I see frequent mention of the term, 'sweet spot', but I don't really know what's meant by it. Are people seeing an actual spot—a circle of clearness surrounded by a ring of relative obscurity—or are they using the term as a metaphor for the notional area of maximum sharpness?
Michael.
I think the concept of the "sweet spot" usually refers to the that area of the view (from the center point, progressing out towards the edges), wherein you do not notice any optical abberations, (other than varying degrees of chromatic in certain situations if the lense elements do not include special characteristics to focus all three primary colors at one point).
As I interpret it, it is when your eyes are coincident with the center of the focal point projected by the occular lenses, and is the sum of such considerations as exit pupil size, eye relief and interpupilary adjustment of the lenses.
At this spot, your sighting is at is optimal positioning.