Beginning Optics Advanced Optics Terms
In-depth, technical instruction on the design, function, and features of binoculars and spotting scopes
Advanced Optical Design
Binocular and spotting scope optical design is comprised
essentially of three components:
1. Objective lenses
2. Prisms
3. Ocular lenses
1. Objective Lenses
The objective lens has one job, to gather light and transmit
it to the user's eyes. In order to gather more light, an
objective lens must be made larger. Transmitting more
light (which is just a fancy way of saying "delivering"
more of the light to the user's eye) can be achieved by
using a higher density optical glass that is carefully cut
and polished (and also by applying anti-reflective coatings,
which are discussed later).
With the relatively small objective lenses in binoculars,
high-quality images can be obtained using standard optical
glass. However, the larger objective lens of a spotting scope
causes many manufacturers to make use of an exotic glass
type (and market the glass with names like "ED" and
"HD" among others) to achieve the highest possible image
resolution, contrast, and color fidelity.
These remarkable glass types reduce or eliminate the
inherent problem of chromatic aberrations. Chromatic
aberrations are the result of a physical reality of color;
different colors move at slightly different wavelengths,
which means they will have slightly different focal lengths
when they pass through optical glass. Chromatic
aberrations diminish the resolution and color fidelity of
normal binoculars and spotting scopes. They show up as
greenish and/or purplish ghost images, and are especially
apparent under low light conditions.
The prisms in a binocular or spotting scope have two jobs:
- They revert the image, which would otherwise be
presented to the user upside-down and backwards.
- They shorten binocular length (light bounces in the
prisms; its path is shorter through the whole of the
binocular than it would be without them).
Porro prism or roof prism?
Porro prisms have rich depth and wide field of view and
usually employ large prisms. However, Porros aren't very
robust and many people feel they handle poorly. Porros are
generally heavier than roof prisms due to the prism size
and the use of larger prism housings (the housings are
larger due to the bigger prisms, but also from the optical
design of the binoculars).
Roof prisms can achieve image quality similar to porro
prisms, but it costs more (and involves phase correction
coatings, discussed later). Also, most roof prisms generally
can't replicate the 3-D feel of good quality Porros.
However, roof prisms are generally more popular because
of their ruggedness and superior handling (more
streamlined and lighter weight).
What determines prism quality?
The density of prism glass is important in determining its
ability to deliver high-quality images. Also important is the
care taken in grinding and polishing the prisms. There are
two kinds of glass used to make Porro prisms; boro-silicate
(more commonly known as BK-7) and barium-crown
(known as BaK-4) glass. BK-7 glass is of inferior quality
to BaK-4 glass, and is commonly used in the less
expensive binoculars.
The type of glass used in roof prisms is of less consequence
than with Porro prisms. Roof prisms involve more
complicated engineering and stricter tolerances on their
design - these factors have a greater determination on
quality. Many manufacturers use BaK-4 glass in their
roof prisms, but some manufacturers don't release any
information on the prism glass type used - it's a trade secret.
Determining prism glass type
BK-7 glass prisms transmit a distinctive light beam
shape, seen when you see the exit pupil of a binocular
or scope at a distance of about 8 inches from your eye.
BK-7 prisms lose some light as it passes through
the prisms.
BaK-4 prisms (or other more exotic prism types)
transmit more of the light through the prisms, and
hence feature exit pupils with a clearly defined circle.
Phase correction?
Many modern roof prism binoculars advertise
"phase-corrected roof prisms", but what are those? This
special kind of coating corrects for an inherent flaw in all
roof prism designs.
After light passes through the objective lens, it is reflected
off the mirrored surfaces of the roof prism and split into
two out-of-phase beams of light. Light reflected from one
roof surface is 1/2 of a wavelength different from the light
hitting the other roof surface. This is sometimes referred to
as "out of phase" or "phase shift". Although the light waves
are subsequently forced back together when they reach the
viewer's eye, there is a slight reduction in image resolution
and contrast.
The phase correction coating, which is applied to the
mirrored surfaces of the prism, forces the light beams back
into phase, thus improving a roof prism's resolution and
contrast. The coating also enhances color fidelity.
3. Ocular lenses
The ocular lenses magnify the images that the objective
lenses have transmitted. Ocular lens design incorporates
between three-to-six different lenses, but overall quality is
determined mostly by the care in manufacturing and
polishing of the glass and also the quality and quantity of
anti-reflective coatings employed.
Many of the important optical specifications (such as field
of view, eye relief, etc.) are determined primarily by the
design of the ocular lenses.
Advanced image quality elements
Exit pupil
The shaft of light
that meets your eye
when you use a
binocular or scope is
its exit pupil.
The exit pupil is
seen by holding the
binocular or spotting
scope a short distance
from your face.
The exit pupil should appear as a clear circle surrounded by
a uniformly dark background. Exit pupil is calculated by
dividing the objective lens by the magnification and is
measured in millimeters. An 8x42 binocular will have an
exit pupil of 5.25mm.
Why does exit pupil matter? The human eye pupil (which
is controlled by the muscles of the iris) can change in size
from roughly 2-8mm in diameter, depending on the
lighting situations. The eye pupil dilates to about 2-3mm
in regular lighting conditions, dilates out to about 4-5mm
in lower light situations, and further dilates to 7-8mm in
near-dark conditions. A binocular will appear brightest
when its exit pupil is equal to or larger than your eye
pupils. This is most important when viewing in
low-light conditions.
Optical coatings
When you look at the lenses of a binocular or spotting
scope, you'll notice tints in the glass that are usually
purplish/greenish in color. What you are seeing are the
anti-reflective coatings that have been put on the lenses.
These coatings serve to reduce light reflection and
scattering at the air-to-glass surface. When light strikes
uncoated glass, a percentage of it (4-5%) is reflected back
from the surface, and with 10-16 air-to-glass surfaces in a
pair of standard binoculars or a spotting scope, almost 50%
of the light passing through uncoated optics would be lost!
By applying just one layer of anti-reflection coating, loss
due to reflection can be reduced to 2-3%, and by applying
multiple layers of coatings, light loss can be reduced to a
mere .5% per surface!
Optical coatings are made from certain metallic compounds
(most often involving the compound Magnesium Fluoride,
but many manufacturers have proprietary coatings whose
ingredients are trade secrets) that are vaporized and
applied to the optical glass in very thin layers (measured in
microns; millionths of a meter) inside a vacuum chamber.
The quality and quantity of optical coatings matters a
great deal in determining how bright and sharp a
binocular or spotting scope will be.
There are some standardized terms concerning the
level of coatings applied to binoculars and scopes.
With optical coatings, more is better! With more coatings
comes increased resolution, contrast, color fidelity, and
increased light transmission.
fully coated optics: all air-to-glass surfaces are coated
with an anti-reflective coating film. Many modestlypriced
binoculars offer fully coated optics and have good
but not great image quality.
multi-coated optics: one or more surfaces are coated
with multiple anti-reflective coating films. Image quality
with multi-coated optics can be quite good, except
perhaps in lower light settings.
fully multi-coated optics: all air-to-glass surfaces are
coated with multiple anti-reflective coating films. Fully
multi-coated optics offer the highest image quality.
Important optical terms
Resolution: The ability of a binocular/spotting scope to
separate and distinguish thin lines with clarity. Resolution
is essentially the same as image sharpness.
Resolution test: A chart on paper
containing a series of sets of lines
at progressively smaller spacing
and used to ascertain the limiting
number of lines per millimeter
that a binocular or spotting scope
is capable of resolving clearly.
Contrast: The ability to distinguish differences in brightness
between light and dark areas of an image. Because we see
much of the color spectrum, contrast also refers to the ability
to distinguish differences in dimensions of hue, saturation,
and brightness or lightness. Optics with superior contrast
transmit colors that appear very dense and saturated.
Transmission: The percentage of light that passes through
the binocular or spotting scope and reaches the user's eyes.
With modestly priced optics, transmission generally ranges
from 85 - 90%. More expensive optics can achieve
transmission of 95% or more
Chromatic aberrations: Because different colors move at
slightly different wavelengths, they will come to focus at
slightly different lengths when they pass through optical glass.
The resulting false colorations (seen most often as purplish
and greenish ghost images) diminishes resolution and color
fidelity. Chromatic aberrations will be negligible with
binoculars and scopes that use better optical coatings and/or
higher quality glass.
Astigmatism: The lenses used in a binocular or spotting
scope usually have a curved shape, and thus all light rays
passing through will not converge on the same focal plane.
If this physical reality isn't remedied in the overall optical
design, a binocular or spotting scope will provide images
where either the center image or the edge image is in focus,
but not both (without refocusing).
Astigmatism cannot be
eliminated completely, but it
can be kept to a minimum.
Users will want to avoid
binoculars or spotting scopes
that exhibit too much astigmatism,
as it cuts into the image quality.
Distortion: The disability of a binocular or spotting
scope to deliver an image that is a true-to-scale
reproduction of an object. There are principally two types
of distortion to be concerned with; barrel distortion (where
images bow outward and look bulged), and pincushion
distortion (where images bend inward). In both cases, the
distortion is due to a poor or compromised optical design
and any binocular or scope that exhibits distortion should
be passed up.
Alignment and collimation: In a binocular or spotting
scope, the optical components must, for the best
performance, be situated as they were initially designed.
Poor manufacturing and/or poor handling of the
equipment can cause any or all of the components to
become misaligned, resulting in diminished performance.
In a binocular, the optical components (primarily the
prisms) in both barrels must be pointing in the exact same
direction, known as collimation. Viewing through
binoculars that aren't perfectly collimated (whether they
became miscollimated through poor construction or
mishandling) can cause great eye strain and fatigue.
Porro prism binoculars are much more susceptible to
collimation issues than roof prisms.
You can test a binocular for collimation by looking through
them at a horizontal line (a door frame at about 15 - 20 feet
works very well) and then slowly and carefully pulling the
binoculars away from your face so that you can start to see
where the two exit pupils intersect. The horizontal lines in
each exit pupil should match up correctly. If they do not,
they are out of collimation and need repair.
How to use binoculars properly
Adjusting for your interpupillary distance.
In order to see one singular image, you must adjust the two
barrels of the binocular to your interpupillary distance (the
distance from left pupil to right). To do this, turn the
barrels of the binocular on their central hinge until you see
one singular field of view with
no disturbing shadows.
If you wear eyeglasses / sunglasses, rest the eyecups of the
binocular/scope right up against your glasses. Since your
glasses sit away from your eye, they generally provide the
proper distance for seeing the full field of view comfortably.
The rubber eyecup must be folded down to be able to rest
on your glasses. The retractable eyecup will stay flush with
the eyepiece to accommodate glasses. If the rubber eyecups
aren't folded down or the retractable eyecups are twisted
out, it will appear as if you are looking through a tunnel.
If you don't wear eyeglasses / sunglasses, you will rely on
the eyecups to provide the proper distance for seeing the
full field of view. The rubber eyecup stays "as is" and fits
right around your eyes, while the retractable eyecup must
be twisted- or pulled-out fully in order to see the full field
of view. If the rubber eyecup is folded down or if the
retractable eyecups aren't retracted out, you will note
disturbing black "crescents" in your field of view.
Many retractable eyecups offer multiple positions or
"stops". With these eyecups, experiment to see which
position is most comfortable for you.
Using the diopter / Focusing your binoculars.
Most binoculars feature a center focus wheel and also an
adjustment ring (usually on the right eyepiece) known as
the diopter. The diopter adjusts for differences between
your individual eyes (many people have one eye that is
"stronger" than the other). The procedure for attaining
proper focal balance using the center focus wheel and the
diopter is:
1. With your right eye closed (or with your hand over the
right objective lens), focus your left eye on an object
approximately 20 yards away with the center focus
wheel until it is in sharp focus.
2. Now adjust for your right eye. To do this, close your
left eye (or, again, place your hand over the left
objective lens) and focus your right eye on that same
object 20 yards away. Focus using the diopter until
the object is sharply focused.
3. The binoculars are now set for your eyes. Make a note
of your diopter setting if you are sharing your binoculars
with someone else.
What to disregard and why
There are a number of specifications and product
"features" that are often confusing or misleading which
should just be ignored or avoided.
Specifications to ignore
Twilight factor: This specification gives a measure of
viewing efficiency in low lighting. The bigger the number,
the more efficient (sharper) the binocular is in low light.
Twilight factor is calculated by taking the square root of
the power times the objective lens diameter, so the value is
usually between 12 and 25. Twilight factor is a dubious
specification because it says nothing about actual optical
coatings or glass quality, nor does it take into account the
light transmission of the binocular.
Relative brightness: This specification is a measure of
overall image brightness, and is calculated by squaring the
exit pupil of the binocular. Relative brightness is misleading
in that there are different binoculars that can have the same
values. A 7x35 binocular and a 10x50 binocular will both
have a relative brightness value of 25, but the 10x50s gather
much more light than the 7x35, and will often present a
brighter image. It also doesn't take into account different
light transmissions.
"Features" to avoid
Focus-free binoculars: Binoculars that offer an "instantfocus",
"permanent-focus" or "focus-free" feature are
advertised in many department stores. The main reason to
avoid this type of binocular is that optical quality is very
poor as a result of its focus-free design. Focus-free binoculars
are also often difficult to use if you wear eyeglasses.
Ruby coated lenses: Often advertised in sporting goods
stores and department stores, these are seen as bright
reddish-orange coatings on the objective lenses of several
binocular models. Ruby-coated lenses reflect most red out
of the optical system. This skews all colors to the cool end
of the spectrum and takes away from the overall brightness
of the binocular.
So why use these binoculars? Shortening the color spectrum
increases contrast and resolution somewhat. However, better
glass and better coatings are capable of excellent contrast
and resolution without giving up color and brightness.
Trade-offs
Unfortunately, there is no such thing as the perfect
binocular or spotting scope. In the design phase there are
many inherent trade-offs that manufacturers, and hence
consumers, have to make.
The main trade off that you have to make involves
objective lens size. The larger the objective lenses, the
brighter and sharper the binocular/scope, thus the more
useful it is, especially under low-light conditions. The
larger the objective lens, however, the heavier and bulkier
the binocular/scope will be, and unless you plan to hire a
porter, remember that you have to carry it!
Higher quality optical glass is by design heavier, so when
it is employed the binocular/scope will weigh more.
Better manufacturers will use lighter housing materials to
offset the weight of the glass components rather than not
using the heavier glass.
There are a number of trade-offs with binoculars and
scopes regarding higher magnification. With higher
magnification binoculars comes a diminished field of
view, a shallower depth of field, and more chance of
image shakiness. With higher magnification spotting
scopes comes diminished optical quality with magnified
heat waves and atmospheric dust & debris.
There are trade-offs inherent to some of the optical
specifications as well. There is an inverse relationship
between eye relief & field of view and close focus & depth
of field. The greater the eye relief, the narrower the field
of view (the wider the field of view, the shorter the eye
relief). Similarly, a binocular with an extreme close focus
distance will generally have a shallow depth of field.
More expensive scope eyepiece designs can offer good
compromises and give the user very good (but not great)
specifications.
The final word
For many people, this guide may only be the beginning
of the journey into learning about binoculars and spotting
scopes. There is a lot more information available, you
need only to look for it. Should you have any questions or
comments about this guide, please don't hesitate to contact us.
