I don't recommend using two teleco0nverters together, not at all. There's simply too much bouncing around of the light within the lens, it'll almost guarantee a loss of sharpnes. Rather than use two, I would recommend going to a single 3x or 5x converter. Another option would be to find an inepensive mirror (catadioptric) lens. there are a few problems associated with cat lenses, but far fewer than using multiple teleconverters together.
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