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Message: <blockquote data-quote="elkcub" data-source="post: 1701662" data-attributes="member: 14473">It may be that there is a causal relationship between field curvature (i.e., a "flat field") and "rolling ball" perception, but to my knowledge this psychological phenomenon is primarily associated with the aberration of distortion.  Field curvature has to do with the the focal surface of the image, which is, essentially, a spherical cap.  Depending on the expert, some amount of field curvature is considered beneficial for the observer.  Still, some people want to eliminate it altogether.  Distortion has to do with image magnification as a function of field angle, — where increasing magnification results in pincushion distortion and decreasing magnification results in barrel distortion.  (Only the unique case of constant magnification results in a rectilinear image.)  Some amount of pincushion is beneficial to the observer for minimizing rolling ball perception; still, some people want to eliminate it altogether.Because the two parameters are design independent, a completely flat field binocular may be given sufficient pincushion distortion to minimize rolling ball perception.  Vertical images viewed at the edge will curve outwards to some extent, but still be in focus.  Conversely, an instrument that produces a perfectly rectilinear image across the field will necessarily produce some rolling ball perception.  It may or may not be given field curvature, depending upon the design, so that the edges may be somewhat out of focus.  What it comes down to, therefore, is that the user must ultimately decide what amount of these two aberrations is most effective for his/her own viewing needs.  All the rest is either strong opinion or commercial rhetoric.   <img src="data:image/gif;base64,R0lGODlhAQABAIAAAAAAAP///yH5BAEAAAAALAAAAAABAAEAAAIBRAA7" class="smilie smilie--sprite smilie--sprite2" alt=";)" title="Wink    ;)" loading="lazy" data-shortname=";)" />Ed</blockquote>

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