UILabel default kerning different from CATextLayer

Question

I have a UILabel with the string 'LA'. I also have a CATextLayer with the same characters in an NSAttributedString assigned to its string property. The kerning in the UILabel is noticeably different from the CATextLayer. Here's the code.

- (void)viewDidLoad
{
    [super viewDidLoad];

    // 
    // UILabel
    //
    UILabel *label1 = [[UILabel alloc] initWithFrame:CGRectMake(20, 50, 280, 100)];
    label1.text = @"LA";
    label1.backgroundColor = [UIColor clearColor];
    label1.font = [UIFont fontWithName:@"Futura" size:90.0];
    [self.view addSubview:label1];

    //
    // CATextLayer
    //
    UILabel *label2 = [[UILabel alloc] initWithFrame:CGRectMake(20, 130, 280, 100)];
    label2.backgroundColor = [UIColor clearColor];
    CATextLayer *textLayer = [[CATextLayer alloc] init];
    textLayer.frame = label2.layer.bounds;
    textLayer.contentsScale = [[UIScreen mainScreen] scale];
    [label2.layer addSublayer:textLayer];
    NSMutableAttributedString *string = [[NSMutableAttributedString alloc] initWithString:@"LA"];
    CTFontRef aFont = CTFontCreateWithName((__bridge CFStringRef)@"Futura", 90.0, NULL);
    [string addAttribute:(NSString*)kCTFontAttributeName value:(__bridge id)aFont range:NSMakeRange(0, [string length])];
    textLayer.string = string;
    [self.view addSubview:label2];
}

Here's an image of the results.

Why is the kerning different between these two methods and what am I doing wrong in the CATextLayer example?

Answer 1

UIKit generally uses WebKit for its text rendering (as visible in this crash log), most likely for performance reasons. If you really need super-precision then there are some custom UILabel reimplementations using CoreText as its back-end.

EDIT: As of iOS7 this is no longer true since UILabel uses TextKit for its rendering which is based on CoreText as well.