Allan Eckert wrote:
I don't know what Kirby is talking about. He seems to be answering someone else's question.
It reads pretty clearly to me.
The camera has a sensor. Additionally, it has an array of focus-sensors that are fixed in position relative to the main sensor. Which of these focus-sensors was used by the auto-focus system to determine the focusing distance is what is displayed by the feature under discussion.
The point I'm trying to make is that the "focus point" in the discussion refers to which exact focus-sensors in the camera were used to achieve auto-focus. Unless your camera and your subject did not move between the time of focus-lock and the time of exposure, there will not be an exact correspondence between the spot on the sensor that is associated with that focus and what is depicted under that spot in the image.
In my practice, I find it much more awkward and time-consuming to select a focus point while the shot is framed than it is to focus with the central cross-type focus-sensors, lock focus, and then frame the exposure. So for me -- and I think many others -- knowing which focus-sensor was used by the auto-focus system doesn't tell me anything: there is no correspondence between the focus-sensor used and the image.
In the linked blog post, the author shows an image with the selected focus-sensor super-imposed over the model's upper eyelid. IME, it is assuming too much to assume that the autofocus system used that spot on the model's face when it determined the focusing distance.
There are programs that will analyze the actual image and attempt to show what parts of the image are in sharpest focus. Aperture does not do that.
This is -- as I indicated -- tangent to the OP's question. But the way s/he worded the question made me think that s/he things there is a certain one-to-one correspondence between the focusing-sensor used and the objects depicted under that focusing-sensor when it is superimposed over the image. There is not. I found that important to know, and so I pass it along.