DotTune: AF tune technique without taking photos

Started Jan 31, 2013 | Discussions thread
Mako2011 Forum Pro • Posts: 23,288
I learned

Marianne Oelund wrote:

Leonard Shepherd wrote:

Although this link is five years old it is one of the few from Nikon that confirms analysing two reference points on detection lines are used in "normal" autofocus - this is the basis of phase detect autofocus

That article states quite clearly, a number of times, that it is the distance between the two images, not merely between two specific points, which is used to determine focus error.

Do not be misled by the simplified diagram showing only two rays arriving at each image. That is only to keep the diagram from becoming cluttered, not to imply that other rays go unused.

Phase detect autofocus relies on identifying the right subject in the right place on the detection lines. If it gets this wrong (as it sometimes does) the auto focus system is likely to focus at the wrong distance.

Still wrong, as ever. The distance between the two images can be determined, regardless of whether or not they are aligned to specific points on the detection lines. Of course, there is a maximum allowed displacement because the lines are of finite size, but as long as corresponding image details fall within the detector spans, the relative displacement (and thus focus error) can be accurately measured.

The system is far less limited than you imagine. You cannot induce misfocus simply by re-pointing the camera slightly within the span of the detection lines.

Confusion may arise if there are multiple similar details, since in this case the system may not know which details to match up; this is the periodic-pattern limitation.

I learned a bit (a lot really) in this post.  Thank you!

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