The Sony 500mm f/4.0 lens just got tested by the DxOmark team (Click here to read). They write: “Overall, the Sony looks promising but it’s a pricey option.” Note that this is the first time DxO tests a super-telephoto lens and that’s why it’s hard to draw a conclusion with no competition scores.
Only thing we know for sure is that you have to sell your car to buy a lens like that. Check the price at [shopcountry 11116].