AVCaptureDevice 曝光模式

Exposure Modes

AVCaptureExposureModeContinuousAutoExposure 
---自动调节曝光

AVCaptureExposureModeLocked 
--使用当前曝光

下面是设置为持续自动曝光的例子：

if ([currentDevice isExposureModeSupported:AVCaptureExposureModeContinuousAutoExposure]) {
CGPoint exposurePoint = CGPointMake(0.5f, 0.5f); // 曝光点为中心
[currentDevice setExposurePointOfInterest:exposurePoint];
[currentDevice setExposureMode:AVCaptureExposureModeContinuousAutoExposure];
}