Camera Auto-Align with the nearest object with a determinated tag

Hey guys

I was writing a source, but unfortunately I can't get any logic to write a source to align the camera with the nearest object with a determinated tag, if it is near to the center of the screen.

I try to explain this better:

I have a camera moving and rotating.

I want that if it founds a particular gameobject that is near to the camera and has a particular tag, it will aligned to it, and it remains untill, another gameobject doesn't take its position or it will destroyed.

How can I explan to Unity?

I tried with Camera.main.ScreenToWorldPoint ();

This is the logic:

if the object has the setted tag (example enemy) then (simple, i can do this)

if the object is the nearest then (simple I can do this)

if the object is near to the center of the screen then (here is the problem) align the camera to the object.

I don't know how to detect, if the object is near to the center of the screen. But i don't know how to detect if the gameobject is near of the center of screen

Please help

Thanks byeeee!