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Posted By: Walter F. Mathers on: 05/08/2002 09:01:21 CDT
Subject: A Relative Technical Question on Focal Length?

Message Detail:
Ken,

I just haul'd out two of my long glasses and found them both to be focus'd in at about 27 inches each. They are able to be slide out another two inches each until they reach the point of being fully extended (all tubes in full extension)

A ship's glass I possess is more powerful than the two mentioned above and is but a two draw model. It extends quite a bit longer than the 29 inch focal lengths of the other two glasses. On the other hand, I have a pocket glass that extends to only about ten inches when focus'd.

I can say that, even with the 26 or 27 inchers, you'd need a shorter draw for objects closer to you and a longer draw for objects viewed just over the horizon (by mere fractions of an inch), as in viewing the masts of another vessel lying just over the curvature of the Earth, or approximately about eight and one half miles distance.

Walt
3.3.3.

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Signal Corps Association (1860-1865)
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