I had this idea while messing around with the portal gun. My idea was that every time you got near a portal AND you were facing towards it, your fov would transition to a higher level. I thought it might be a cool cinematic effect to have as an optional setting.

When you get closer to the portal your FOV might go up from 5% to 30% of your original fov. and if your facing 180 degrees away from the portal that might be reduced to 5% to 10%. and these percentages would increase to 5% - 30% as you turned towards the portal.