LensAlign - why does each lens need adjustment?

[TimeLapseTulsa]

Recently Scott Bourne reviewed the LensAlign Pro made by these people:

LensAlign® Focus Calibration System (http://www.lensalign.com/)

This is a device for determining with considerable accuracy whether there is a front-focus or back-focus bias in the autofocus of a camera. I understand how it works, but not why you need to do this for each lens you use.

I thought the autofocus system of a modern DSLR was essentially a feedback system, in which the autofocus sensor provides the information on how good the focus is, and the camera then interprets that and activates the focusing motor to move the lens in or out until maximum sharpness is achieved in the focusing sensor. So there's nothing in the lens itself that has any effect on this, except that the lens must move appropriately in response to the camera's commands. But there's no sensor in the lens - all of the "smarts" in the autofocus system are in the body.

I can see that if the main image sensor and the focus sensor are not exactly the same focal distance from the lens, then autofocus might have a front or back bias. It would be the same problem as in manual focus if the main image sensor and the ground glass at the end of the penta-whatever reflections are not quite at the same focus point. But I don't understand why one autofocus adjustment made for the body wouldn't serve for all lenses.

But it's clear I must be wrong about this, because the LensAlign site clearly states the adjustment must be made for each lens, and in fact the high-end cameras that permit fine-tuning of autofocus also remember a setting for each lens.

And in the same review, Bourne mentions a lens that had to be sent back because it had a front-focus bias, which doesn't make any sense to me. All this means that I don't understand autofocus - maybe it's not a feedback system at all.

So could someone who really understands this please explain (in excrutiating techincal detail :-) ) how autofocus really works, or at least why the fine-tuning would be different for each lens? This is going to bother me until I understand it.

Thanks very much.

TLT

[F8&Bthere]

Nobody else wanted to try to tackle this? Thread's a bit dated, but FWIW....
I am considering purchasing the Lite version of that product.
Here's my (limited) understanding of auto focus tuning:
What you described as a feedback system is pretty accurate. But I guess you can say a given body and lens don't always see eye to eye. I can't go into a lot of detail because I'll for sure word it wrong or expose my ignorance on the subject, but I'll just say that lenses and bodies are manufactured within a certain tolerance. Instead of 100% mass produced copies being dead on accurate, there is some minor variance that considered by the mfg to be acceptible. When you put a lens and body together, you might be lucky and have focus spot on, or you might be close enough that you won't notice it unless you are very critical over time, or you might have a bit of a mismatch. If the latter is the case, and you have one of the modern mid to high end DSLR bodies, you may be in luck because many of them have a lens AF fine tune feature. So that, combined with a tool like Lensalign, can help you correct your lenses (well actually your body's communication or synchronization with the lens) individually.
I notice when I am doing portraits at wide aperture/thin DOF, using AF as I often do, when the subject is looking at an angle to the camera, that I think I've got the front eye in perfect focus, recompose, snap the shot, and in review I'm disappointed- sometimes it's ever so slightly off. So then I keep fumbling over whether it was my error or that particular lens being out of sync with that body.
Check out this link for a better explanation:

LensRentals.com - "This lens is soft" and other myths (http://www.lensrentals.com/news/2008.12.22/this-lens-is-soft-and-other-myths)

The freaky thing about that article for me was that sending a lens or a body into the manufacturer for calibration doesn't always solve the problem either. You would have to send in your body and all your lenses to be safer. But now with this new feature in some cameras, you can do it yourself as needed.

[TimeLapseTulsa]

Well, since the original post I had come to the conclusion that I was wrong about the focusing mechanism. It isn't really a full feedback mechanism. What the camera appears to do is use the AF sensor to do a phase-shift measurement, and then calculate where the correct focus point should be, and then tell the lens to make that adjustment. But apparently it only does that once, and doesn't double-check or fine-tune the result. So the result can vary with different lenses.

I understood how a body could be out of adjustment if the AF sensor isn't physically at quite the right place, or in manual focus if the pentaprism isn't exactly where it should be, because then they would not be at the same focal point as the main sensor. But with full continuous feedback the result should be the same for all lenses. And indeed, I believe the Lens-Align site even says that for manual focus one adjustment works for all lenses. And that makes sense.

But with the limited feedback of AF, or I guess more properly the lack of continuous re-calculation of focus, the way each lens reacts will be different, so each lens has to have its own fine AF adjustment.

I think when you go into live view and switch to contrast-based focusing, all of this goes away because you are looking at what's actually hitting the main sensor, and the camera really can just adjust the lens until the result is perfect. But that's slow. I think that's why they use the phase shift method when the mirror is down - it's very fast. I do wonder, though, if you use AF in portrait work, might it be better to use live view where focusing speed isn't important?

[F8&Bthere]

Ya see, you're ahead of me then. Because I was having a hard time of my own wrapping my mind around how the AF system really works. But curiously enough on one of the Lensalign viseo tutorials I watched online, the host had his camera tripod mounted and in Live View mode (which he was touting as the easier way to assess and adjust) and he demonstrated via the live view display on the rear LCD how his camera-lens combo was still slightly missing focus. His camera, if I remember correctly, was a Canon 1Ds III.