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Message: <blockquote data-quote="elkcub" data-source="post: 1533823" data-attributes="member: 14473">Ron,My slightly different way of saying it is that the observer uses information in the image to control precise focus on the retina (i.e., visual accommodation).  A key source of such feedback information is known to be chromatic aberration.  Consistent with what you said, this control mechanism would help to overcome focus uncertainly due to an increase in DOF with smaller pupil size.  And, along with greater illumination contrast, the whole biofeedback mechanism would work to optimize visual clarity.  Since it may be assumed that a binocular-telescope image on the retina is focused using similar criteria, some minimum CA from the instrument may also be beneficial, if not necessary.  Of course, even using ED glass in the objective there is no concern about having too little, since some fringing can always be seen.  Still, it points up that the ultimate optimization can't be done by optics alone, but has to take into account the observer's oculomotor and perceptual systems.Are we on the same wavelength?  Ed</blockquote>