@Override
    public void update(float tpf) {
        if (spatial == null) {
            throw new IllegalArgumentException(
                    "The spatial to follow is null, please use the setTarget method");
        }
        target.setLocalTranslation(spatial.getWorldTranslation());
        camNode.lookAt(target.getWorldTranslation(), upVector);

        target.updateLogicalState(tpf);
        target.updateGeometricState();
    }
move the camera toward or away the target

@param value the distance to move
jme3-core/src/main/java/com/jme3/app/ChaseCameraAppState.java