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Riflescope Manual for Terms & Technology

Riflescopes are generally referred to as closed sights, whereas the rear sight and front sight are referred to as open sights. The mil-dot scope shows the target and reticle in focus and enlarges the image to visible. With open sights, the eye can’t focus on all three points simultaneously because the rear sight, front sight, and target are at different distances.

Riflescope Manual for Terms & Technology

Different visor types

Key figures on the lens

Riflescopes are named after the parameters magnification and lens diameter. The designation 4×32 means, for example, a 4x (4x) magnification and an objective diameter of 32mm.

The values 4-16×44 denote optics with variable magnification from 4x to 16x (4x-16x) and an objective diameter of 44mm. The magnification is indicated by the first number before the “x,” and the lens diameter is indicated by the second number after the “x.”

The reticle

The typical reticle is the marking in the mil dot scope, the crosshair, or the luminous point (generally the luminous marking ). There are different designs for different purposes; for example, target markings for hunting purposes differ from military or police precision shooters and sport shooters. There are also different versions within the various areas of application, e.g., for estimating distances or for use in twilight or night hunting.

The target mark is in the ocular (oculus = eye) or objective plane. If the crosshair lies in the objective plane with variable magnification, the threads and bars grow in the same ratio when the target object is enlarged. This can restrict the field of view and obscure parts of the target object. The reticle’s size does not change when it is in the eyepiece plane; only the size of the target object changes there.

To represent the reticle, two threads are drawn orthogonally to each other. To do this, a wire is inserted into the glass, or the markings are etched in. In this way, further markings can be made, which play an important role in assessing the distance (e.g., angular units).

To recognize the marking on the target object better in the twilight or on a bright moonlit night or to be able to adopt the reticle to the respective environment generally, there are telescopic sights with an illuminated reticle. The lighting is supplied either electrically or by natural light. Many UTG optics have, for example, a multicolor reticle with 36 colors so that the reticle can be adapted to any background.

The parallax error

To many shooters, parallax appears to be a mysterious phenomenon, but it is an everyday occurrence.

Here is a practical example to understand the effect:

  1. Cross both index fingers and aim the cross with one eye at a fixed object, for example, in the middle of the adjacent target or a light switch. If you now move your head, the cross will certainly move away from the target.
  2. If, on the other hand, you place your crossed fingers directly on the target opposite or on the light switch, the cross will always remain on the target when you move your head.

The deviation shown in 1 is called parallax error. This error always arises when trying to match two image planes. The following applies: The further the image planes are apart and the further the viewing angle deviates, the greater the parallax error.

Suppose you bring two image planes, the target, and the finger cross, physically onto one plane, as in 2. , the parallax error disappears. That brings us to parallax compensation.

The parallax compensation

You cannot physically place the crosshairs of riflescopes on the target object to avoid a parallax error. This is also not necessary as the lens creates an image of the target object inside the pipe. If the reticle is mounted in the lens image plane, the two image planes of the target object and crosshair lie together, and there is no parallax error.

In a riflescope with parallax compensation, the parallax is set to a certain distance by shifting the lens or reticle. Even if the view into the optics is shifted, the direction of the reticle does not change.

The twilight number

The natural visual acuity of a human eye is reduced to around a third of the daytime visual acuity at dusk. The degree of the ability of a mil-dot scope to recognize the target object in detail in twilight is expressed with the twilight figure. The higher the twilight factor, the more useful the glass is in the twilight.

The twilight factor, i.e., the calculated light intensity, stands for the theoretical image brightness. However, one cannot compare differently makes solely based on the twilight factor, since factors such as the construction of the lens groups, the glass used for the lenses, the quality of the lens coating, and especially the exit pupil also influence the light transmission.

To be able to perceive details over long distances in the twilight, a glass with high magnification and a large lens diameter is used. The theoretical value of the twilight factor can be calculated as follows:

  • Twilight factor = root (magnification x lens diameter)
  • Example: A riflescope with the values 10×40 has the twilight factor 20.0.

Mathematically, this means that a target object can be easily recognized at a distance of 200 meters.

The cheapest sighting distance GEE

The cheapest bullet distance GEE describes an intersection of the projectile and the line of sight at a physically defined distance. The function of a rifle scope is to give the shooter a straight line to aim. The projectile, in turn, flies on a trajectory similar to a parabolic trajectory according to its speed. The bullet’s trajectory and line of sight cross at two points that the shooter can choose from depending on the intended use.

The first intersection occurs when the bullet leaves the weapon and passes the line for aiming at its curve. The second point is called the most favorable shooting distance (GEE) and corresponds to the point at which the bullet passes the line of sight while descending. The riflescope’s reticle is usually set to the distance of the GEE or to a distance between the two intersection points. A classic shooting distance on the hunt is about 100m.

The field of view

The field of view is the space that can be seen at a certain distance, the viewing angle. In the case of telescopic sights, the calculation basis is usually 100 m away, and in the case of binoculars, it is usually 1000 m away. The size of the field of view is measured in meters or degrees (viewing angle). The simpler it is to monitor moving target objects and observe large objects with a wider view.

Due to the technical design of the optics, the field of view decreases with increasing magnification, i.e., with increasing focal length, and orientation becomes more difficult. If you want to compare the field of view of different riflescopes, the optics must have the same or almost identical magnification.

In general, the quality of a magnifying glass should only be determined by considering the field of view and the edge sharpness together. Inferior quality optics from the low price range are sometimes advertised with a large field of view, which is only made possible by a reduced edge sharpness.

Likewise, a provider can advertise with excellent edge sharpness, which was realized at the expense of the field of view. Hence, both features are in a direct relationship to one another. However, you will not find such products on this page.

The lens coating

High-quality riflescopes have a coated optics, a lens coating, also known as an anti-reflective coating. The glass surfaces are vaporized in several layers with fragile coatings, which have a reflection-reducing property to achieve this. By reducing the reflection, the loss of light at the gas-glass transition is significantly reduced. In this way, a light transmission of more than 90 percent can be achieved at the transition layer. However, these values are only achieved with high-end glasses.

The central tube

The outer diameter of the central tube determines which assembly, i.e., the connecting piece between the weapon and the telescopic sight, can be used. Since essential technical elements are built into the central tube, the diameter also determines how much space is available for the components and how many clicks, such as the parallax and the side and height adjustment, can be moved. The most common sizes are 1 “(25.4mm) and 30mm. Also, some tubes are prepared for a specific assembly and can only be used with this, such as for the STANAG standard or a Zeiss rail.

The fixed and correct assembly of the ZF is the prerequisite for a clean shot; if the assembly is not carried out carefully and professionally, the ZF slips or wobbles, the best preparation, and the most expensive equipment no longer bring you any advantage.

The exit pupil

The maximum dilatation of the eye’s pupil often reduces to 5-6 mm with advancing age. As a result, the light output of a 7 mm exit pupil can no longer be fully absorbed by the eye in the 8 x 56 glass case used in the example. Older hunters often prefer optics with a 6 mm exit pupil for hunting in the twilight or night, e.g., a glass with the values 10 x 60.

The highest possible twilight performance can only be used in the eye’s pupil, and the exit pupil has the same diameter. The fact that we humans differ in anatomy and that the maximum opening of the pupils can change with age leads to a different perception of the light intensity in the same glass.

In daylight, the human exit pupil usually has a diameter of 1.5-2 mm. If you now look through an eyepiece with a larger exit pupil (5 – 7 mm), you have more leeway before you get outside the diameter of the exit pupil.

For the perception of objects in the twilight, in addition to the twilight number, the size of the exit pupil on the eyepiece, i.e., the lens group into which the user looks, is an essential characteristic.

The greater the theoretical image brightness in the marksman’s eye, the greater the diameter of the entrance pupil must be.

The exit pupil is a purely mathematical value and therefore does not allow any conclusions to be drawn about the quality of the telescopic sight.

To determine the exit pupil, the quotient of magnification and objective diameter is calculated.

Exit pupil = lens diameter: magnification number

The values 8×56 result in:

Exit pupil = 56: 8 = 7 mm.

The value 7 mm roughly corresponds to the visual pit of a healthy human eye.


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