But doesn't using that device still require the photographer to look at the images and figure out what is in sharpest focus? . What if someone is incapable of looking at something and being able to tell what is really sharp and what isn't? . How would that kind of person do MA with your device?
I know that my questions may come across as critical and fault-finding, but that is not my intention. . I am genuinely curious about how to use your device (and other similar devices) if you can't look and see what is sharp and what isn't.
You do have to take photos and compare. The main point about targets like this is that your intended plane of focus is an easy target that you focus onto. The diagonal ruler has the 0 point right at that plane of focus, so it should be apparent if the 0 point is not in focus (and if a tick mark on the ruler is sharper in front of or behind). The only main critical point is that I have found it's sometimes hard to find what could be sharper with a step 1 MFA (but in most situations, that's getting too critical anyway).