Are you talking “Focal Distance?” If you adjust or animate that parameter you can basically “roll the focus” during the course of the animation. You’ll find it in the “Object” tab. If you want to increase the amount of blur then you’ll have to lower the f-stop. Sometimes to really low numbers to make a difference (like 0.25, etc..)
In real world optics when you lower the f-stop you’re opening the camera’s iris allowing more light in, allowing you to achieve depth of field by focusing on an object by using a combination of focal length and iris settings (generally meaning you need to be zoomed in on an object to achieve DoF). In C4D’s physical renderer you’re simulating real optics by allowing similar controls. But unlike the real world you can take the f-stop down below f/1.0 to allow you to achieve DoF in smaller environments where objects may be close together, and you don’t have to adjust the focal length of the camera.
You could also play with the DOF Map Front Blur and Rear Blur settings under “Details”. Those are present in both renderers.