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Message: <blockquote data-quote="OPTIC_NUT" data-source="post: 3219011" data-attributes="member: 121951">A note on the transferability of 'sweet spot' measurements.Because the common definition involves a standard which is in fact threshold someone can detect,the error in measurement is oddly the same as the measurement itself, so it has little repeatabilityand accuracy according to normal standards of metrology.   As a result, this cannot be transferrable from one person to another (30 arc-sec for one person, 60 for another),or even for one person during the day or across days or different lighting.  By definition, it is not an accurate metric. Two different eyestrain conditions duringtesting, for example, give a false impression.I compare with 'usable sharpness', reading to the equivalent of 90 arc-seconds.It is a zone a little wider than 'sweet spot', but much more transferable to differentpeople, times of day, and days, since the margin of error is less than the measure,(for curves and stereo effects I can detect down to ~40 arc-sec apparent)and because the steepness of the fall-off puts the number pretty close for two different people.You may want a better focus, but it would be nearby and much more reliable to declare as well.Using a more repeatable standard makes it a lot easier to compare and identify'standard', 'improved', and 'advanced' fields in the historical collection, and tellsme what to expect inside for eyepiece design. "Sweet spot" fluctuates during a busy day.</blockquote>

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