var vec = new THREE.Vector3(); // create once and reuse
var pos = new THREE.Vector3(); // create once and reuse

vec.set(
    ( event.clientX / window.innerWidth ) * 2 - 1,
    - ( event.clientY / window.innerHeight ) * 2 + 1,
    0.5 );

vec.unproject( camera );

vec.sub( camera.position ).normalize();

var distance = - camera.position.z / vec.z;

pos.copy( camera.position ).add( vec.multiplyScalar( distance ) );