The following images demonstrate the effect of different camera depth of field values. The examples below use a sphere which has a radius of 1 (remember that scene scale size can affect the DOF values that you use).
No depth of field
Size 0.01. Aspect Ratio 1
Size 0.25. Aspect Ratio 1
Size 0.5. Aspect Ratio 1
Size 1. Aspect Ratio 1
Size 1. Aspect Ratio 0.5
Size 1. Aspect Ratio 2
Size 0.5. Aspect Ratio 1. Blades 3
Size 1. Aspect Ratio 0.5. Blades 3. Blade Curvature -5
Size 1. Aspect Ratio 0.5. Blades 3. Blade Curvature 0.5
Size 1. Aspect Ratio 0.5. Blades 4
Size 1. Aspect Ratio 0.5. Blades 4. Rotation 45
Distance 8.5. Size 1. Aspect Ratio 0.5. Blades 3. Blade Curvature -5