Let me simplify then. For the telescope you used to generate this graph, calculate the width of the CFZ , and the focal length change of a single step (in microns). Now, in your graphic, how many CFZ widths error are contained in the extreme spread (roughly 1000 steps) of the values you generated?

Thus, estimating a step position based on that graphic, is statistically more likely to produce an out-of-focus image, than one in focus.

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