I’ll use a SolidWorks file that I used for rendering the logo of Young Dutch Design.

Add a camera by right clicking on the Lights, Camera and Scene folder in the Feature Tree.

Set up the view you prefer and set the target point. This is the point the lens will focus on so I’ll manually select the first grey sphere. In the image below you’ll see a top view of the sphere’s.

Scroll to the bottom of all the options and turn on ‘Depth of Field’.

This is the most important part. Set the d value so that the solid blue rectangular surface is on the part that need the focus. Set the f value such that the surrounding components are quickly faded/off focus.

Hit the apply button on top and start rendering after you’ve set the scene and the appearances. If you need some info about this part read Rendering realistic chrome with SolidWorks.

This is my result! Be careful, don’t make the f value too small because it will look very unnatural.