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I have a 500mm f/8 mirror lens and 135mm f/2.8 telephoto left over from my (long gone) SLR, the 135mm works OK as a 9X finderscope and the 500mm is great for eyepiece projection of the sun through a 20mm Kellner.
You can use camera lenses at low power for wide angle views but they're a bit of a waste:
(a) they are not designed to be diffraction-limited, so don't expect good images at high power.
(b) in focal lengths up to 200mm they are designed to provide very fast focal ratios - but faster than f/4.5 is a waste with most eyepieces.
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