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A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: Correcting Night Vision
From: Frank Reed CT
Date: 2005 Aug 11, 21:53 EDT
From: Frank Reed CT
Date: 2005 Aug 11, 21:53 EDT
Jared you wrote: "I'm not sure that it is a "focus" issue per se." It's a failure to focus due to spherical aberration at larger pupil aperture, so it qualifies as a "higher order" focus issue. The point of the article's discussion of night eyeglasses is that a prescription can take better account of whatever spherical aberration exists in the eye and yield sharper star images and also a fainter limiting magnitude. I highly recommend the article: pages 28-42 of the September, 2005 issue of Sky & Telescope. Barnes & Noble usually has it available if your local library does not. And: "I have lasik damage, and during the day when my pupils are contracted I see better than I do at night." Most people see better during the day. Even Maskelyne back in 1789! Lasik damage, of course, is different from the usual spherical aberration. Spherical aberration yields worse focus but not the halos, glare, etc. that people with eye damage see (due to lasik or cataracts other causes). You also wrote: "Distortions in the cornea are actually common, and new procedures for corneal topography actually map the front and rear of the cornea separately to accomodate for them. Some of the ray tracing gizmos actually "project" a dot into your eye, and you use a joystick to mark a spot where you see it. That position is then compared to a mathematical position where a perfect eye SHOULD have seen the dot. And almost everyone has irregularities within the eye that show up during this kind of mapping." There are lots of details on this in the S&T article. Well worth reading. While I'm here, there's a typo in my original message in this thread. I wrote: "(20/20 is 1.0 arcminute resolution by definition, 10/10 is 0.5 arcminute resolution --which is right at the theoretical limit for ordinary resolution)." Of course, that should say '20/10 is 0.5 arcminute resolution', not '10/10'. -FER 42.0N 87.7W, or 41.4N 72.1W. www.HistoricalAtlas.com/lunars