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Message: <blockquote data-quote="Binastro" data-source="post: 3651807" data-attributes="member: 111403">Don't know about DOF but I was able to measure distances of a mile, maybe more, with the 317mm Dall Kirkham, 4650mm focal length, just by the focus position of the eyepiece compared to the stars.So I don't think the DOF was that great, although it could be.I don't think the 317mm aperture was stopped  down at high magnification.I suppose it was similar to a 317mm base rangefinder with high magnification, although I haven't done calculations to compare.I also had an approximately 6ft rangefinder possibly Barr and Stroud, which measured miles I think. I can't remember the maximum accurate range or the accuracy.P.S.Apparently the Americans used stereoscopic rangefinders and the British Barr and Stroud were coincidence rangefinders.The stereoscopic and coincidence acuities were about equal in tests.4.5m American and 3m British instruments.Ranges 4,000 yards to 12,000 yards approx.Laser rangefinders are used nowadays but can give away the observer's position.I suppose the DOF in binoculars and telescopes depends on the observer's accommodation and acuity?</blockquote>