title: zooming camera in threeJS without trackball controls or other camera control library
body: <p>I'm trying to use threeJS to control a camera in my scene. I currently have the camera set up to orbit in a circle around my object using the left and right keys on my keyboard. But does anyone know how I would zoom? Here's my current code:</p>
  
  <pre><code>camera = new THREE.PerspectiveCamera(45, window.innerWidth / window.innerHeight, 1, 10000 );
  camera.position.set(0,20,35);
  var rotSpeed = .02;
  
  function checkRotation(){
  
      var x = camera.position.x,
          y = camera.position.y,
          z = camera.position.z;
  
      if (keyboard.pressed("left")){ //MH - find a way to do this in a switch statement 
          camera.position.x = x * Math.cos(rotSpeed) + z * Math.sin(rotSpeed);
          camera.position.z = z * Math.cos(rotSpeed) - x * Math.sin(rotSpeed);
      } else if (keyboard.pressed("right")){
          camera.position.x = x * Math.cos(rotSpeed) - z * Math.sin(rotSpeed);
          camera.position.z = z * Math.cos(rotSpeed) + x * Math.sin(rotSpeed);
      } else if(keyboard.pressed("up")){
          //zoom in
      } else if (keyboard.pressed("down")){
          //zoom out
      }
  
      camera.lookAt(scene.position);
  
  }
  </code></pre>