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Best for Birding...
 OPTICS - STATISTICALLY SPEAKING For your information we present a comprehensive list
of product
features and specifications for the binoculars
we stock.
Binoculars are essentially a pair of miniature scopes built
into the one body. To reduce their overall size, binoculars manage
light gathered using internal prisms. Two prism designs are in
use:
*Porro prism folds the light through an S-shaped path
*Roof prism splits the light into two beams to be recombined
later
The two prism systems can be distinguished by comparing the
sideways distance between the objective lens and the sideays distance
between the ocular lens of a pair of binoculars.
Porro prism binoculars show a distinct distance (classic binocular
shape) whereas Roof prism binoculars are virtually in alignment
leading to a more compact design. Porro prism-equipped binoculars
are generally cheaper as they're easier to manufacture. Cheaper
porro prism binoculars use BAK7 borosilicate glass for their prisms,
which can produce non-circular exit pupils, meaning there's the
possibility of disconcerting grey borders on the periphery of
your vision. This is caused by light not being reflected fully
by the prism's internal elements. Better porro prism binoculars
use BAK4 barium-crown glass prisms offering better optical qualities.
Binoculars are defined by two numbers eg: 10x42. The first
number refers to magnification, the second to the aperture of
the objective lenses described in millimetres. Therefore 10x42
means you're 10 times closer to what you're looking at and the
objective lenses (closest to what you're looking at) are 50mm
in diameter.
Additional specifications such as field of view and twilight
factor also appear in the tables below.
Field of view is best thought of as 'how wide is the image?'
The term can be expressed in various ways. As 110
metres at 1000 metres, for instance - which means that these particular
binoculars will simultaneously view two objects 110 metres apart
that are both a 1000 metres distant. Another way of thinking
about this is that these two objects could be 11 metres apart
and at 100 metres distance. Alternatively, the field of view is
expressed in degrees. The conversion from metres at 1000 metres
to degrees is: degrees = (metres at 1000 metres) / 17.5 degrees.
There is also a distinction that should be drawn between the apparent
and the true field of view. Although you may be viewing
what appears to be a large apparent field of view, you may in
fact be actually viewing a very small field of view. The apparent
field is largely defined by the optical design of your equipment.
Twilight factor is used to describe the performance of binoculars
and spotting scopes in low light conditions. It is a mathematical
formula: the square root of the sum of magnification times the
aperture. It should be noted that the size of the exit pupil and
the overall light transmission (sometimes defined in terms of
its brightness index) of the instrument are also important criteria
for using binoculars in low light use.
Most binoculars include mechanisms for interpupilliary distance
and dioptric adjustments to improve your observing comfort. Some
may also be attached to tripods using an adapter or mounting bracket.
Spotting scopes provide an image that is the correct size
up - astronomical scopes are upside down - they are designed to
view objects on or near the earth's surface. Scopes usually have
interchangeable eyepieces and most can be connected to 35mm SLR
cameras by way of T-mount adapters suitable for all cameras. Spotting
scopes must be mounted on tripods as their higher magnifications
limit their use without a steady mount, as such, they can be attached
to tripods for steadier viewing. Many scopes have special lens
coatings in an effort to prevent excessive ultraviolet and/or
infrared light from reaching the human eye.
Spotting scopes are best defined as refractors, that is, they
use a fairly large objective lens to gather light. To reduce chromatic
aberration, most refractor scopes use either apochromatic or achromatic
objective lenses. These lenses are difficult to manufacture and
therefore, larger aperture refractors are expensive to construct.
Scopes offer either a straight through or angled viewing position
- both positions have their benefits.
Tricky Technical Terms
Magnification
Magnification for scopes and binoculars is defined as the
focal length of the objective lens divided by the focal length
of the ocular(eyepiece). For spotting scopes - and astronomical
telescopes used in daylight - the maximum useful magnification
is equal to the size of the objective lens in mm. That is,
an 80mm spotting scope has a maximum magnification of 80x. But
this only applies on an ideal day. If there is any wind, or sun
shimmer, then air turbulence may limit the maximum useful magnification.
Most scopes are used between 15 and 30x magnification, zooming
up to 60x (for instance) when the weather allows. After all, it
should be noted that you're also magnifying any air turbulence,
pollution and heat haze by the same magnification.
Coated optics
A difficult term to define as it is used to describe a lens
that has been coated with a surface treatment of some kind - this
is usually an anti-reflective treatment. Uncoated, ordinary, optical
glass reflects approximately 4% of the light falling upon it.
In a complex optical system this may result in a significant loss
of light to your eyes. It may also mean an undesirable loss
of contrast and ghost images or haloing.
The manufacturer deposits a coating on the surface of the
lens under carefully controlled conditions. Once correctly applied,
the anti-reflective coating suppresses reflected light energy
and redirects it into transmitted light (this is the goal of the
optical system). Quality lens coatings will do this job for all
the wavelengths of visible light. On the other hand, some lens
coatings are put on lens elements to exclude certain wavelengths
for safety reasons (UV coatings) and to protect the optical surface
from scratches.
Coated lenses may be recognised by viewing with a white light
source behind you, and looking at the light source's reflection.
Twin white reflections means an un-coated lens. Two faint coloured
reflections indicates a fully coated lens. One coloured and one
white reflection means only one surface is coated. Beware
though. some manufacturers' claims of coated optics may in fact
mean that only the two exterior surfaces of an entire lens system
have been treated. Fully coated optics should mean that every
lens-air surface has been treated.
Exit pupil
The small circular beam of light that leaves the instrument
- hopefully to then pass into your eye. The maximum useful diameter
(regarded as about 7mm) is set by the capability of your eyeball's
own pupil. If your pupil cannot dilate enough to accept all of
the eyepiece's exit pupil, then some of the light is wasted --
reducing your effective aperture. The minimum useful exit pupil
is constrained by your eye's ability to detect and focus such
a narrow beam of light.
Interpupillary distance
Separation between the centres (pupils) of your eyes. This
is normally between 58 and 72mm in adults.
Dioptric adjustment
Used to compensate for slight differences between each of
the user's eyes it is measured in diopters. Dioptric adjustment
is made possible by making one of the eyepieces (usually the right
one) capable of minor re-focusing, independently of the main focusing
mechanism.
Specifications
Ph +61 (0)3 9813 5488 / Fax +61 (0)3 9813
1744
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