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Amazon.com : Lightdow 500mm f/8 Telephoto Mirror Lens for PK K20D K10D K200D K100D K-5 K-7 K-20D Work with Any Pentax/Ricoh DSLR/SLR K-X Adapter : Electronics
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Mirror…Mirror… A guide to choosing and using mirror lenses Part 1 – Wayne John Grundy's PhotoTech Blog
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optics - With this lens and mirror combination, should there be a third image formed? - Physics Stack Exchange
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