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For the same objective diameter, another feature of slow (F8, F10, etc), long focal length 'scopes is that focusing is not quite as critical. Using the analogy of camera lenses, you get a “better” depth of field, or “wider zone” of focus with a long focal length (F8 +) ‘scope. In a short focal length ‘scope (F4, F6), the zone of critical focus is quite small in comparison.
Cheers
Dennis
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