ER specs are always given as an exact number, meaning (as I understand it)the proper distance from the eye to the eyepiece for viewing the full FOV. In reality though, there is a window (or range) in which the view is, let's say, full. Under a given set of conditions, this full view window has a definite close limit and far limit, which would seem to constitute an actual range of ER. (It's easy to detect these limits by simply moving the bins from too close to too far from one's eyes. Nearer than the close limit or farther than the far limit both result in loss of field and/or "blackout".) In terms of "ease of view", the width of this ER range would seem to be quite significant. I suspect that it could come into play on a regular basis in field use for most viewers.
To what extent does this "ER range" vary from model to model? If so, is it inherently linked to magnification, ER, objective diameter, etc?
Also, would it vary due to differences in eye-pupil diameter?
-APS
Thanks Kevin for the extensive reply. If the "ray bundle" converges at a specific distance, that makes sense for a precise distance spec. But ...
There is a perceivable "range" though, right? So then, what causes it? Does the "range" possibly represent the distance in which the "ray bundle" is of a diameter less than one's eye pupil diameter (at a given moment)? Would that also imply that bins with larger EP have a smaller range? Or, would it make sense that the "ray bundles" of various bins converge on different angles depending on the eyepiece design (a gradual angle would result in a wider ER range), meaning that there are perhaps other over-riding factors?
Regardless of the actual cause(s), I'm primarily interested in knowing how this perceivable range varies from one binocular to another. Is it perhaps better (in general) in some bins than in others? And to what extent? And are there inherent bin design factors (other than, or in addition to, eyepiece design)? Most importantly, is this possibly a factor in the "ease of view" so often mentioned in field comparisons?
Thanks, APS