At our December Capture SIG meeting, Alan used a Lightroom Plugin as well as some software called PIE to dig deeper into the EXIF data on some of our photos. There were a few times when the software indicated that no focus points were actually active when the photo was taken. While that seemed reasonable in some cases, intuitively it seemed like that indication was coming up too often.
After the meeting, I did some online research and some experimentation with my Canon 80D. What tipped me off was this thread in an online forum:
Show AF Focus Point on Playback
One of the entries in the thread indicated that the active focus points are only recorded in the EXIF data if 1) you continue to press the AF-ON button (if you are using back button auto-focus) while pressing the shutter release or 2) you aren’t using BBAF (back-button auto-focus).
I confirmed the author’s information on my own 80D. First, I configure the camera to highlight the active focus points in red in the viewfinder. Then, I used the AF-ON / back button AF to achieve focus and then released the AF-ON button. On the resulting photo, the image playback shows no red squares.
If I repeat the steps and keeping press the AF-ON button while pressing the shutter, the red squares DO show up on playback.
If I disable BBAF, I always get the red squares on playback.
I concluded from this that the display of the red squares on image playback is being achieved by reading some EXIF data from the image file and it is likely that same data that allows computer software like that demonstrated by Alan to display the achieved focus points.
I think it’s useful to know which focus points were active later on because that might help you diagnose what went wrong (or right) with your photo.