ios - Access device camera exposure value before taking a picture -

i'm using wonderful gpuimage framework brad larson , have @ point instance of avcapturedevice (property inputcamera gpuimagevideocamera class) want read actual exposure value before taking picture, in metadata information created after picture taken. there way value live?

set avcapturesession, can exposure data in real time from:

- (void)captureoutput:(avcaptureoutput *)captureoutput didoutputsamplebuffer:(cmsamplebufferref)samplebuffer  fromconnection:(avcaptureconnection *)connection{     lightsamp++;    if(lightsamp>30){       cfdictionaryref metadatadictionary = cmgetattachment(samplebuffer, kcgimagepropertyexifdictionary, null);       nsdictionary *metadict= (__bridge nsdictionary*)metadatadictionary;        nslog(@"exposure %@",metadict );       lightsamp=0;     } } 

}