public Camera(Vector3 position, Vector3 lookAt, float focalLength, float screenWidth, float screenHeight)
{
Position = position;
LookAt = lookAt;
FocalLength = focalLength;
ScreenWidth = screenWidth;
ScreenHeight = screenHeight;
Vector3 relPos = position - lookAt;
float dist = Vector3.Distance(position, lookAt);
sphericalPosition = new Vector3(dist, XMath.Acos(relPos.Y / dist), XMath.Atan2(relPos.Z, relPos.X));
}