AF tunning what actually happens?????

Started Jan 24, 2014 | Discussions thread
Roy Kikuta Senior Member • Posts: 1,587
Re: AF tunning what actually happens?????

Horshack wrote:

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.

Do you have more details - for example, how does the AF module/CPU communicate with the "silent wave" (ultrasonic?) focusing system in an AF-S lens?  Since AF-S lenses contain CPU's, I assume the AF module in the body communicates digitally with the CPU in the lens - has anybody hacked the lens CPU or signal to actually understand what's going on?  Is the fine tune "correction" applied during the phase detection/calculation, or is it applied in the signal sent to the lens - stated another way, is the fine tune "correction" applied in the body's AF module - or is it actually executed by the lens CPU?  I've heard that only one lens drive command is applied - with no secondary phase check(s)/lens drive command(s) applied - in order to speed up AF operations - is this true?  I'm not criticizing your explanation - I'm simply looking for more details because it's a subject that's dear to my heart

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