Ideas for limiting what is viewable.

Just looking for some ideas I might not have thought of, given that I'm not really an expert in visual effects.

The scenario is I have a 3d isometric view. But all I'm using to get that is the camera angle.

The scenario is the game has a lot of siege weapons. A couple of which can target things out of the visual range that an isometric view provides. So I decided the best approach all things considered is to go with a different perspective when using a siege weapon. One that allows the camera to go outside the isometric view. And to keep it consistent use this for all siege weapons even ones that are within ranges that would not be problematic.

What immediately comes to mind is using a depth of field and probably Bokeh together. Any other ideas on good effects to use for something like this?