Why is my camera not centered? - ios

Question

I've noticed that the camera property my SKScene is nil. I remedied that by creating a SKCameraNode setting that equal to the camera property, setting its position and adding it to the scene. However, when I do this all of a sudden my objects are no longer in the center of the scene. They are all shifted in some way that does not make sense to me. What am I doing wrong?
class GameScene: SKScene {
override func didMoveToView(view: SKView) {
/*
// This is the problem code
let cam = SKCameraNode()
cam.position = CGPoint(x: self.frame.midX, y: self.frame.midY)
self.camera = cam
self.addChild(cam)
*/
let sprite = SKSpriteNode(imageNamed:"Spaceship")
sprite.xScale = 0.5
sprite.yScale = 0.5
sprite.position = CGPoint(x: self.frame.midX, y: self.frame.midY)
self.addChild(sprite)
}
}
Object appears in middle of scene, as expected.
Now the object is shifted to the left. Why?
UPDATE
The following code is giving me the behavior I want. The key is to reposition the camera every time the size of the SKScene changes. Thanks for the suggestions, they got me on the right track.
class GameScene: SKScene {
override func didChangeSize(oldSize: CGSize) {
if let view = self.view{
camera?.position = convertPointFromView(view.center)
}
}
override func didMoveToView(view: SKView) {
let cam = SKCameraNode()
cam.position = CGPoint(x: frame.midX, y:frame.midY)
self.camera = cam
self.addChild(cam)
let sprite = SKSpriteNode(imageNamed:"Spaceship")
sprite.xScale = 0.5
sprite.yScale = 0.5
sprite.position = CGPoint(x: frame.midX, y: frame.midY)
self.addChild(sprite)
}
}