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Message: <blockquote data-quote="John Dracon" data-source="post: 2855995" data-attributes="member: 14799">Dave - a non-scientific word yet useful word to decribe having the optical elements moving so that the desired focal points are repeatable is the word "timing."The current binocular designs have to be able to move the optical elements back and forth with accuracy and repeatability,  if they are to be useful. To accommodate the difference between eyes in resolving power (there are of course other eye issues which cannot be resolved by adjustments , e.g.  astigmatism), the diopter adjustment (normally on the right barrel) also comes into play.  Some binoculars, normally the more expensive, incorporate the diopiter mechanism internally, but this reguires very sophisticated mechanisms and mechanical relationships. Most internal focusing designs enhance waterproofness. The classic way of dealing with it is found in CF binoculars with oculars which move back and forth, and the right ocular can be adjusted still further.  This requires close tolerances as the ocular casings have to slide in a sleeve, and minute bits of whatever can be drawn into the mechanism causing galling and other problems. The excellent B&L Zephyr binoculars and many other external focusing CF binoculars have this problem.  If one side hangs up, then a timing problem in focus is created. Even my Zeiss alphas such as the 7x42 BGTA and 15x60 BGTA are subject to timing issues.  The only design that I have used that seems to avoid this without extraordinary costs is the IF binocular.  The Zeiss 8x30 Safari model illustrates this easily.All the optical elements save the oculars are fixed internally.  The oculars movements are snug and creates water and dust proof interiors. This designs allows also collimation to be unaffected when the oculars are moved in and out.  But of course the IF binocular is hardly a handy birding binocular.  When comparing the least expensive Zeiss, the Terra to the most expensive  model, the HT, Zeiss incorporated the effective and time tested diopter adjustment, the external friction system.  Undoubtedly less expensive to do than the system in the HT.IMO many of the simpler designs minimize timing problems.John</blockquote>

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