Single element lens calculations

[noahyt]

Hi! I am just getting into the world of large format cameras, so I'm not quite sure to look for this type of information.

I am trying to build (on a very small college-student budget) a basic camera.

I watched this https://www.youtube.com/watch?v=-G3eI2kH8b8 video and was intrigued by the simplicity of his design, notably the fact that he eschews normal lenses for a single piece of ground glass.

However, as with other threads that I have read, he does not explain how he knew that this element would be able to throw an image large enough to cover the image plane and stay in focus.

As I want to make only the most rudimentary camera (I'm not looking to be able to tilt/shift, or even necessarily have super clear corners) I'd like to find out how to calculate what type of single element will create a given image circle size.

Can this be calculated from just the focal length, the diopter? I've been looking around surplus shed http://www.surplusshed.com/ and it seems like you can get pretty specific for the type of lens you want. The problem is, I don't even know what I'm looking for without the image circle size listed.

Thanks!

[Tim Meisburger]

I'm not sure about calculation, but you can text empirically very easily if you have a lens (a regular magnifying glass lens works well, especially when stopped down to f8 or more). Hold the lens near a white wall across from a window, and move it in and out until an image out the window is formed on the wall. If you focus on a distant object outside the window then the distance from lens to wall will be your nominal focal length, and the size of the image will be your image circle. Actually, using this technique you may often have vignetting of the image circle, so it might be necessary to project onto a white card (I use poster board) and move lens and card closer to the window to get the full image circle.

My guess is any magnifying glass will give you enough image circle for 4x5. (see the thread on Chinese Pictorial Lenses http://www.largeformatphotography.info/forum/showthread.php?35097-A-new-line-of-Chinese-pictorial-lenses!&highlight=Chinese+Pictorial+Lenses )

[ic-racer]

Easiest to buy some cheap lenses and test them. If you want to design lenses, look around for free software. http://www.opticalsoftware.net/index.php/how_to/lens_design_software/winlens3dbasic

[Jim Jones]

The box camera of a hundred years ago used a simple meniscus lens. Such a lens performs better than a typical magnifying lens. The focal length was usually about as long or a little longer than the diagonal of the image area. www.surplusshed.com has a good variety of meniscus and other lenses. The close-up lenses that screw into the threads on the front of camera lenses are another source of meniscus lenses for larger cameras. A site with a wealth of links to camera making is http://home.online.no/~gjon/.