Re: Binocular Question

From: Ed Cannon (
Date: Wed Apr 11 2001 - 21:06:22 PDT

  • Next message: Kevin Fetter: "not the same object"

    I want to contribute here, as I've been a serious *handheld* 
    binoculars user for five years now and have also done considerable 
    online "research" about them.  From what I've learned, I agree with 
    Rob's conclusion -- for those with 7mm dark-adapted pupils -- that 
    a 7mm exit pupil (7x50, 8x56, 9x63, 10x70) is best.  (For newbies, 
    "exit pupil" is the binoculars' objective lens size in millimeters 
    divided by the power or magnification.  For example, for 7x35 and 
    10x50 binos, it's 5mm; for 7x50s, it's 7.14mm.)  However...
    HOWEVER, I wonder how many of us get our pupils dilated that much, 
    and how frequently.  In moonlight or light-polluted skies, they 
    don't dilate that much.  Also, as Rob mentioned, maximum pupil 
    dilation decreases with age.  So in many cases (especially old 
    folks observing from mid-city or just about anyone observing under 
    a full Moon!) it could be that a 6mm or even 5mm exit pupil is very 
    good (e.g., 7x42 or even 7x35, 8x50 or even 8x40, or maybe even 
    10x50, which I use).  (Footnote:  There are a couple of reasons,
    having to do with motion of target and/or your own shaking hands, 
    to have an exit pupil somewhat larger than your eye's pupil.)
    There are five very informative articles about choosing binoculars 
    for astronomy on this page of the Sky & Telescope Web site:

    In two of those articles it's argued that the best single measure of 
    binoculars is a "visibility index" (magnification times objective 
    lens size), but that can NOT be the only factor considered.  Rob 
    mentioned the common-sense factor of weight.  There are excellent 
    10x70 binoculars on the market, but unless you are two meters tall 
    and weigh 120 kg, they'll tire you out rapidly.  You would have to 
    use them with a mount.  And if you're getting "over the hill" (i.e., 
    older, such as I!), you probably don't need that large an exit pupil 
    anyway!  One interesting comparative table online by Brooke Clarke 
    develops a "holding index" for a lot of models -- it's an attempt to 
    quantify the shaking factors due to magnification and weight, 
    alongside of the "visibility index":

    He has a lot of satellite-observing links also:

    Another good article about binoculars, especially under "Exit Pupil", 
    is at this location:

    One other consideration for watching LEO satellites (or anything in
    motion) is field of view, and within other constraints, wider field 
    of view is better!  Also, if you need to wear eyeglasses while using
    binocs, you need longer "eye relief".
    Ed Cannon - - Austin, Texas, USA
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