For those looking for a larger lens selection at the long end at least the Sony a9 handles Canon glass fairly well using single point AF. Almost all reports seem to agree that the Sigma MC-11 is the most capable AF adapter for use on the Sony a9 at this time, but the Metabones could easily catch up with a firmware update.
Even Sigma lenses seem to work pretty well on the Sony a9, but obviously not as well as native glass. It’s always best to get E-mount glass when you can, but if you already own a lot of Canon glass then an adapter is probably a good investment. I wouldn’t go out of my way to secure Canon glass for use on the Sony a9 myself.
Sigma MC-11:
B&H Photo / Amazon / Adorama
Metabones Canon EF/EF-S Lens to Sony E Mount T Smart Adapter V:
B&H Photo / Amazon / Adorama
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Sony a9 Action Shooting Kit: B&H Photo / Amazon / Adorama
Sony a9 Accessories Kit: B&H Photo / Amazon / Adorama
Sony a9: B&H Photo / Amazon / Adorama
Sony FE 100-400mm: B&H Photo / Amazon / Adorama
a9 Battery Grip: B&H Photo / Amazon / Adorama
a9 Battery NP-FZ100: B&H Photo / Amazon / Adorama
GP-EX1 Grip: B&H Photo / Amazon / Adorama
MQZ1 Battery Charger: B&H Photo / Amazon / Adorama
Sigma Canon Glass