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Binocular Buying Tips
by Roland Herrera
(back to my Astronomy page)
Introduction
I have 4 binoculars (x8.5 x10 x12 x15). I use binoculars for Astronomy, and
general viewing while travelling. I love just looking around my garden at
nothing in particular through a good pair of binoculars. Over the years I have
bought and sold quite a few binoculars. The tips I present here are for
intermediate users ; those who wish to know a little more about high end optics.
If you've not heard of exit pupil diameter (measured in mm) or field of view,
then you're a beginner... try reading up about binos first then come back here
for some practical tips. The reason I wrote this page is because I would have
liked to have known the things I now know before buying some binoculars (binos)
that I bought in the past!
Bino-Theory
- Binocular Advantages ; contrary to most modern high tech gadgets you can
buy, Binos do not go out of date very easily. Sure, Flouride lenses are
introduced, but the improvements are small. Binos have remained unchanged for
many years. We are still a long way away from achieving a perfect picture with
alpha binos... what they achieve is a better picture... take my latest Leicas;
an on axis star test reveals many spikes, rather than a pinpoint of light.
Towards the edge of the field of view the star looks like a flattened pancake
or a curved line... or something like an eighth of the circumference of a
circle. Defects are many, but the view improves slowly. Binoculars are a
compromise between optical quality and a practical package with sensible
weight.
- Porro prism binos transmit more light than Roof prism binos because the
light path is shorter mainly because there is one less prism surface to
reflect off. Also, roof prisms split the light beam up which then has to be
recombined in phase. If they are recombined with phase shifting the result is
less light and contrast.
- Porro prism binoculars, for the above reason, are traditionally better for
Astronomy. However, in recent years Porro prisms have gone out of fashion, and
mainly thanks to bird watchers, Roof prism design and quality has grown to such an extent that the best now
equal or surpass Porro prisms.
- Roof prisms use the following designs : Schmidt / Pechan / Schmidt-Pechan
/ Amici / Abbe-Koenig. This last type, the Abbe-Koenig does not displace the
output beam from the input beam, and is generally brighter. Zeiss's top of the
range Victory binos usually use a phase corrected Abbe-Koenig design. Their
mid range Conquest might use a phase corrected Schmidt-Pechan prism design.
- Aperture. Binocular objectives are measured in mm. Naked eye can observe
down to magnitude 6. 50mm binos (like 10x50s) can go down to magnitude 10.3
and 100mm binos to 11.8 so a good binocular lies half way between naked eye
observing and a telescope.
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