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A discussion about rifle scopes. Text file.
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A discussion about rifle scopes. Text file.
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Combat Arms
2869 Grove Way
Castro Valley, California 94546-6709
Telephone (415) 538-6544
BBS Phone: (415) 537-1777




Scopes for Dopes




I have compiled and written the following to help the reader learn
something about rifle and pistol scopes. Since rifle and pistol scopes are
the same for our purposes (and since I do not sell 3 pistol scopes a year),
this talk will remain limited to rifle scopes mounted on .223 and .308
weapons such as the Colt AR-15, HK-91, SSG and FN-LAR. The information is
also valid if you mount the scope on your typical hunting gun but I do not
sell that kind of weapon and know little about them.

First of all, let's discuss FIXED POWER scopes. If your shooting
conditions do not vary a great deal (meaning that you are generally
shooting at the same distance most of the time), you should choose a scope
with a fixed magnification. A high quality fixed power scope in 4 to 6
power will cover everything you need to do out to 500 yards without having
to fiddle with things and readjust the power setting on a variable power
scope. I recommend that you consider a 6 power scope with about a 42
millimeter objective and a 4 power scope with a 32mm objective.

VARIABLE POWER scopes are useful at low power in order to provide you
with a wide field of view for close distances or wooded areas. The higher
magnifications are used for longer distances in open areas for greater
detail. Variable power scopes range from a 1 to 4 power scope to 2 to 10
power scope. The problem with variable power scopes is with the user. If
you constantly shoot using the high power setting there you are wrong to
buy the more expensive variable power scope. If that's the case, a fixed
power scope is less expensive and is the right tool for the job.

Another consideration with variable power relates to the field of
view. Field of view is the measurement of the diameter of the field of
vision seen by the observer while looking through the optical device. This
measurement is determined at different distances from the instrument,
depending on whether the product is a riflescope, spotting scope or set of
binoculars. As a general rule, the higher the magnification of a particular
instrument and the smaller the diameter of the objective lens, the more
restricted the field of view will be. If the power gets too great, the
field of view will be too small. At higher powers, a slight movement of the
instrument results in a radical change in the viewing area.

The field of view may also be influenced by several practical, rather
than optical, considerations. Any time the distance from the eyepiece to
the eye (called "eye relief") is extended, there is a corresponding
narrowing of the field of view. This may be necessary in products such as
riflescopes in order to prevent the riflescope from contacting the eyebrow
during the recoil of the gun.

In pistol scopes or other extended eye relief applications, the field
of view will be further reduced. Riflescope field of view is determined at
100 yards (or meters if a European scope), while both spotting scopes and
binoculars have their fields of view measured at 1,000 yards (or meters).

Why do some scopes cost so much more than others? The secret is in the
optics. Higher quality scopes use the very best optical glass to make the
lens. The best raw glass blanks are then polished by skilled craftsmen into
lenses of very close tolerances. The very best glass lenses are made in
Europe.

The accuracy with which any optical instrument registers an image
depends on the quality of lens grinding and polishing. The primary silica
glass material must be free of all impurities and the lens surface finish
and curvature must be absolutely precise. The ability of the instrument to
distinguish fine detail will be in direct proportion to the care taken in
lens manufacture. Since lens quality is something that cannot be properly
judged by the human eye, one should not expect to purchase a quality optic
at a bargain basement price. Simply put, the more a lens manufacturer must
put into the production of its products, the greater must be the price.
Because optical lenses are highly polished, it is a fact of optical science
that approximately five percent of the light that enters or leaves that
lens will be reflected back upon itself. Should a particular optical device
contain 10 or 12 different lens elements (like a rifle scope), the total
reflected light could conceivably be as much as 50-60 percent.

To prevent this loss of image, lens manufacturers coat their lenses
with a special anti-reflective coating especially formulated for maximum
light transmission of over 90 percent. This coating is of a specific type
and thickness. The process involves molecular bombardment of the coating
material in a vacuum onto the lens surface. This coating greatly reduces
internal reflection and increases image contrast, which enhances the detail
that the eye can see.

It is impossible for 100% of the light entering the front of the scope
to reach your eye. A cheap scope will have a darker image than one with a
great refractive coating job done to the lenses. Also, the better scopes
will enable you to have a brighter image and to continue to see under
poorer lighting conditions, such as at dawn and dusk. One trick to help you
in shopping for a scope is to look at the objective (the front end of the
scope that gathers the light). The poorer the coating put on the lenses,
the more the lens will act as a mirror and reflect your image. The better
manufacturers use a magnesium fluoride coating on the lenses. Good scopes
have a poorer mirror image, thus transmitting more light and a sharper,
brighter image of the target to your eye.

This coating material is easily discerned by its color, which is often
seen as purple, straw or a faint green. The most common type of coating is
magnesium fluoride but more exotic multi-layer coatings provide enhanced
image transmission. The time, cost and quality control involved in the
coating process seriously affects the final price of the unit. Since it is
impossible to determine the coating's thickness or the type of coating used
without laboratory instruments, you must trust the manufacturer's
reputation. Bear in mind, that below a certain retail price line, the
quality of the lens grinding, polishing and coating are unknown quantities
and are generally inferior.

Color fidelity is important and is related to lens quality and coating
preciseness. Looking through a cheap scope, the colors will not appear
true. This is critical in a wooded or jungle environment which already has
reduced light.

You want a scope which will withstand the recoil of the weapon and has
its reticles ("crosshairs") centered. The scope should be dust proof and
moisture proof. There must be very precise click adjustments to the scope.
It is best if the scope has , 1/3, or minute of arc adjustments per
click. Thus, one click on a scope with minute of arc adjustment with
change the point of impact inch at 100 meters (109.36 yards). This fine
adjustment becomes important at longer distances. If your shooting
regularly includes distances greater than 500 meters, choose a scope with
minute of arc adjustments, unless the precision is unimportant.

Beware of the overuse of the term "twilight factor" in a scope's
description. It is arrived at by a simple formula which only takes
magnification and objective lens diameter into account. This is misleading
because it leaves out important information about the lens coating, the
quality of the scope tube, and the quality of the blackness inside the
tube. Optical data say nothing about the quality of the scope! Just as the
caliber and magazine capacity of a weapon tell you nothing about the
accuracy of the piece, optical data does not necessarily equate with
quality. If a salesman starts pitching numbers at you regarding optical
data, walk away in a hurry because he simply does not know what the hell
he's talking about but has merely memorized some data to impress you.

The way to correctly use the twilight factor is to understand that the
pupil of the human eye during daylight conditions will be open to
approximately 2 to 5 millimeters and will increase its opening to 7mm in
dark conditions to allow more light to enter the eye. A rifle scope will
"funnel" light into an exit area for the eye to view the target. This exit
area, referred to as the "exit pupil" by optical engineers, should
correspond to the pupil of the human eye in dark conditions. The exit pupil
of the scope can best be understood by holding the scope about two feet
away from your eye. The small disk of light that appears in the rear ocular
lens (the engineering term for the eyepiece) is the actual area your eye
looks into the scope, not the entire rear eyepiece - only that small pupil
area of light. The pupil of the human eye and the exit pupil discussed
above should correspond very closely. so that the full amount of light
concentrated at the exit pupil of the scope will enter the eye of the
shooter.

For example, a 6x42 scope (6 power magnification with a 42mm
objective) will "funnel" its 42mm of light gathered from its 42mm objective
six times (the power of the scope). The resulting final image is then
concentrated into the 7mm exit pupil. To find the size of the exit pupil
image in millimeters, simply divide the size of the objective in
millimeters by the power; in our example 42 6 = 7. This 7mm exit pupil is
equivalent to the maximum normal opening of the pupil of the human eye
(forget about the pupil diameter caused by opiates!). A 8x56 scope will
"funnel" more light because of its 56mm diameter and will therefore appear
brighter then the 6x42 scope. Extending this theory to its illogical
conclusion, a 10x70 scope would be even brighter still, but how the hell
could you mount a scope with a 70mm objective? Thus we can see that a 8x56
scope or a variable power scope with a 56mm objective set at 8 power is
about the brightest we can expect. Such a scope will enable you to see your
target under rather unfavorable dark conditions.


Now that you know all of that, we can define twilight factor as the
square root of the product of the magnification and the objective in
millimeters. For example, a fixed 6 power scope with a 42mm objective has a
twilight factor of:
_______ ___
x 42 = 52 = 15.9

Zeiss says the twilight factor can give you some measurement of the
visibility of details in twilight. The higher the twilight factor, the
better the visibility in poor light, with all other conditions the same.
But this says nothing about the quality of the glass, the lens coating, the
tube strength, etc. So, the better manufacturers suggest that you do not
rely on the twilight factor when comparing scopes, especially when
comparing different brand names.

Early optical instruments had bodies made of brass but this material
proved too heavy and tarnished easily. Modern optical devices use bodies
constructed of steel, aluminum or synthetics.

Should the scope tube be aluminum or steel? Steel offers strength and
resistance to thermal expansion but can rust and requires a higher level of
maintenance unless it is specially coated. Steel weighs more but it is
stronger. Many gunsmiths will tell you that there is no substitute for
steel.

Aluminum has widespread usage in the optics field because it is
lightweight, rustproof and resistant to wear. Aluminum expands more than
steel when heated. This must be compensated for by a more rigid mounting
system for the lenses so they will not more when subjected to variations in
temperature.

Synthetic materials offer lower weight, lower maintenance, high damage
resistance and total thermal stability. The use of synthetic is found more
in binoculars and spotting scopes. Expect to see riflescope tubes made from
graphite and other materials.

The modern scope mount clamping assembly places a high demand on the
stability of the sight. On the other hand, the better manufacturers have a
line of aluminum tubes that can withstand the pressures put upon them by
the clamps. There are various grades of steel and aluminum and the simple
truth is that the cheaper scopes use cheaper materials. Choose steel if
ruggedness, stability and absolute accuracy are the criteria. If weight is
the consideration, choose the alloy tube. The tube should be finished with
several layers of plating and a baked enamel outside finish. This protects
your investment in the scope. You don't want a finish that will wear and
expose bare metal to the air.

Who makes the better scopes? The Rolls Royce of scopes is made by Carl
Zeiss of West Germany. They are in first place. There is a tie for second
place, in my opinion, held by Swarovski, Schmidt & Bender, Hensoldt,
Kahles, Steiner, and Nikon. These are all European manufacturers. The tie
for third place goes to Leupold, who makes all of their scopes in America,
and to Nikon, who makes its scopes in Japan. The higher priced Leupold
scopes edge them towards second place. There is no sense in wasting your
time discussing the quality of Nikon except to say that they are new to the
rifle scope business. The Nikon scopes I have sold have been good value for
the money and are priced affordably. The Nikon 3-9X scope has a 40 mm
objective, minute of arc adjustment for windage and elevation, weighs
only 16.06 ounces (455 grams), is 12.32 inches in length and has a field of
view of 11 to 34 feet at 100 yards (as a function of the power setting).
These values for the Nikon are included because they are typical of those
found in the better scopes. In my considered opinion, Redfield is in fourth
place and Bushnell and Tasco are in fifth. Somewhere farther down the line
is Simmons. If you are putting a scope on the AR-15, consider Tasco's
armored scope. Believe it or not, it is perfect for the job and is one of
Tasco's finest scopes. It is also Colt's choice for the Delta HBAR and
retails for $129.95 plus the $59.95 mount, thus saving you considerable
money. I am amazed at Tasco's quality in this one particular scope (called
their World Class series). Too bad it does not extend across their entire
line.

My suggestions are to choose a fixed power, lower magnification scope
for use in the brush. The maximum power scope you can really quickly shoot
off hand is 6 power, so a 6x42 scope is an excellent choice. Some people
feel that magnifications higher than six power simply magnify the shooter's
errors and make the shooter reluctant to fire a quick shot without support.
The 8x56 style fixed power scope really should be used with support. The
variable power scope in the 3-9 power category generally has a 42mm
objective and there are quality variable power scopes with 56mm objectives.
The 3 to 9 power variable should be set to 6 power at dusk and during dark
shooting conditions. Remember that 42mm 6 = 7mm and 7mm is the magic
number for the maximum normal opening of the pupil of the human eye! Use
the 8 power setting if your variable power scope goes up to 12 power. The
lower power settings of the variable permit better offhand shots while the
higher powers make target identification easier. Select a scope with a 56mm
objective only if you intend to shoot under dusk or dim light conditions,
such as in a wooded area or under a jungle canopy and need such light
gathering capability.

A note regarding Zeiss scopes. All Zeiss scopes have a 30 year
warranty, multiple layer lens coatings. Diatal indicates a fixed power
scope while Diavari means a variable power scope. The Zeiss scope will
withstand 1,000 g's in recoil forces with very negligible change in the
point of impact. The scopes all have minute of arc adjustment for windage
and elevation. There is no change in the point of impact with changes in
magnification on variable power Zeiss scopes. All are fog free and sealed
for protection from weather. Scopes with a 1" tube have their calibration
in inch at 100 yards. Their 26 and 30mm scopes are calibrated for 1 cm at
100 meters.The 1" tube scope are more compact than the metric scopes.

It would hardly be fair to have explained all of this and say nothing
about binoculars, so let's turn our attention to them for a moment.
Binoculars are really two separate monoculars, hinged along a central axis,
about which the separate monocular halves can rotate to accommodate the
varying separation between individual eyes so that the binoculars will fit
any person.

Center focus binoculars feature a central focus knob that moves both
halves of the binocular simultaneously to adjust the focus of the
instrument. Usually, one eyepiece has an individual plus or minus
adjustment to make up for the slight differences in eye focus. Individual
focus knobs eliminate the central focus knob and each half of the binocular
is focused separately.

The major difference between binoculars is in their basic prism system
design. Traditional binocular configuration is the offset "z" shape of the
individual binocular halves. These are called "porro prism" designs. The
porro prism method of making binoculars gives the manufacturer a rather
wide tolerance in fabrication without sacrificing optical quality. This
means theat porro prism style binoculars generally cost less. The drawback
is that porro prism systems are heavier and physically bigger binoculars.

Another design method is called the "roof prism" style of binocular
construction. They eliminate much of the porro prism design's bulk. Because
both lenses of a roof prism binocular are in a straight line, the usual "z"
shapped binocular tube associated with the porro prism method is
eliminated. This results in a sleeker, straighter tube. Very precise
toleerances are necessary when building a roof prism instrument and this
results in higher costs. A negative aspect of the roof prism design is that
it results in an overall loss 10 to 12 percent of the light transmission
due to the nature of the prism system used. For the vast majority of
people, this loss is inconsequential and is more than compensated for by
the reduced weight and bulk of the more compact roof prism design.

Here are the prices of the Zeiss and Schmidt & Bender scopes. This
will give you some feel for what a truly good scope costs. If quality is
important to you then stick with one of the better European manufacturers.
Quality costs money. If you think you can get high quality at a low price,
your parents lied to you. It ain't possible!

ZEISS RIFLE SCOPES

Fixed power - 1" tube
5210909920 4 power with 32mm objective 600.00
5210919920 6 power with 32mm objective 650.00
5210929920 10 power with 36mm objective 770.00
Fixed power - 26mm or 30mm tube
5210819908 4 power with 32mm objective 600.00
5210839908 6 power with 42mm objective 700.00
5210859908 8 power with 56mm objective 900.00
Variable power - 1" tube
5210979920 3 to 9 power with 36mm objective 1050.00
5210069920 1.5 to 4 power with 18mm objective 900.00
Variable power - 26mm or 30mm tube
5210979920 1.5 to 6 power with 42mm objective 1000.00
5210069920 2.5 to 10 power with 52mm objective 1160.00

SCHMIDT & BENDER RIFLE SCOPES

Fixed power steel scopes:
001515 1 power with 15mm objective 690.00
043600 4 power with 36mm objective 730.00
064200 6 power with 42mm objective 770.00
085600 8 power with 56mm objective 870.00
124200 12 power with 42mm objective 890.00

Variable power alloy scopes:
014200 1 to 4 power with 20mm objective 930.00
016420 1 to 6 power with 42mm objective 1020.00
021056 2 to 10 power with 56mm objective 1230.00
01642S 1 to 6 power with 42mm objective - Sniper grade 1680.00
04025S 4 power Sniper with 25mm objective 1360.00

*** *** *** *** *** *** *** SPECIAL OFFERING *** *** *** *** *** *** *** **

Because the better line of scopes described above are really new to
the San Francisco Bay area, Combat Arms is offering a special 20% discount
to any BBS customer on any Zeiss or Schmidt & Bender scope if paid in
advance in cash. A discount of 15% applies if paid by credit card. This
offers expires December 31, 1989.



ACKNOWLEDGEMENTS


I am grateful to Zeiss and Schmidt & Bender for the material they
provided. Also thanks go to Durwood Hollis for his article called "The
Clear Picture on Optics" in the September/October, 1988 issue of Shooting
Sports Retailer magazine.


Richard M. Bash - Owner
Combat Arms
Castro Valley, California
January 14, 1989


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