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Hi Xespok.
In most cases with Diptera I use Canon 100/2,8 macro with 24mm additional ring. Autofocus always off, also no hand focus. I put the fucus distance according to size of fly and than move myself altougether with camera to (or out) of fly.
But I think that choise of 100mm as any other choise is only one of possible compromise in searching the highest probablibility to get shot of unknown object with rather unpredictable behaivor. In some cases 50mm macro is much better or 300mm or 24mm...
Nikita