I'm building an eclipse camera from a long cardboard tube. The longest lens I can find on Surplus shed is 1400mm. I found online that the size of the sun can be calculated as f/110, which should give me a 12mm image of the sun. I would like twice this or more. Can I just put a double concave lens in combination to extend the focal length? I used to use plain DCX lenses to make macro fiters for my 35mm camera and increase the effective but not actual focal length, but in this case I don't mind if the setup is actually telephotographic or not, I just want more magnification.
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