If someone watches your video for less than 20 seconds, does it affect audience retention?

[thecardifflad]

I know that it takes 30 seconds to register as a view, so if someone watches it for 20 seconds, it doesn't register as a view. By this logic, it shouldn't affect retention right? I only ask because I'm planning on purchasing ads, and am worried that some people will watch it for a couple of seconds and click off. I think people who watch for 30 seconds will stick around for the full video, so I'm not worried about that.

Any input is highly appreciated.
Thank you.