In external views I rather change the distance to the aircraft than the field of view because a very big FOV creates ugly distortions.
Here is how to do this with the same buttons/axis:

<axis n="7">
  <desc>Field of view</desc>
  <direction>upward</direction>
  <low>
    <repeatable>true</repeatable>
    <binding>
      <command>nasal</command>
      <script>
    if ((getprop("/sim/current-view/name")=="Cockpit View") or (getprop("/sim/current-view/name")=="Cockpit")) {
       view.increase();
    } else {
      setprop("/sim/current-view/z-offset-m",getprop("/sim/current-view/z-offset-m")-0.5);
    }
      </script>
    </binding>
  </low>
  <high>
    <repeatable>true</repeatable>
    <binding>
      <command>nasal</command>
      <script>
     if ((getprop("/sim/current-view/name")=="Cockpit View") or (getprop("/sim/current-view/name")=="Cockpit")) {
      view.decrease();
    } else {
      setprop("/sim/current-view/z-offset-m",getprop("/sim/current-view/z-offset-m")+0.5);
    }
      </script>
    </binding>
  </high>
  <dead-band type="double">0</dead-band>
  <binding>
    <factor type="double">1</factor>
  </binding>
</axis>