Scope Innards

How Today’s Riflescopes Work And Are Constructed.

To understand how scopes work, we need to understand a little about optics. Telescopes first appeared in the early 1600’s, more than 250 years after convex magnifying lenses were first used in eyeglasses to help older people read. The first telescopes only had two lenses, one at either end of a simple tube, an objective (front) and ocular (rear). Unfortunately, using two convex lenses resulted in an upside-down image. Substituting a concave lens for the ocular made the image appear right-side up, but reduced magnification. Eventually somebody placed a third convex lens between a convex objective and ocular, finding this not only resulted in an erect image but could produce even more magnification.

Other people thought of putting a telescope on a rifle but needed an aiming point and discovered a reticle could be placed in front or behind the middle “erector” lens. (Many of the first reticles were made of fine hair, the reason they’re still called crosshairs today, despite being made of metal or etched on glass.)

Originally all riflescopes were adjusted by moving their mounts, but eventually the mounts used screws to move the scope back and forth. Windage adjustments were made by turning the screws in and out, but the elevation adjustment often featured a spring on the bottom of the scope, to push it against a screw in the top of the ring.

Internal adjustments first appeared in the late 1800’s, but at first were usually used for elevation changes only, since it was cheaper and easier to put a pair of opposing screws in one mount for windage adjustments. The earliest internal adjustments pushed the reticle back and forth, the most practical method in the longer, thinner scopes then in use. (The length was due to relatively primitive optical glass, incapable of bending light sufficiently to place lenses closer together. Thin tubes were necessary to keep weight at a reasonable limit.)

As optics improved, scopes shrank in length and grew in diameter, making it possible to place another tube inside the main tube to hold the erector lens and reticle. The most common arrangement is to attach the ocular end of the erector tube to the outer tube with a gimbal, the same basic pivot system used to hold a globe of the earth. This allows the erector tube to be pushed by screws mounted in the scope’s main tube, just as the entire scope used to be pushed back and forth inside its mounts. The tops of the adjustment turrets are actually the tops of the screws pushing against the erector tube, while a spring (or springs) presses the erector tube against the screws. It’s that simple.

Even the largest riflescopes are relatively small telescopes. Put an erector tube inside the main tube and things get crowded. Consequently the springs pushing the erector tube against the adjustment screws (naturally called “erector springs”) are pretty small.

Most scopes use a single flat spring. Since the adjustment turrets sit on the top and right side of the scope, the spring is placed at the “7:30” position on the lower left side. In some scopes the spring’s held in place by a tiny angle at one end, fitting into a slot in the erector tube. In other scopes a screw holds the spring and some scopes use dual flat springs.

Unfortunately, flat springs can grow weaker with use and in some scopes the spring is not strong enough in the first place. This is why many shooters tap the top of the turrets to “settle” the adjustments. In reality, whacking the scope with the heel of a hand or bumping the recoil pad on the ground often works better, but tapping the turrets apparently seems more technical, even though it doesn’t always work. Another technique is to turn the turret a few clicks past the correct spot, then turn it back again, but with weak springs this still doesn’t work all the time.

When the dial’s turned to the right there shouldn’t be any reason to tap the turret, because the adjustment screw is directly pushing the erector tube. (Or at least it is in American-style scopes. The adjustments in some European scopes have left-hand threads.)


The adjustment screws of this rusty old Weaver are the
same basic adjustment system used in scopes today.


This century-old Lyman scope has adjustments in the mount. The spring
connected to the mount base pushes the scope against the elevation screw.


This is the camming arrangement in an older Weaver 3-9X variable.
In many modern scopes the lens-cam slots are inside the main tube.

Coil Springs

In some Burris scopes the interior flat spring is replaced by a coil spring inside a small housing on the side of the main tube. A couple of other companies place erector springs at the eyepiece end of the erector tube. Swarovski’s version has four coil springs in a housing, while the Simmons TrueZero system uses a fitting that’s a spring in itself.

All three alternatives leave more room inside the main tube than the traditional flat spring. In some scopes both Burris and Swarovski have used the extra room for larger interior lenses, claiming more light transmission, but that’s not necessarily true, since the amount of light entering a scope is primarily controlled by the objective lens. (Extra light transmission was also claimed for 30mm-tubed European scopes when they first started showing up in America. At the time Euro scopes tended to have better optics anyway, so many people believed it, but the amount of light passing through 30mm scopes is also primarily controlled by the objective lens.)

The real advantage in extra room around the erector tube is greater adjustment range, helpful when clicking the elevation up on longer shots. Using larger interior lenses limits adjustment range, and doesn’t really make scopes brighter.

Variable scopes work by moving erector lenses back and forth inside the erector tube, with the distance between the lenses controlling magnification. Each lens fits inside a cylindrical housing, with a stud on the outside of the housing. The exterior tube has two angled slots, and the lens-housing studs fit inside these slots. When we twist the magnification ring we’re turning the exterior tube and the slots cam the erector lenses back and forth, changing magnification.

The exterior tube, however, usually takes up more room inside the scope, the reason variable scopes normally don’t have as much adjustment range as fixed scopes. The extra moving parts also make variables less reliable than fixed-power scopes, part of the reason today’s “tactical” variables are heavier than conventional hunting scopes: Their innards are beefed up to prevent break-downs.

Two other problems affect variable scopes, both involving reticle placement. Remember, the reticle can be either in front or behind the erector lenses. In either position the reticle must be where the light rays focus, or it looks blurry.

As light rays pass through a convex lens they form a cone. The point of the cone is where they focus, the reason a magnifying glass concentrates sunlight into a tiny point of light and heat. The cone also creates the “exit pupil” as light leaves a scope, and our eye needs to be at the cone’s focal point to see the entire field of view. (Luckily, we don’t normally aim a rifle at the sun, so our eye doesn’t fry.)

The same thing happens inside a riflescope. The objective lens’s cone of focus is about halfway to the erector lenses, and the focus of the erectors lenses about halfway to the objective. A reticle placed in the objective lens’s focus is called a “first focal-plane” reticle, and when placed in the erector’s focus is called a “second focal-plane” reticle. These terms are often contracted to FFP and SFP.


Unscrewing the objective bell allows us to see the erector spring inside a
50-year-old 2-1/2X scope made by Light Optical in Japan (the same company th
at makes Nightforce scopes today). This particular scope has a first focal-plane
reticle, also visible in the photo.


First focal-plane variables are often preferred in Europe, because
they grow along with the image when the magnification’s turned up.
This FFP reticle was photographed in the main Zeiss factory.

In internally-adjusted scopes, the reticle’s mounted at either end of the erector tube. In fixed magnification scopes the placement doesn’t matter, but with variable scopes it does.

A first-plane reticle doesn’t change position or size on a target when magnification changes, because the light moving through the scope has passed the reticle before entering the erector system. The advantage of zero reticle movement is obvious, but the reticle not changing size can be either an advantage or disadvantage.

Many European hunters like FFP reticles because they grow along with the image when the magnification is turned up. In many European countries it’s legal to hunt far beyond the usual twilight hours of North America, sometimes all night, but artificial light and illuminated reticle are usually illegal. Since cranking up a variable also makes animals more visible in dim light, FFP reticles are considered an advantage.

Many North Americans, however, consider FFP reticles too coarse at higher magnifications, since we crank up variables when shooting at longer ranges, not for shooting in moonlight. We prefer SFP reticles because they shrink, providing a more precise aiming point on long shots.

The primary trouble with SFP reticles is the slight mechanical error in the alignment of the erector lenses as they slide back and forth. Since light passes through the erector tube before reaching the reticle, the reticle can apparently shift position on the target. This shift can be demonstrated either by shooting or using a collimator (bore-sighter), and was such a problem in early variables almost all used FFP reticles for many years.


The outsides of scopes are nice-looking, but what goes on inside is more
interesting (above). Erector lenses must be very precise, due to their
relatively small size (below). These are for Zeiss variables.


As a result, some American scope companies came up with tapered reticles, with a fine center that still allowed reasonably precise aiming with the magnification cranked up. Weaver used a reticle with three closely spaced crosshairs, both vertically and horizontally. At low magnification you aimed with the “cluster” of crosshairs, at high magnification with the intersection of the center crosshairs.

As a result, variables didn’t really become popular in the USA until the 1960’s, when manufacturers figured out how to make erector assemblies precise enough to minimize point-of-impact shift with SFP reticles. They didn’t totally succeed, but came close enough for scopes to be sighted-in at maximum magnification, yet still shoot close to point of aim when turned to lower magnifications for close-range shooting.

Today, significant reticle shift is rare even in inexpensive variables, and so tiny in high-quality variables it basically doesn’t exist. However, some shooters are returning to FFP scopes because of “ballistic” reticles with multiple aiming points. In FFP scopes the spacing of the aiming points doesn’t change at different magnifications, and in SFP scopes it does.

All scopes feature some means of refining the focus, which also reduces parallax, the apparent reticle shift on the target when the shooter’s head moves back and forth. Parallax is caused by the reticle not being exactly in the focal plane. Unless we always place our eye behind the centerline of the scope (few shooters do) any scope shows some parallax at some ranges.

The reticles of all scopes are set at the factory to be free of parallax at a certain range. Scopes of 10X or less have so little parallax, focusing the eyepiece reduces or eliminates the problem. Scopes above 10X normally require an extra means of focusing.

For many years the solution was an adjustable objective bell, essentially the same arrangement as the eyepiece, with the bell mounted on threads so the objective lens could be turned in and out. This is mechanically simple and relatively inexpensive, but grasping the front end of a scope is clumsy, especially when looking through the scope to observe the results. Many scopes now feature a side-focus knob opposite the windage turret. These shift a lens inside the scope so are more expensive, and sometimes touchier to use, but more convenient.


Adjustable mounts remained in use even after World War II. This Bausch & Lomb
mount (above) used opposing cones to make windage and elevations adjustments.
The scope is a Balfor 4X. This disassembled scope (below) is a basic fixed-power,
with objective and ocular lenses, and a single erector lens and erector spring.


Up, Down, Left, Right

Windage and elevation adjustments have become much more reliable in recent years, thanks to the emphasis on long-range shooting, a trend beginning with affordable laser rangefinders. The earliest internally-adjustable scopes had simple marks on their adjustments, not the “clicks” we’re used to today, but clicks started becoming common after World War II.

Many early click turrets featured a spring-steel post pressing against the serrated edge of the dial. Most of today’s scopes use a similar system hidden inside the turret, or a tiny spring-loaded ball sliding into detents. It doesn’t really matter, as long as the system is precise and doesn’t wear easily.

Firmer clicks don’t necessarily mean more reliable adjustments—the erector spring also plays a key role—but mushy clicks sure don’t help. The first Japanese-made Bausch & Lombs had perhaps the finest optics of any “American” scopes offered at the time, but they also had weak erector springs and mushy clicks. Several people I know (including me) gave up on them until the problem was fixed a few years later.

Many people assume optical quality is the most important aspect of a riflescope, and any scope with good optics is also fine mechanically. Neither is true, and the finest glass in the world is worthless if we can’t adjust our scope to work correctly.

Back when scopes could be easily taken apart it was possible to actually see some of this stuff, but 50 or 60 years ago the innovation of inert-gas removal of humid air from the inside of scopes made it mandatory to prevent curious owners from dismantling their Weaver. Old scopes, however, can be purchased for a few bucks, especially if they’re not totally functional, and several of the photos were taken of old scope innards. If you’re really curious it’s an inexpensive way to further understand how scopes work.
By John Barsness

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