My previous career (well, one of them) was in technical writing: 5 books and uncounted chapters and articles. You can probably tell by my grammer how much I miss having a full-time editor.
Unfortunately, my writing style was 1) set a deadline 3 months from now, and 2) realize 2.5 months from now that I hadn’t done doodly on the article due in two weeks. I need more structure

Roger

You’re a gem and a genius! You make all this geeky stuff fun, funny, and even useful!!!
I can’t thank you enough!

If you ever tire of being a lens geek and businessman, there is undoubtedly a great future for you in technical writing, science fiction, or comedy!

Oh. One other thing. If you get bored and need something to do, I’d like to see equivalent information on the Nikon D800/e system and lenses. And the reason for Stonehenge.

Thanks!

Dave

If you mean there’s is some confirmation reading that says “this is the correct point”, then I disagree. Phase detection by principle is less accurate the closer it gets to the focus point. Unlike contrast detection, it does not choose a ‘best reading’ point.

There are at least 3 control loops inside a lens, and at least 1 inside a camera. But the overall behavior depends on settings, camera, lens, and a host of other factors including light temperature, strength, others. I’ll add that there’s also a differentiation in lens feedback in different systems and even within system in different lenses. Nikon uses at least 4 different types of position sensors. Canon at least 3. Making a blanket statement that is supposed to cover a gear driven lens with perhaps 700 steps and an older magnetic sensor and also cover a USM driven lens with 3500 steps and a new optical position sensor is making a very, very broad statement indeed.

I don’t absolutely disagree – you may well be correct. But I fear you’re trying to paint a gray picture black and white and I’m hesitant to do that. I’ve seen to many conflicting patents and too much conflicting data cloaked in too much corporate secrecy to be comfortable with that.

But in the interest of not arguing let’s just assume you’re correct and there is a closed feedback loop. If so it’s a fairly inaccurate closed feedback loop, and an inaccurate closed feedback loop isn’t all that different from a semi closed loop in behavior.

Roger

SD results with these lens manually set past infinity before autofocus showed far more inconsistency, sometimes front focusing, and then sometimes and back focusing. The amount of microadjustment required using this “closer” pre-autofocus manual adjustment is much less as follows:

8-15mm F4L -2
85mm F1.8 +4
17-40mm F4L +3

Using this procedure I can achieve very good accuracy and repeatability with the 85mm @ F1.8, which has a very small depth of field. There are some misses for sure, but it’s general due to the phase detection system failing to properly detect and lock on the subject area I’m focusing on due low contrast area and/or edge definition.

I thought I would pass this on to get your thoughts and see if this is something you’ve run across during your testing.