The Sigma 50-500 has a pretty good macro mode for larger targets (like butterflies). I have a bunch of shots in various threads of both birds and butterflies taken with it; this is a set of butterfly shots I posted last week: https://photography-on-the.net …/showthread.php?t=1361520.
The two-Tamron combination would work well too, and would get you to 1:1 macro, which the 50-500 does not, but is needed for shooting smaller bugs.
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