What's the purpose of a hood being vented?

[Kin Lau]

So that when someone asks you "Did you just take my picture?" You can say "I'm just venting" 😀

 

[Traut]

Does not.

Actually, I think it helps with drainage.

 

[FrankS]

The problem with the heavystar hoods is that they are not angled/tapered so they end up blocking part of the viewfinder even though they are vented. The Leica hood for the Summicron 35 and 50, as well as the hood for my Mamiya 6 75mm lens are vented and tapered, and practically disappear in the viewfinder. The rectangular vented hood for the 50mm Canon 1.9 collapsible lens is also angled.