Good morning!
I have a problem with focal lengthening in DIY fixed focal length lenses.
For example, I have two plano-convex lenses with a focal length of 500mm. Using them simultaneously obviously shortens the focal length. Wanting to make a duplet lens from these lenses, with a distance between them of 100mm, the focal length decreases, all the way to 278mm.
I would like to know from the respected forum members if there is any rule/method of adding additional lenses to the system, so that in the end the focal length is larger and not smaller.
Going back to the example. What lens or how should the lens system be constructed to get a focal length of 1000mm.
Thank you very much for your time.
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