I use the 100-400L and I really like it. Its incredibly versatile. Im not the best photographer from it actually if you want images I can link you to my pbase account. Any faults are "mine" and not the lenses.
As someone noted above. Without even getting into IQ specifics its safe to say that any other zoom option (sigma or Tamron) will AF slower. I know from testing the Tamron 200-500 myself when i was shopping that its AF is noticeably slower. Thats not an issue with larger, slower moving targets ( Deer, Great Blues) or static subjects but for birds in flight or birds that dont stay static on branches long...It is.
The 100-400L AFs faster than its contemparies.
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Another option for you is as someone mentioned the Canon 400mm 5.6 prime. Same price range bracket as the Bigma, the Tamron 200-500 and the Canon 100-400L. The downside is like anyother prime vs zoom trade-off.
I myself am trying to see if I can locate and do a trade for it. My 100-400 for someones 400prime for the reasons above but mainly because it AFs faster and Im a sucker for bird flight photography.
So in the end try to pick the lens that best suits your needs.
If you want a great birding lens and good wildlife lens and are willing to sacrifice the versatility of zooms. Look at the 400L 5.6.
if you want more adapability than the Canon 100-400L or Bigma or Tamron 200-500. The biggest difference between these lenses are their AF speeds. Which would be the Canon, then the Bigma, then the Tammy.
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