I am in process of developing a small game where a space-ship travels through a layer (doh!), in some situations the spaceship comes close to an enemy space ship, and the whole layer is zoomed in on the two with the zoom level being dependent on the distance between the ship and the enemy. All of this works fine.

The main question, however, is how do I keep the zoom being centered on the center point between the two space-ships and make sure that the two are not off-screen?

Currently I control the zooming in the GameLayer object through the update method, here is the code (there is no layer repositioning here yet):

-(void) prepareLayerZoomBetweenSpaceship{
    CGPoint mainSpaceShipPosition  = [mainSpaceShip position];
    CGPoint enemySpaceShipPosition = [enemySpaceShip position];

    float distance = powf(mainSpaceShipPosition.x - enemySpaceShipPosition.x, 2) + powf(mainSpaceShipPosition.y - enemySpaceShipPosition.y,2);
    distance = sqrtf(distance);

    /*
        Distance > 250 --> no zoom
        Distance < 100 --> maximum zoom
     */
    float myZoomLevel = 0.5f;
    if(distance < 100){ //maximum zoom in
        myZoomLevel = 1.0f;
    }else if(distance > 250){
        myZoomLevel = 0.5f;
    }else{
        myZoomLevel = 1.0f - (distance-100)*0.0033f;
    }

    [self zoomTo:myZoomLevel];
}

-(void) zoomTo:(float)zoom {
    if(zoom > 1){
        zoom = 1;
    }

    // Set the scale.
    if(self.scale != zoom){
        self.scale = zoom;
    }
}

Basically my question is: How do I zoom the layer and center it exactly between the two ships? I guess this is like a pinch zoom with two fingers!