radically add sharpness; and allow one to use a much faster system; try maybe F125 ;F90; F64; F45 as a start. One can still use a stop; which when placed correctly will further boost the systems performance. This is how we did it in 4h club's Photo club in grade school; in a small farm town.

4. ### kelly_flanigan|1

We used a refrigerator box; or a washing machine box that we made light tight.

5. ### al_wildey

Thanks Donald & Kelly--you've got me off to a great start! Can you point me to any on-line sources from which to get this and additional info? Suggestions from others?

Cheers!

6. ### nancy_bueler

Al, are we going to see the results? How will you fit the pics on the scanner
(tee-hee).

7. ### glenn_thoreson

Sounds like an interesting project. If you do a web search for pinhole photography, you will pull up more stuff than you can digest. Good luck!

8. ### jbq

What Kelly said. While shooting through a plain pinhole is fun, using a lens is actually a practical solution that'll yield sharper brighter images (the downside is that some objects will be less sharp than others).

9. ### daisuke nakabayashi

Hi Al,

I used a large format lens shutter for the aperture for a camera obscura installation that made a few years ago. Since I did not know before hand how the weather will be (it was in San Francisco), I decided to use the iris/aperture of a shutter to adjust the opening. This worked well since it also demonstrated how the size of the opening affected the sharpness of the image in relation to the amount of light it let in. I used a Compur #1 lens shutter (same size as Copal #1) with the lenses removed.

Here some images I took inside the installation: http://justdai.com/wppd04/index.html

10. ### al_wildey

So, what is the focal length with a 2.2mm aperture in a 6' x 10' box and NO
diopter used?

11. ### j.w.

Since you indicated that you're making a camera obscura, can we assume that this will be used primarily for visual use, rather than as an ultra-large format camera?

If so, you should consider experimenting with a pinhole size larger than what would be considered optimal. Although the image may be less sharp than with an ideal pinhole, it'll be brighter. There's probably an optimal tradeoff between brightness and sharpness, which is best determined with a few tests.

You could fashion the equivalent of a lens-board bracket in the side of the trailer, and experiment with various sized holes till you get the best compromise between brightness and sharpness.

12. ### al_wildey

Sorry I didn't respond sooner--I was out of town for two days.

Actually, I intend to use the camera for both image acquisition AND as a camera obscura (opening it up and having school kids inside to observe the image formed).

Again, I'm looking for some calculation to figure out the "focal length" of this simply as a pinhole camera (no additional lens elements). I'll be using variable apertures purchased from Edmund Scientific--it should be pretty cool.

Any other issues anyone foresee that I should consider? Once again, thanks in advance for your help!