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Message: <blockquote data-quote="elkcub" data-source="post: 1689120" data-attributes="member: 14473">Matt,The notion of "exit pupil matching" as expressed by optical folks is really oversimplified and often misleading, — primarily because it overlooks the adaptive processes of the eye, in which pupil diameter is a rather minor player.  It's somewhat like the tail wagging the dog.  Optically, it is true that the larger exit pupil of a 7x42 would potentially allow 3.6 times as much light to enter the eye as a 10x42, and this seems impressive.  However, considering that the eye's sensitivity has an adaptive range of six orders of magnitude (i.e., a million to one) it's really rather insignificant.  The primary function of the iris (pupil), it would appear, is to protect the eye from too much light rather than to allow more in.  This is evidenced by the reluctance of the iris to expand and its great willingness to contract.  Full expansion takes about 30 min., whereas full contraction takes only seconds or even fractions of a second.  Contrary to what is often assumed, the aperture of the pupil is also not a constant function of light intensity, because the sensitivity of the retina changes as a result of adaptation.  Thus, when light intensity increases moderately, the pupil constricts, but then under constant illumination it gradually dilates again. With larger increases in intensity, the pupil will remain constricted as a physical protection.  On the opposite end, as the eye becomes starved for light the iris gradually opens to its maximum, but much more importantly the retina increases sensitivity by orders of magnitude.  If the iris were really all that critical there would probably be no old astronomers.  Personally, I think ergonomic factors such as eye relief, image stability, ease of visual alignment, perceived depth, and apparent field are much more important in selecting a 7x vs 10x binoculars.  Most of these factors were mentioned above.Ed</blockquote>

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