why micro focus adjustment ?

Started Feb 25, 2013 | Questions thread
theswede Veteran Member • Posts: 3,936
Re: why micro focus adjustment not in the camera ?

And then there is a third possible cause which I have been told about and don't know if I should believe. Apparently, most focusing with AF is one try only: The camera PDAF sensors measures the exact focus error in the image, calculates how much the focus mechanism should rotate, rotates the mechanism exactly this amount. And then it takes the photo, no matter if best focus was achieved.

This is called open loop, and is not how most AF mechanisms work. Most are predictive closed loop with a (non disclosed) threshold for how many times they will retry and a (non disclosed) threshold for how correct the AF has to be to be accepted.

And the idea that the focusing is not at all an iterative process also amazes me.

Every manufacturer has their own way to do it, and it also changes from camera to camera, but I have yet to see a recent camera which uses an open loop for focusing. All camera give up pretty fast, but what they do then varies. Some will accept a rather large focus error and still show a green square (and take the photo in focus priority), others will indicate OOF, and some seem to change their minds depending on the weather.

There is almost no documentation available on implementation details from the manufacturers on this to my knowledge, and for good reason - these are trade secrets and good focusing is a key benefit to a camera.


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