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Re: [OM] (OT) Field Glasses recommendations - Chuck comments on exit pup

Subject: Re: [OM] (OT) Field Glasses recommendations - Chuck comments on exit pupil diameter
From: Chuck Norcutt <chucknorcutt@xxxxxxxxxxxxxxxx>
Date: Wed, 24 Dec 2014 11:14:53 -0500
My own Pentax thingies are these, the 8x version
<http://www.amazon.com/Pentax-25-DCF-SW-Binoculars/dp/B0014PTIDK/ref=sr_1_sc_1?s=electronics&ie=UTF8&qid=1419435842&sr=1-1-spell&keywords=Pentax+8x+25mm+Dcf+Sw+Binoculrs>

Perhaps some folks don't know of the relationship between the diameter of objective lenses, magnification and exit pupil diameter... and therefore why it matters. The exit pupil diameter is the diameter of the beam of light coming out of the eyepieces and is given by the diameter of the objective lens divided by the magnification (focal length of objective divided by focal length of eyepiece). That same beam must also pass through the aperture (pupil) of your eye. (The same thing holds for telescopes and spotting scopes)

Now consider some different sizes of binoculars and the size of the exit pupil that is created in each.

7x50 gives a 7.1mm exit pupil. These are often called "night glasses" since 7mm is about the maximum size of the pupil for someone who is totally dark adapted. If you're as old as many of us on this list we probably can't open our pupils quite that wide. Also, if you're not in a dark area, you eyes aren't fully dark adapted and your pupils are not fully open. So, under these conditions you aren't getting the full benefit of the large diameter exit pupil. Some (and maybe a lot) of the light gathered by the 50mm diameter objective is going to waste. In other words, you're carrying a large, heavy object which may not be doing what you hoped it would.

7x35 gives a 5mm exit pupil. Much smaller and lighter and may give you just as much or more light as your eye can use in most situations.

8x25 (my Pentax thingies) gives 3.1mm exit pupil. This is a significant number since 3mm is about the size of a typical person's pupil when in full sun. This binocular is very small and light but delivers all the light that the eye can use under bright conditions. Those 7x35 and 8x50 glasses are just extra dead weight when it's bright.

10x25 (Moose's Pentax thingies) gives 2.5mm exit pupil. Trades off a little more magnification for a little less brightness.

Purchase one or more according to your needs.

Chuck Norcutt


On 12/23/2014 5:01 PM, Moose wrote:
These are what I carry in my camera bag.
<http://www.amazon.com/Pentax-10x-25mm-Dcf-Binoculrs/dp/B0014PYBJQ>

Yes, they look rather like endless cheap, crummy binoculars sold
everywhere. Yes they are inexpensive.

BUT, BUT, I say, they are really first class optics - in a tiny, light,
under $100 package. I spent endless hours of research and miles of
driving to try out binoculars years ago, and ended up with a pair of B&H
8x42 Waterproof Elites. They are spectacularly good binoculars, as
proved for me in extremely difficult birding situations, for a fair
amount of dosh.

But in decent light, these little Pentaxes are within a fine
hairsbreadth of as good. Phase coated prisms, excellent eye relief,
twist up eye cups, nitrogen purged waterproof, small, light and a really
clear, crisp view. As the light gets dim, they do fall behind the much
larger, heavier 8x42s, but that's not as yet bothered me.
--
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