zooming camera in threeJS without trackball controls or other camera control library

Question

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:

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);

}

Answer 1

If you want a real zoom, without moving the camera, then you can play with the field of view (fov) parameter of the camera:

  camera.fov *= zoomFactor;
  camera.updateProjectionMatrix();

See: http://jsfiddle.net/bvcCB/87/

If you want to move the camera near (or far) of the target, then calculate the vector from the camera position to the target, and move the camera position along that vector.

Answer 2

From r69 you can now use camera.zoom:

camera.zoom = zoomFactor;
camera.updateProjectionMatrix();