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why don't extension tubes increase focal length (links?)

Started Feb 26, 2007 | Discussions thread
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mdg Regular Member • Posts: 355
why don't extension tubes increase focal length (links?)

If I were to make a pin hole lens out of a body cap, say, and then use an extension tube the focal length would increase and I would get a narrower field of view.Why doesn't the same thing happen using a convensional mutli-element lens?

Does anyone have any good links which explain the optics of extension tubes, preferably with lots of diagrams?

Thanks

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