There is a lot of these lenses online, this is where I got mine from https://www.amazon.co.uk/gp/product/B073X4RRWP/ref=oh_aui_search_detailpage?ie=UTF8&psc=1 at the time of this video, it is currently unavailable, this is an alternative link – https://www.amazon.co.uk/Walimex-Pro-650-1300-Pentax-white/dp/B001UAIERU/ref=sr_1_24?s=electronics&ie=UTF8&qid=1536498221&sr=1-24&keywords=650-1300+mm+lens
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Thanks for the video, very interesting.