The Optics

Any lens that focuses light (a double convex, plano convex, or positive meniscus) can theoretically be used as a camera lens. Magnifying glasses, lenses from binoculars, close-up "filters," reading glasses, are all potential camera lenses. A good source for purchasing lenses is Surplus Shed, where they typically cost $4.00 to $10.00.

Notice that I used the word theoretically above. There are limitations. Every lens has its focal length. When a lens is used to focus parallel rays of light, the focal length is the distance from the lens to the point of focus. This means that for a single element camera lens, the focal length determines the distance the lens needs to be from the film or sensor. This is very important for constructing your lens and it puts some limitations on the lenses that are useful. Assuming you want to use your lens with a SLR or DSLR, the focal length can be no shorter than about 45 mm because that is the approximate distance from the lens mount to the film/sensor plane. A lens with focal length shorter than 45 mm would have to be mounted inside the camera to reach proper focus on distant objects. That's not going to work. At the other extreme, a lens with a focal length of more than about 400 mm starts to get very awkward.

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