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Re: [OM] OT Binoculars

Subject: Re: [OM] OT Binoculars
From: Chuck Norcutt <chucknorcutt@xxxxxxxxxxxxxxxx>
Date: Sat, 17 Oct 2009 22:06:28 -0400
Moose gave you a pretty good overview but I'd like to mention some basic 
things about binocs that you should understand because it will assist 
you in understanding that a particular binoc is or isn't a candidate for 
your usage.

The first thing is to understand the size convention which is to list 
magnification and objective lens diameter.  For example, the common 7x35 
(and there's a reason why it's common) means magnification 7X and an 
objective (front) lens diameter of 35mm.  These two numbers are 
important since together they determine the size of the exit pupil. 
I'll point you to wiki for a formal definition of exit pupil 
<http://en.wikipedia.org/wiki/Exit_pupil> but from a practical 
standpoint you can consider that there is a cylindrical beam of light 
emanating from the eyepiece and entering your eye.  The diameter of that 
beam is the size of the exit pupil.  For a 7x35 binocular the exit pupil 
is given by 35mm / 7 = 5mm.  So imagine a 5mm thick beam of light 
projecting out of the eyepiece.  But, to do any good, it must fit into 
your own eye.  ie; the pupil of your eye must be open at least 5mm to 
allow that beam of light in without being vignetted.

Now some points about the typical sizes of the human pupil under 
different lighting conditions.  If the human eye is fully dark adapted 
(probably at least 15 minutes in very dark conditions) the pupil will 
dilate to about 7mm.  The sunny 16 daylight adapted eye, however, will 
close down to its smallest size which is about 3mm.

Now consider using the 7x35 on the beach at noon.  The binoc is 
producing a 5mm exit pupil most of which can't fit into your 3mm 
diameter pupil.  The extra size and weight of a 35mm objective lens is 
being wasted.  So consider a 7x21 binoc.  You're still getting 7X 
magnification but now only a 3mm diameter pupil.  In practice, the 
smaller glass is delivering as much light as your eye can reasonably use 
but the tool is much smaller and lighter and better sized to the task.

Now consider a 7x50 binoc.  The exit pupil is 50mm / 7X = 7mm.  Notice 
that the 7mm exit pupil of the 7x50 glass is exactly the right size to 
slip into the pupil of your fully dark adapted eye.  That's why a 7x50 
is also known as a "night glass".  But carrying around a 7x50 night 
glass at noon when a 7x21 will provide all the light your eye can use is 
a bit of a waste.  Sort of like carrying around a big and heavy f/1.2 
lens at a time when f/4 is all you need.

So, for birding you need something between full daylight and shadowed 
hiding places.  In practice that means an exit pupil of 5-6mm diameter. 
  You need as much magnification as you can reasonably hold still.  Most 
folks can handle 7-8X but 10 is getting too high.  Work it all out and 
you'll find that somewhere between 7x35 and 8x42 (or thereabouts) is the 
right specification.  You maybe knew this all along but now you know why.

Finally, another really important spec number is the field of view.  The 
wider the better when trying to track fast moving birds.  Binocs give 
field of view as a linear extent at a given distance.  Telescopes do it 
differently.  Angular field of view is apparent field of view of the 
eyepiece divided by the magnification.  A really wide apparent field of 
view for any eyepiece design is about 65 degrees.  At 8X magnfication 
that would yield approximately an 8 degree field of view.

I can't give you any specific brand and model advice.  I've never spent 
a lot of money on binocs.  I have some inexpensive Bausch & Lomb 8x42s 
and also a pair of Minolta 7x35 which I bought used on the bay for a 
song.  They are my favorites.

Chuck Norcutt


Jim Couch wrote:
> Moose,
> 
> Thanks, very helpful. Not the answer I wanted, but pretty much what I 
> expected -compact,  inexpensive, good for birding, pick two!  (Or in 
> this case, pick one?) :)
> 
> At least I have some idea of what to look for at this point.
> 
> Jim Couch
> 
> 
-- 
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