al_wildey Posted June 22, 2005 Share Posted June 22, 2005 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! Link to comment Share on other sites More sharing options...
silent1 Posted June 22, 2005 Share Posted June 22, 2005 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). Link to comment Share on other sites More sharing options...
kelly_flanigan1 Posted June 22, 2005 Share Posted June 22, 2005 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. Link to comment Share on other sites More sharing options...
kelly_flanigan1 Posted June 22, 2005 Share Posted June 22, 2005 We used a refrigerator box; or a washing machine box that we made light tight. Link to comment Share on other sites More sharing options...
al_wildey Posted June 22, 2005 Author Share Posted June 22, 2005 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! Link to comment Share on other sites More sharing options...
nancy_bueler Posted June 22, 2005 Share Posted June 22, 2005 Al, are we going to see the results? How will you fit the pics on the scanner (tee-hee). Link to comment Share on other sites More sharing options...
glenn_thoreson Posted June 22, 2005 Share Posted June 22, 2005 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! Link to comment Share on other sites More sharing options...
jbq Posted June 22, 2005 Share Posted June 22, 2005 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). Link to comment Share on other sites More sharing options...
daisuke nakabayashi Posted June 22, 2005 Share Posted June 22, 2005 Hi Al,<br><br>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. <br><br> Here some images I took inside the installation: <a href="http://justdai.com/wppd04/index.html">http://justdai.com/wppd04/index.html</a> Link to comment Share on other sites More sharing options...
al_wildey Posted June 23, 2005 Author Share Posted June 23, 2005 So, what is the focal length with a 2.2mm aperture in a 6' x 10' box and NO diopter used? Also, anyone with links to images made by large pinholes/camera obscuras please forward--thanks! Link to comment Share on other sites More sharing options...
j.w. Posted June 23, 2005 Share Posted June 23, 2005 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. Link to comment Share on other sites More sharing options...
al_wildey Posted June 25, 2005 Author Share Posted June 25, 2005 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! Link to comment Share on other sites More sharing options...
Recommended Posts
Create an account or sign in to comment
You need to be a member in order to leave a comment
Create an account
Sign up for a new account in our community. It's easy!
Register a new accountSign in
Already have an account? Sign in here.
Sign In Now