IPad connected to DSLR captures incomplete image

At present, I am using the avfoundation external device API to connect my iPad to a DSLR camera for data collection. On my end, I am using AVCapture Video Data Output to obtain raw data for processing and rendering. However, the pixelbuf returned from the system layer is incomplete, with only a portion cropped in the middle. But using the Mac API is normal. I would like to ask how to obtain the complete pixelbuf of the image on iPad

IPad connected to DSLR captures incomplete image
 
 
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