Buying Binoculars

Terry Lisansky (lisansky@UDel.Edu)
Mon, 21 Dec 1998 04:58:47 -0500 (EST) 

	Buying binoculars, what do you need to know?  To sum it up, large
objective diameter, magnification no greater than 7, good eye relief, good
exit pupil, fully multi-coated glass, bak-4 prisms, and maybe a couple
other goodies.  Simple, right?  Maybe not, so lets get into the details. 
	First of all you need good objective diameter.  For astronomy this
usually means 50mm or larger.  Simply put, the larger the diameter, the
more light can get in.  This is balanced by magnification.  Usually 7
power is the most you can use and still hold your binoculars steady.  Also
this is related to exit pupil.  This is the diameter of the light beam as
it exits the the eyepiece.  Divide the objective diameter by the
magnification and you'll get it.  6 millimeters is about the size of your
dark adapted eye pupil.  50 (the objective diameter in millimeters)
divided by 7 (the magnification) is a little more than a 7mm exit pupil. 
Plenty large enough.  
	Eye relief is the distance back from the eyepiece the image forms. 
If you wear glasses and want to keep them on while viewing, you need a
good relief.  This number is available from the manufacturer.  You may
have to find this out for yourself by looking through the binoculars and
deceiding if its enough.
	Fully multi-coated glass refers to the optical coating that should
be on every air to glass surface.  Every time light hits glass a small
amount is reflected away.  You don't want this.  You want the light to
pass through with a minimum of trouble.  When a surface is coated, this
reflection is minimized.  With a multiple coatings, it's minimized even
more.  When light has several air to glass passages to make, there are
lots of possibilities for light to be reflected everywhere but straight
through like you want.  So you can see why it's important for each surface
to be coated.  A quick way to judge this is to look at the objective lens
surface with a light source behind you.  For every light source colored 
reflection you see, you have an uncoated surface.  If the light behind you
is white, each white reflection in an uncoated surface.
	Bak-4 prisms use the best glass.  This is a must for good light
transmission.
	Other things you might like are a tripod mount or rubber coated
barrels.  Both can make life easier.  Also look at the guarantee.  Does
the manufacturer with a 30 day guarantee have the same faith in their
product as one that has a lifetime guarantee? 
	All these things are not going to be found in $25 binocluars.  But
you don't need $1000 for a good pair either.  Start your buying adventure
by looking in catalogs and stores and seeing whats available.  Look for
what I've just discussed.  Maybe you can look through various models at
star parties and ask the owners how their particular model performs.  If
you can find a store with more than one model, look through all that are
in your price range.  Things you might consider are, how heavy are they. 
Will you be comfortable holding them after a few minutes?  Is the image in
focus from edge to edge or is it sharp in the center and blurry around the
edges.  Does the image have good contrast.  And can I really tell the
difference between the $300 model and the $500 pair.  
	The differences between cheap models and more expensive ones are
obvious.  But as the price gets higher the differences become less.  Only
you know if you can justify that $500 pair when the image through the $300
pair was almost as good.  Price alone is not always an indication of a
good image.  If the image through a $500 pair isn't as good as the $300
pair, why spend the extra money? 
	Armed with some basic information you can be an informed buyer and
avoid the pitfalls that you shouldn't have to fall into. 


Terry Lisansky - N3JJB				http://udel.edu/~lisansky
		Give me a fast ship........
			For I intend to go in harms way.