So I found a lens that works to focus an LCD at around 9 cm distance. Any technique to determine / measure the focal length of that thing (because I'd like to obtain a second one with similar parameters)?


Oh wow, you just hold it against a white wall and try to get the projected image that comes in from a window sharp. So it's indeed around 9 to 10 cm.

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Octodon is a nice general purpose instance. more