用于获取iPhone设备摄像头所感知的环境光强度：

• 值越大，光强度效果越明显

#import <ImageIO/ImageIO.h>
-  (void)captureOutput:(AVCaptureOutput *)captureOutput didOutputSampleBuffer:(CMSampleBufferRef)sampleBuffer fromConnection:(AVCaptureConnection *)connection
{
    CFDictionaryRef metadataDict = CMCopyDictionaryOfAttachments(NULL,sampleBuffer, kCMAttachmentMode_ShouldPropagate);
    NSDictionary *metadata = [[NSMutableDictionary alloc] initWithDictionary:(__bridge NSDictionary*)metadataDict];
    CFRelease(metadataDict);
    NSDictionary *exifMetadata = [[metadata objectForKey:(NSString *)kCGImagePropertyExifDictionary] mutableCopy];
    float brightnessValue = [[exifMetadata objectForKey:(NSString *)kCGImagePropertyExifBrightnessValue] floatValue];

   NSLog(@"%f",brightnessValue);
}