I don't understand. Let's say I have a perfect camera, a perfect lens and a point source of light at a distance of 10 feet. The point source of light emits an infinite number of rays in all directions. Some of these rays hit the lens, which bends them so they all converge on a single point on the camera's sensor. If I now move the light to 9 feet, the lens will bend the rays so they converge either before or behind the sensor. If I adjust the lens to focus a 9 feet, the light rays again converge to a point on the sensor. If I then move the light back to 10 feet, the point of convergance will again be either before or behind the sensor. What's the problem? Just refocus the lens.
The problem, it seems to me, is determining when the adjustment has the point of convergence on the sensor. Using live view, when you see the light reduce to a point on the viewing screen, you are in focus. But using autofocus, things are different. Again, using a perfect lens and perfect camera and a point source of light at 10 feet, the rays hit the lens and it bends them so they converge on the sensor. But there is a mirror that reflects most of the rays up to the viewfinder and the rest down to a focusing sensor on the bottom of the camera. Since the camera is perfect, the distance from the lens to the imaging sensor and the focusing sensor is exactly the same and the rays going down to the focusing sensor also converge to a point. If I move the light to 9 feet, the focusing sensor will on longer see a point so it will tell the lens to start focusing. When it again sees a point, it will tell the lens to stop. When the picture is taken, the mirror and shutter get out of the way and the rays go to the imaging sensor and converge as a point there also.
Now, we switch to another copy of the same lens. This one is not perfect. If I focus it manually using live view, there should be no problem. When I see a point of light on the viewing screen, I'm in focus and the light is converging on the imaging sensor. However, if the camera is perfect, in auto focus mode, it should tell the lens to start focusing and when it sees a point of light on the focusing sensor it will tell the lens to stop focusing. At this time, since the camera is perfect, the light rays.will also be converging to a point on the image sensor.
In high school, I had an algebra teacher that presented a beautiful algebraic "proof" that 3 equaled 4 and challenged us to prove it wrong. When we had given up, he showed us where he had hidden a division by zero (this was a lead in to calculus). So far, do I have a division by zero in my thoughts above?
OK. So, let's deal with an imperfect camera. We should still be able to manually focus using live view. However even in the case of a perfect lens, when the the light rays converge to a point on the focusing sensor, they will not converge on the imaging sensor. They will converge either before it or behind it.
Now one of the questions I have is how the fine focus tuning works on the cameras that have this feature. Does it physically move something, either the focusing sensor, the imaging sensor or the mirror, so that when the rays converge on the focusing sensor they also converge on the imaging sensor? Or does it tell the lens to go "just a little more or a little less" when the rays converge on the focusing sensor, thereby causing them to converge on the imaging sensor instead?
There has been discussion about a mechanical adjustment inside the 5100 that moves something (I'm assuming the focusing sensor) so that the focusing sensor and the imaging sensor are exactly the same distance from the lens. He makes it sound easy, but I keep hearing the Mythbusters sayng,"DON'T try this at home. We're what you call (pregnant pause) EXPERTS". But it seems that if this adjustment were made so that the distance was spot on the same from the lens to each sensor, it wouldn't matter if the lens was or was not perfect; the camera would tell the lens to keep focusing 'til focus was achieved on the focusing sensor. And since the camera is now perfect, focus would also be achieved on the imaging sensor.
Does anyone know of anyone know of a repair facility that would make this kind of adjustment - ie. "blueprint" the camera? I don't have a big enough pair of brass ones to try it myself.