Browsing the Dog Trick Academy, I came across a link to an interesting article about how dogs see.
I've often noticed than when something is thrown for Penny to fetch, she waits for a moment before setting off. (And how wonderful it is that many years down the track after her cruciate operation, she is able to run once again. We don't do much throwing for her, though. We're still aware she has a weakness in that leg.)
If it's a long way away, she will circle in a way that makes me think she's depending more on smell than sight.