Lens questions re telephoto adapter project

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Esinem New Member • Posts: 3
Lens questions re telephoto adapter project

I'd like to try this project, as cheaply as possible, but know nothing about lenses, so help would be appreciated: DIY Thermal Camera Telephoto Converter Don't worry that this is about thermal imaging as it seems to be only the lens material that's different.

I have seen cheap ZnSe CO2 laser plano-convex lenses on eBay etc. but the choices of diameters and focal lengths are limited. If you know of better sources, do let me know. I don't think this needs high quality lenses as the image resolution is so low, just cheap'n'cheerful.

So, my questions are:

1) I'm finding it hard to find small diameter ZnSe lenses cheap. What effect would using a larger diameter eye piece lens have, e.g. 20mm?

2) It seems hard to find cheap FL 15mm lenses. What effect would it have using something around FL 25mm for the eye-piece and around FL 75mm for the front one? I'm guessing that would give 3x magnification based on the formula. Would it mean a bigger inter-lens distance and less light transmission like a normal camera lens? If so, would it cause a significant loss of performance?

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