🚀 go-pugleaf

In article <1993Jan24.184101.27715@thunder.mcrcim.mcgill.edu>, mackenzi@McRCIM.McGill.EDU (Paul Mackenzie) writes:
|> 
|> I assume there must be some people in this group who know about
|> binoculars - can anyone tell me what the numbers listed with
|> a particular unit mean?  I've seen a few in stores that say
|> 8 x 40mm or something like this, and I've also seen things like
|> field of vision is 340' at 1000 ft.  Can anyone tell me how to
|> decipher the magnification out of these so I don't buy a cheap
|> pair of field glasses?
|> 
|> Thanks,
|> Paul

The first number is the magnification, and the second number is the diameter of
the objective lens in millimeters.  The wider the lens, the more light it can
collect, and the brighter the images.  So 8 x 40 simply means 8x magnification
with 40 mm wide lens.

I suppose that the higher the magnification, the better for birding.  Usually high
magnification comes with wide lenses, plus weight and the cost goes up. 
 The monster binoculars go up to something like 20 x 100.  I think
they're mostly used for astronomy.  Are they too unweildy for birding, anyone?

I have a pair 10 x 50 Bushnell binoculars w/ Insta-Focus.  It's good 
for astronomy, and it's really great for birding, too.  It's not too heavy, but it
is a little tiring if you hold it up for a long time.  It's no big deal though.  I
bought it a few years ago for about $60.  Most high quality 10 x 50 binoculars go
for about $200.  Even good 7 x 35 binoculars go for $100 plus.
My $60 binoculars is good enough for me.  In my case, I would
rather spend money on a spotting scope instead of replacing my binoculars for a
better $200 one.

In other words, the less expensive models are sometimes good enough.  Unless, you
are highly perceptive and notice the less-than-optimum optics, or if you have the
money.

Before you buy a new pair of binoculars, it's a good idea to test them out.  For
example, you can look through the binoculars a foot away.  The circle of light
should be centered in the eyepiece lens.  If you look at a horizontal line, it
should also look horizontal through the binoculars.  If you look at a white wall,
it should look white through the binoculars, not yellow or blue or whatever.
  Just make sure that there are no
grossly misaligned optics, and scratched lenses.  Just a precaution.

However, that 340' at 1000 ft, I can't tell you about.  I read it somewhere, but
I forgot about it.  Someone else?

Field glasses are a little different than binoculars.  These were around before
prisms were used, which made binoculars much more compact than field glasses.
  Field glasses were two telescopes mounted side-by-side.  So
they turned out to be more than one feet long.  Very unweildy.  It might be nice to
have in your home though.

Does anyone know if there is a big difference in the image quality between a $60
binoculars and a $200 binoculars?  I haven't tried out the more expensive models.
Maybe I'm too complacent with my own pair of $60 binoculars.


-- Chau  

Paul asks the meaning of 8 x 40 for bins.  The 8 is the
magnification and the 40 is  the diameter of the objective
lens.  The value 40 / 8 (i.e. 5) is a measure of the width
of the field of view.  Thus 7 x 42 have a lower
magnification, but a wider field of view -- better for
following birds through trees relatively close to.

Regards,

Mick

In article <mick.728017855@picus>, mick@dcs.bbk.ac.uk (Mick Farmer) writes:
> Paul asks the meaning of 8 x 40 for bins.  The 8 is the
> magnification and the 40 is  the diameter of the objective
> lens.  The value 40 / 8 (i.e. 5) is a measure of the width
> of the field of view.  Thus 7 x 42 have a lower
> magnification, but a wider field of view -- better for
> following birds through trees relatively close to.
> 

The field of view is not directly defined by magnification. It is
a function of both magnification and eyepiece design. Thus a 10x50
binocular can have a wider field of view than a 7x50, if designed
to do so. The problem is that the quality of the image can get
quite poor at the edge of the field in a wide field design unless
you take a lot of care (i.e. expensive). Within any give power
binoculars (say 7x35s), each will probably have a different field
of view though all are 7 power. 

The diameter divided by the power (40/8 in the above example) has
nothing at all to do with field of view. It gives what is called
the "exit pupil" of the system (pretty much the diameter of the
rays of light coming out of the binocular and into your eye). 
By and large, the larger this number is, the brighter the image will
be in the dark. Numbers larger than 7(mm) provide no advantage since
most eyes cannot open their pupils wider than 7mm. Thus you see 7x50s
but not 5x50s. There are other factors affecting image brightness, but
exit pupil is probably the most important. The pupil of the eye
contracts in bright light (to maybe 3 or 4mm?), so in daylight exit
pupils larger than this do not give brighter images. This means that
in daylight 10x50s and 7x50s will give images which look equally bright,
but at night things will look brighter in the 7x50s (assuming good eyes).
Maximum pupil size decreases with age, so as you can see there are a few
other factors to consider!

Now back to birds...
===============================================================
Bob Atkins  AT&T Bell Labs  email (direct)  att!clockwise!rma
===============================================================

mick@dcs.bbk.ac.uk (Mick Farmer) writes:

>Paul asks the meaning of 8 x 40 for bins.  The 8 is the
>magnification and the 40 is  the diameter of the objective
>lens.  The value 40 / 8 (i.e. 5) is a measure of the width
>of the field of view.  Thus 7 x 42 have a lower
>magnification, but a wider field of view -- better for
>following birds through trees relatively close to.

I'm pretty sure this is incorrect.  The value 40/8 (5)
is a measure of how wide the image leaving the binoculars is.
In other words, it measures not how wide a field of view you get,
but how much light you can see.  8 x 40 bins and 7 x 35 bins
(everything else equal) give you the same brightness of image,
but could have completely different fields of view.
Any ratio over 7 is wasted, because the human eye's maximum
pupil width is 7 mm.
-- 
Adam Winer              | The number you have reached is imaginary.
WINER@HUSC.HARVARD.EDU  | Please rotate your phone 90 degrees and
                        | try again.

> >Paul asks the meaning of 8 x 40 for bins.  

In article <winer.728073920@husc.harvard.edu>,
winer@husc10.harvard.edu (Adam Winer) writes: 
>   The value 40/8 (5)
> is a measure of how wide the image leaving the binoculars is.

This is correct, though it might be better to think of it as the ray
bundle.  The technical term is "exit pupil diameter."

Cheaper binoculars have square exit pupils because of vignetting in
the prism.  In these, the "diameter" is actually the size of the
diagonal of the square, and you get only 2/pi of the light you would
expect. 

> In other words, it measures not how wide a field of view you get,
> but how much light you can see.  8 x 40 bins and 7 x 35 bins
> (everything else equal) give you the same brightness of image,
> but could have completely different fields of view.
> Any ratio over 7 is wasted, because the human eye's maximum
> pupil width is 7 mm.

In fact, for birding, any diameter over about 3 or 4 mm is wasted.
Check out your pupil size in twilight some time.  (This opinion is
contrary to a lot of received wisdom, but I stand by it.)

Exit pupil diameters up to 7 mm _may_ be useful for surveillance or
astronomy or other observing in full darkness, but even then not
everyone's eyes will open that wide.
-- 
Steve Willner            Phone 617-495-7123         Bitnet:   willner@cfa
Cambridge, MA 02138 USA                 Internet: willner@cfa.harvard.edu
  member, League for Programming Freedom; contact league@prep.ai.mit.edu

mackenzi@McRCIM.McGill.EDU (Paul Mackenzie) writes:


>I assume there must be some people in this group who know about
>binoculars - can anyone tell me what the numbers listed with
>a particular unit mean?  I've seen a few in stores that say
>8 x 40mm or something like this, and I've also seen things like
>field of vision is 340' at 1000 ft.  Can anyone tell me how to
>decipher the magnification out of these so I don't buy a cheap
>pair of field glasses?

>Thanks,
>Paul

The first set of numbers-8x40mm: The first number stands for the
                                 magnification power. The higher it is,
                                 the closer objects appear, but the harder
                                 it is to keep the object steady. The 
                                 second number is the diameter of the large
                                 lens on the end of the bincos. The larger
                                 this lens, the more light gathered, and the
                                 clearer objects will seem.
The second set-340'/1000': The field of view is the amount of area you can
                           see at once through the glasses. With a 340/1000,
                           you can scan an area 340' across, 1000' distant 
                           without moving the binocs.

These sound like solid glasses for birding to me (though I prefer something
with a wider field of view). When looking fora good set, make sure they are
waterproof and come with fully coated optics (coated optics cut down on the
amount of light lost due to reflection off the lens...the more light making
it to your eyes the better!!)

Chris
Kansas Dept. Of Wildlife and Parks
chavel@matt.ksu.ksu.edu