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So then I felt it was worth experimenting with the 2x III extender - but the only way to mount it to the 1.4x III extender was to use a small spacer (EF-12) between them. Now I was working with 1,803.20 mm - almost the sort of magnification a larger telescope offers.
combination was as follows: Canon EOS M mirrorless (APS-C) + EF-to-EF-M lens adapter + EF 1.4x III extender + EF-12 Tele-extender + EF 2x III Extender + EF 100-400mm f/4.5-5.6L IS USM II lens = 1,803.20mm (equiv).
