iPhone mapview区域缩放无法正常工作

| 这是我的代码：

CLLocationCoordinate2D location;
location.latitude = 50;
location.longitude = -50;
MKCoordinateRegion region = MKCoordinateRegionMakeWithDistance(location, 5, 5);

region.center=location;
[mapView setShowsUserLocation:TRUE];
[mapView setDelegate:self];

[mapView setRegion:region animated:TRUE];
[mapView regionThatFits:region];
[self setView:mapView];

它将放大，但是放大得太远。因此，如果我将其更改为：

    CLLocationCoordinate2D location;
location.latitude = 50;
location.longitude = -50;
MKCoordinateRegion region = MKCoordinateRegionMakeWithDistance(location, 200, 200);

region.center=location;
[mapView setShowsUserLocation:TRUE];
[mapView setDelegate:self];

[mapView setRegion:region animated:TRUE];
[mapView regionThatFits:region];
[self setView:mapView];

它将一直缩小。我有什么想念的吗？