AF tunning what actually happens?????

Started Jan 24, 2014 | Discussions thread
Horshack Veteran Member • Posts: 5,803
Re: AF tunning what actually happens?????

The AF tune value alters the camera's interpretation of what's in focus (phase detection).

To understand this better, first go to this page and play with the java applet to get a sense on how phase detection works by moving the "lens position" slider around. Now move that slider so that the coin is in focus - notice how the phases of the split beams (red and green) are now aligned on the main sensor. For a camera+lens which requires no tuning this "perfect" alignment will yield sharp photos. However for a camera+lens which requires tuning this "perfect" alignment will actually yield unsharp focus, and the reason for this is because the optical path by which those phase beams are split and recombined can be affected by lens abberations, AF sensor misalignments, etc... so that what the camera thinks is "perfect" alignment with respect to the phase beams is actually the wrong alignment. This is where AF tune comes in - the AF tune value adjusts the camera's interpretation of those phase beams, so that what looks like "perfect" alignment to the camera is instead treated as misalignment, and what looks like misalignment to the camera is instead treated as "perfect" alignment. The + or - of the tune value affects the direction of this phase re-interpretation (front/back focus), and the specific value affects the degree of that re-interpretation. A very rough analogy would be eye glasses.

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