Path: utzoo!attcan!utgpu!jarvis.csri.toronto.edu!mailrus!ncar!ames!uhccux!mikeb
From: mikeb@uhccux.uhcc.hawaii.edu (Mike Burger)
Newsgroups: rec.birds
Subject: Objective Size
Keywords: scopes
Message-ID: <4549@uhccux.uhcc.hawaii.edu>
Date: 11 Aug 89 22:59:34 GMT
Distribution: usa
Organization: University of Hawaii
Lines: 46


	Objective size (the second number in something like 7X50)
deterimines brightness, as most know.  There is a simple way to
decide what is useful that applies to binoculars and scopes.
	The human eye can open up under dim conditions to
about 7 mm.  In daylight it is considerably smaller.  What
the binoculars or scope is doing is taking the larger objective
lens, and optically shrinking down that cylinder of light to
the size of a human pupil.  This is what the light gathering
power of a scope is all about.   What goes in as say a 50
mm circle comes out as a much smaller one that fits into
the opening in the human eye.
	The difference in the exit diameter of the light and
the entrance diameter at the objective is simply the magnifying
power of the scope.  Thus for a 7X35 binoc, it goes in
35 mm and comes out 5 mm, a comfortable fit for a human
eye in ordinary light to moderately dim light.  In the
evening when the eye can open to nearly 7 mm, the larger
7X50 type becomes an advantage since it goes in 50 mm and
comes out 7.14, nearly perfect for the largest normal openning
in the iris under dim light.
	In bright light the human iris is shut down to
say 3 mm and the 7X50 and the 7X35 both more than fill
the circle of the iris and the view is the same, though the
weight is not.
	As you can see, with 20 power if you want maximum
light gathering power that the human eye can use in dim
light you want an objective lens 140 mm in diameter, FIVE
and ONE HALF INCHES!  Such a monster would not be very
portable.
	Above about 10 power with say 10X80 binocs, the
exit diameter of the light from any scope will all fit
into the human eye for any reasonable diameter objective
lens.   Thus any increase in size will be a practical
increase in brightness under any lighting conditions.
80 mm will be obviously better than 40 mm.
	Other considerations such as portability and
quality of optics (sharpness) rapidly become more
important as magnifying power exceeds seven or eight
power.  Be sure to also check out color correction and
color fringes on all binocs and scopes.  Width of field,
how much is seen side to side at a given power, is
largely determined by the quality and design of the
eyepiece.  This can also very widely on scopes and
is very important.  At 20 power even "wide angle"
scopes see very narrow, hard to point fields.