Re: binocs

From: Steven Rogers (srogers1@austin.rr.com)
Date: Sat Jun 15 2002 - 20:51:35 EDT


On Saturday, June 15, 2002, at 06:53 PM, Linda wrote:

> Hi list mates,
> I am planning on buying binoculars to help me view the satellites, etc.
> I would appreciate suggestions on power, objective lens size, etc.  I
> need any suggestions that would help me make a wise and useful choice.
> Thanks in advance for your help.

7x50 is usually a typical choice for looking at stars and satellites. 
The "7" is the magnification, and the "50" is the objective size in 
millimeters (the distance across the big lens on the front).  The bigger 
the lens on the front, the more light the binoculars will catch - but 
also the size of the cone of light coming out of the binocular into your 
eye becomes larger for a given power. If the size of the cone of light 
leaving the binocular eyepiece is bigger than the iris in your eye, then 
some of the light will be wasted there.  It just shines on your eyeball 
rather than making a brighter image. 50mm is about as big as you can go 
with 7x for the average size pupil.

You might consider 7x35 if 7x50 is too heavy to hold comfortably. 10x 
binoculars are common, but most people find them too difficult to hold 
steady except in special situations (say birdwatching, where you can 
support your elbows). 7x35 would be a typical choice for looking at 
objects during the daytime - say a sporting event or bird watching. 
They're certainly not bad for sky watching - you'd be giving up just a 
little in terms of image brightness.

You can tell a little about the quality of a pair of binoculars by 
holding them out a couple of feet from your eye and looking at the 
bright area in the center of the eyepiece. If you can see a bright 
diamond or square shape floating in the circle, that's a sign of a less 
expensive kind of prism that's used to invert the image. If the image is 
uniformly bright, the prisms are probably of higher quality.

"field of view" is also something to keep an eye on. Generally, the 
greater the field of view, the more it will feel "normal" to look 
through the binoculars, as opposed to looking down a long tube that 
restricts your vision.

One of the most helpful things in making a choice is to visit a store 
and put a bunch of different brands in front of your eyes, see how the 
focus feels, see how heavy they are, and see how they feel in your hands.

Steve

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