ios 圆角按钮边框颜色不均匀

如图 很明显上下边要更白一些 圆角的地方要散一些 感觉按钮好劣质 这样看

这是为什么 如下代码

_schoolBtn = [[MyCustomButton alloc] init];
_schoolBtn.frame = CGRectMake(SCREEN_WIDTH-15-67, 13, 67, 23);
_schoolBtn.titleLabel.font = [UIFont systemFontOfSize:13];
_schoolBtn.layer.borderWidth = 0.5;
_schoolBtn.layer.cornerRadius = 11.5;
_schoolBtn.layer.borderColor = [UIColor whiteColor].CGColor;
_schoolBtn.layer.masksToBounds = YES;
[_saveView addSubview:_schoolBtn];


如果不设置圆角就是正常的  求解惑

---

_schoolBtn.layer.borderWidth = 0.5;
设置成1的倍数

_schoolBtn.layer.borderWidth = 0.5;