How can lens focus ever be "off" in a way that AFMA fixes it?

[Mt Spokane Photography]

I didn't consider that another type of closed loop is camera checks PD array, tells lens to move to a position and lens reports done. While technically closed loop, it isn't as simple as the first case, and exposes the system to more sources of error.

Engineering is all about compromise, so the fact that Canon does it the way it does points to there being a benefit to the more complicated method, my guess would be speed.

It's similar to how the contrast detect focusing in live view is "slow", contrast detect doesn't deliver as reliable a "your focus is off by this much" sort of error signal, so the loop is closed by the contrast detect, hence the hunting you see while in live mode.

So thank you all for clearing this up for me!

One last question: how does Nikon do it? Does it also have an AFMA type tool?

Thanks!

Phase detect operates in basically the same way for all Cameras. It is old technology. A closed loop will still be inaccurate as well as slow. Remember, the check done by the PD array does not mean the subect is in focus.

Contrast detect, which uses the actual sensor photosites to compute focus is the closed loop type of focusing. Its painfully slow because of all the checking and back and forth searching. It often starts searching in the wrong direction.

Some of the newer Canon models now use a hybrid focusing method. Phase Detect pixels are embedded in the sensor, and tell the contrast detection system where to look for sharpest focus. The drawback for DSLR's is that the mirror must be up, so there is no visibility thru the viewfinder to be able to track the subject.

Mirrorless cameras get around this by using EVF technology which is improving with every new version. They are still very slow to focus where large sensors are involved. Small sensors might only use three to five lens positions to cover focus, so they are much quicker. The very deep depth of field makes this work.