How do I make depth of field very blurry in an image?

Item A (a small tree) is 7’ away, Item B (the subject of my render) is 90’ away, and Item C (a background building) is 650’ away. How do I make A and C unrecognizably blurry?
In depth of field, I have it enabled with pick focus set at Item B and aperture set to 1, but I can still count the leaves on the tree and mullions on the building’s windows. What can I adjust to blur the building more?