Schmidt & Bender 4-16x56 Polar T96

Variable magnification

Optical products with variable magnification are more versatile as they're designed with a wider range of magnification and larger viewing angle. With their high number of lenses in housing, optics with variable magnification are larger, heavier and have lower permeability of light. Variable magnification is rarely present in binoculars, but it's mostly used in riflescopes and spotting scopes. Advantages of the variable magnification regarding usability with riflescopes and spotting scopes outweigh disadvantages – in short one fits all. High quality variable binoculars are very rare.

Lens diameter

Lens diameter represents the second value in product’s name/designation. For example, 10x42 optics have 42 mm diameter of the lens at the front (those that are closer to the viewing object). It is known that the bigger the lens, the more light goes through and the image we see is brighter. All of this, however, depends on the magnification and quality of a certain optical product. Although the bigger lens diameter in binoculars is better, the size adds up on the weight, making it more heavy and difficult to handle. With riflescopes, bigger size of a lens diameter also means more problems with mounting.

The most common lens diameters are 24 mm, 42 mm, 50 mm and 56 mm.

Source: ZEISS

­Max. exit pupil diameter

Minimal eye relief

Minimal eye relief is a minimum distance between the eye and eye-piece lens (rear of the scope, some manufactures call the eye piece ocular). For safety reasons, due to the recoil of rifle, minimum eye relief for rifle scopes has to be at least 80mm. Larger rifle calibers produce higher recoil, so it is adviced to have a minimal eye relief of 90mm or even more. Due to safety reasons new rifle scopes have longer eyerelief than older. Longer eye-reliefs were achieved with an advance of optical tehnology.  

Because of safety reason some rifle scopes for safari rifles have extended eye-relief of more than 120mm. Scout and pistol scopes more than 500mm eye-relief, since they are usually mounted far away from the user.

Safety should be always on 1. place when handling guns and shooters often ignore, so make sure that you don`t get a »black eye« injury and choose the scope with a minimal eye relief of at least 90mm. 

Source: Revija Lovec

Maximal Field of View

Maximal Field of view is the range of your visual area, for rifle scopes its measured at 100m. For example if maximal field of view is 13m/100m, means that you have on 100m a maximal view of 13m. Rifle scopes with Variable magnification (click for the info in specifications), maximal field of view is meant when using lowest magnification setting. The higher the magnification the smaller the field of view.

Field of view is important feature when choosing a rifle scope for you activity. Larger field of view will give you wider view/image, which is important factor when you are aiming or hunting moving targets. Opposite, smaller field of view gives you narrower view/image, this option is more suitable for non moving/static targets. Better quality rifle scopes have larger maximal field of view.

Just for information, parameter of lens diameter doesn`t give you a info about field of view. Lens diameter has influence only on the brightness. For example, you can have 2 rifle scopes that have for example lens diameter of 56 mm, but they can have different maximal field of view.

Source: Revija Lovec

Twilight Factor

In the past, twilight factor was an important value in determining the brightness of the optics. The manufacturers were using the same kind of technology and materials of the lenses therefore the optics were comparable. Nowadays, they use different types of lenses and modern coatings so the twilight factor has lost its meaning, because the brightness of the optics depends more on the quality of the coatings than on the twilight factor.

Twilight factor is calculated by the square root of multiplying magnification and lens diameter.

Twilight factor of 8x42 binoculars is a square root of 336, meaning 18.33. All the binoculars with this kind of magnification and lens diameter have the same twilight factor, but not the same brightness. If you look through an old pair of binoculars made in 1950s and a new pair with the same magnification and lens diameter you could see the difference in brightness even though they share the same twilight factor. The new pair is significantly brighter due to better lens materials and coatings.

Though manufacturers still specify the twilight factor, we recommend you to ignore it as it’s not important.

Light transmission

Light transmission specifies an amount of light that is let through the build of optical product. Every crossing through each lens means a certain loss of light (0.1% with best coatings, up to 5% without coatings). Higher light transmission rate is very important when using optics at dawn or twilight. Good optics normally have light transmission up to 90%, whereas top-notch ones have 95% light let through.

Although the quantity of light reaching the eye depends on the size of an exit pupil, light transmission determines transparency of the lenses, whether the image is dark and cloudy or bright and clear.

Light transmission can be increased with applying different coatings on the glass surfaces. However, it depends on the coating type and number of layers. Multi-layered coatings mean higher light transmission.

Uncoated glass reflects about 4% of the light (top line), single coating reduces the reflectivity to approx. 1.5% (middle line), multi coating reduces the reflectivity to approx. 0.1 to 0.2 % (bottom line)
Source: ZEISS

Reticle illumination

Reticle illumination is the possibility to illuminate the rifle scope reticle with light. Some rifle scopes use reticle with illumination possibilities. There are two main types of reticle illumination on the market:

- illuminated point in the middle of the reticle,

- illumination option to light up the whole reticle.

For hunting, the reticle with central light point (dot) is more recommended. Rifle scopes that use illumination of the whole reticle are more recommended for tactical and sports use. The power of illumination light can be either day strong or visible only in the lowest light situations (in dark). For the driven hunts, IPSC or tactical CQB daytime strong illumination is more usable. Illumination option only visible in low light conditions is better for hunting in low light conditions or even in the dark. Rifle scopes that use as many different intensity levels as possible are better because they offer more possibilities to find a perfect view of the reticle in different lighting conditions.

The most advanced rifle scopes use optical fibers built in the middle of the reticle as a light dot. In high quality rifle scopes, these illuminated dots are of the smallest size. For example, if the illumination is set to OFF, you don`t see this illuminated dot at all and when you turn the illumination ON, the light dot is visible. High quality rifle scopes enable the user to fine tune illumination intensity. This way the reticle is always illuminated just right in accordance with the ambient light condition. Very dim in low light and extremely bright in the daytime. Newer rifle scopes have built in automatic turn off electronics that turns off illumination of the reticle when you don’t use your rifle scope for some time the illumination of reticle goes automatically to OFF. Such scopes have built in motion sensor to determine when the scope is not in use. This preserves battery life.

Reticle illumination option provides you a better accuracy at night and in low light situations. With non-illuminated reticles, you could have problems in low light situations, when you can see the target but not the reticle. The type of illumination for low light usage only is mostly meant for raised hide hunting.

Daytime bright illumination is the best option for driven hunting, IPSC and tactical shooting.

Source: Nikon

Elevation

Elevation is how much up and down you can adjust reticle. For example, if you see in rifle scope specifications elevation is 3.5 m, this means that you can adjust reticle maximal 1.75 m up and maximal 1.75 m down for hits on your target at 100 m. Elevation range is usually specified in MRAD (1 mrad is 10 cm / 100 m) or MOA (1 MOA is 2.9 cm / 100 m). Some manufacturers designate MRADs with an acronym MIL.

Practically all newer rifle scopes have the possibility to adjust reticle left or right (windage) and up or down (elevation). This process is known as zeroing. Upper turret on rifle scope is for elevation adjustment of reticle and side turret on rifle scope is for windage adjustment of reticle. Hunter rifle scopes has the mechanism of both turrets protected with caps which protect turret from water, damage or any other outside impacts. Turrets are easily said a rotatable buttons which you can spin in left or right way.

Every single movement made with the turret produces a »click« sound. Usually 1 click on European rifle scopes moves hit on target for 1 cm at 100 m range (0.1 MRAD / MIL). On American, Japanese and Chinese scopes 1 click moves the hit on the target for ¼ MOA (minute of angle) which is 7 mm at 100 m range.

For long range shooting elevation of at least 2.6 m / 100 m (26 MRAD or 89 MOA) is needed.

Source: Revija Lovec

Windage

Windage is how much right and left you can adjust reticle. For example, if you see in rifle scope specifications windage is 1.5 m / 100 m, this means that you can adjust reticle maximal 0.75 m right and maximal 0.75 m left for hits on your target at 100 m. Windage range is usually specified in MRAD (1 mrad is 10 cm / 100 m) or MOA (1 MOA is 2.9 cm / 100 m). Some manufacturers designate MRADs with an acronym MIL.

With windage adjustment we can compensate wind drift of the bullet from straight trajectory. Wind drift is caused by the effect that a side wind has on a bullet.

Practically all newer rifle scopes have the possibility to adjust reticle left or right (windage) and up or down (elevation). This process is known as zeroing. Upper turret on rifle scope is for elevation adjustment of reticle and side turret on rifle scope is for windage adjustment of reticle. Hunter rifle scopes has the mechanism of both turrets protected with caps which protect turret from water, damage or any other outside impacts. Turrets are easily said a rotatable buttons which you can spin in left or right way.

Every single movement made with the turret produces a »click« sound. Usually 1 click on European rifle scopes moves hit on target for 1 cm at 100 m range (0.1 MRAD / MIL). On American, Japanese and Chinese scopes 1 click moves the hit on the target for ¼ MOA (minute of angle) which is 7 mm at 100 m range.

It is recommended that rifle scope has windage of at least 1.5 m / 100 m (15 MRAD or 50 MOA).

Source: Revija Lovec

Turret Type

Turrets are easily said a rotatable buttons on rifle scopes which you can spin in left or right and move the reticle in the scope. This way you achieve a match of point of impact and scope reticle. This is called zeroing. Market offers different rifle scopes for different purposes and they have also different type of turrets. Determination, how will turrets look like depends on rifle scopes purpose, price, manufacturer and country of origin.

Every single movement made with the turret produces a »click« sound. Usually 1 click on European rifle scopes moves the hit on target for 1 cm at 100 m range (0.1 MRAD / MIL). On American, Japanese and Chinese scopes 1 click moves the hit on the target for ¼ MOA (minute of angle) which is 7 mm at 100 m range.

There are 4 different types of turrets on the market

Target turrets are known as the most precise type. They are the best choice for the precise shooters, due to very small and precise adjustments. Usually they are available in 1/4 or 1/8 MOA (1 MOA = 0.29 MRAD = 2,9 cm / 100 m). They are protected with caps and usually they are very high for easier manipulation.

 BDC stands for Bullet Drop Compensation. BDC turrets offer the easiest way to dial number of clicks to compensate for bullet drop when shooting at longer distances. These turrets are usually found on hunting scopes for long-range hunting. These turrets are very similar to Tactical turrets; however, the main difference is they don’t have numbers for clicks engraved on them, but only numbers for the corresponding distances. On such turrets, 1 stands for 100m, 2 for 200m, 3 for 300m, and similar. Color dots, instead of numbers, are also very common; however, in such cases, the hunter must memorize which color corresponds to a given distance to the target.

BDC turret can only match one ballistic trajectory, and BDC turrets are always suitable for use with only one ammunition. When a shooter changes the ammunition in his rifle, the BDC turret must be changed to correspond to a new ballistic curve.

The best modern BDC turrets also have additional function of a Zero Stop (the turret doesn’t rotate under zeroing distance of the rifle) and locking mechanism.

 Scopes meant for tactical applications have uncovered tactical turrets that offer easy control over reticle elevation and windage. Click values on such turrets are visible and number of clicks for bullet drop compensation can be dialed fast and easy. Such turrets are especially useful in tactical situations where, usually, every target is at a difference distance and not more than a couple of shots are shot at each target. Such situations demand continuously changing of reticle position in the scope elevation and windage range. Turrets covered with caps are unpractical in such situations.

Since the main purpose of tactical turrets is easy and often changing of elevation and windage position of the reticle, such turrets usually have numbers of clicks (or elevation in MOA or MRAD) clearly written on them. Resetting the turret to 0 when the rifle is zeroed is also a standard feature on such turrets. Tactical turrets are also distinguished from other types of rifle scope turrets, as the number of turns plays an important role in their usability.

 Tactical turrets on basic scopes, usually, have a high number of turns from one extreme of the elevation / windage range to another. Such turrets are called multi-turn turrets. However, tactical turrets on more advanced tactical scopes have a limited number of turns. The most common are single turn – ST and double turn – DT turrets. Double turn type turrets also have an indicator, which clearly shows in which turn the turret is located. Such advanced tactical turrets are especially useful when there is no room for error when dialing number of clicks for proper elevation adjustment.

Advanced ST and DT tactical turrets often have a Zero Stop – ZS functionality, so when a user wants go back to turret position at which rifle is zeroed, the turret rotation is stopped at that setting.

 Capped turrets are the most usual turret type on hunting scopes, where the turret covers are removed only on rare occasions, when the scope is zeroed. Due to the caps, the turrets are well-protected against water, dust, and unintentional damage. On most of the hunting scopes and on some wide-angle tactical scopes, the turrets under the caps have low height. On target scopes, this is different, since most turrets are high and easy to dial. In modern rifle scopes, turrets caps also hold the spare battery for the illumination.

 Turrets on the target scopes are also covered with caps; however, such scopes have much higher turrets than hunting scopes, which also have turrets covered with caps. The reason for such difference is that turrets on target/sport scopes are dialed often, when shooters shoot on different distances. However, during transportation or when shooting in the rain, high target turrets still get protected with caps.

Source: Revija Lovec

Elevation Per Turn

Elevation Per Turn is when you turn a rifle scope turret for 360 degrees to the same spot where you started. For example, when you see in specifications – elevation per turn is 10 MRAD or 100 cm / 100 m, this means when you make 1 turn with turret, the point of impact on the target will be moved for 100cm on 100m range.

If elevation per turn is 24 MOA, this means that 1 turn the point of impact will be moved for 24 x 2.9 cm / 100 meters.

This information is mostly important for sport and tactical rifle scope where you often change the elevation. It’s much easier to use when the values for a turn are rounded (for example 20 MOA), but on tactical rifle scopes this mostly isn’t possible, because of big elevations, so the turrets had to be Multi-Turn (MT), what would even more complicate the use.

Source: Revija Lovec

Nr. Turret Turns

 Specification Nr. Turret Turns tells you how much turns (maximum) you can make with turrets on a rifle scope. So when you start to turn a turret until it stops.

ST = Single Turn (means that you can make maximum 1 turn on turrets)

Single turn turrets are mostly older tactical turrets and BDC turrets.

DT = Double Turn (means that you can make maximum 2 turns on turrets)

Double turn turrets are mostly tactical turrets. Usually, they have mechanical turn indicators that show you in which turn you are. This type of turret is used on the most advanced tactical rifle scopes.

MT = Multi Turn (means that you can make 3 or more turn on turrets)

Multi turn turrets are mostly target and hunting/capped turrets. Some of them have lines for turn indication, but even with lines, it’s hard to know in which turn you are.

Source: Revija Lovec

Day time usable illumination

Daytime illumination of the rifle scope reticle serves a different purpose than twilight illumination, and in such rifle scopes, high intensity levels are a necessity. The illumination of the reticle is meant for rapid target acquisition, since a bright red dot is the best possible aiming point. Shooters’ eyes are instinctively drawn to a bright red dot in the center of the field of view. Such strong illumination is feasible only in rifle scopes with magnification lower than 1.5x or ideally 1.0x. Such wide-angle rifle scopes with a real 1.0x magnification and daytime bright illumination of the reticle can even be used with both eyes open, similar to reflex/red dot sights.

Rifle scopes with daytime illumination are the best choice for IPSC, CQB, tactical shooting and driven hunting in bright daylight.

Source: Zeiss

Auto –TurnOff

Some of the red dots and rifle scopes with reticle illumination have Auto – TurnOff function. This means, in case that you forget to turn off illumination or when you leave your firearms on table (you don’t do any movements with rifle scope), illumination automatically turns off. The time when illumination turns off is different from rifle scopes to rifles scopes.

Many of the expensive and high quality rifle scopes with illuminated reticle have an Auto-TurnOff smart motion sensor that detects different angles and automatically turns OFF or even ON the illumination of the reticle, based on the rifle scope position.

This is a nice feature to have, because this increases the battery life. 

Source: Swarovski

Parallax setting

Parallax can be adjustable or fixed and parallax setting tells you if parallax is fixed, on which distance.

Adjustable parallax setting allows you to line up your reticle with your target objective in a proper plane, what brings you a proper focus, better sight image and better accuracy.

Easy explanation about parallax meaning. When your rifle with rifle scope is fixed on a bench and you are looking through the rifle scope. Your reticle is perfectly in line with the center of the target. Now you move your head around und you see also the reticle moves a little bit in relation to the target. This is parallax error. It happens when the target is not perfectly focused. When you adjust the parallax, so that the target is in perfect focus, then there is no parallax error. You can move your eye from the optical axis and the reticle will still stay in line with the center.

Traditionally, rifle scopes had a fixed parallax set at 100 meters or 100 yards; however, with the advance of shooting sports, such approach was not adequate anymore. Scopes with fixed parallax offered best resolution only at one distance (100 meters/yards) at which the target was in focus. At all other distances, the picture was less sharp or even blurry. An even bigger problem with this type of scope was the fact that, when shooting at the target not at the same distance as the scope parallax setting, the shooter had to be very careful about his eye alignment with the optical axis. If the eye is moved away from the optical axis, the reticle on the target will move, which will worsen the accuracy. Since these errors are small, the fixed parallax option is still considered accurate enough for traditional hunting. For sport shooting, though, such small error leads to poor results. Parallax errors become more pronounced with magnifications higher than 12x, and that means the majority of scopes with magnification range under this value need no parallax adjustment.

With the ability of parallax adjustment, accuracy of the riflescope is greatly enhanced at all distances. The shooter, however, has to adjust the parallax setting before making the first shot. Such scopes usually have AO – Adjustable Objective or SF – Side focus acronym in their name. 

Source: Revija Lovec

Parallax adj. Range

Parallax adj. (adjustment) range tells you the minimum and maximum distance you can adjust the parallax on your rifle scope.

Most rifle scopes (mostly) with maximum magnification 12 x have parallax setting fixed, for example, to 100m (EU 100 m / US 100 yards or 91 m). On those, you can’t make any adjustment with parallax.

Rifle scopes with more than 12 magnification zoom have adjustable parallax setting, for example, from 50m to infinity.

There are some rifle scopes with special parallax settings. Schmidt & Bender Exos rifle scopes have CC mode, which means when you turn the magnification on 1, the parallax changes simultaneously from 100 meters to 25 meters, so it is much easier to look with both eyes open and therefore easier to shoot close distance running targets.

Source: Revija Lovec

Diopter range in optics with one ocular (riflescopes, spotting scopes, NV optics, …)

Diopter range is an adjustment on optical products which can correct the prescription on each of your eyes. That way you can see a sharp image without wearing glasses. The diopter ring is normally located on the eyepiece and by turning it your image appears sharper.

Every optical product has different diopter range, from positive (+) to negative (-).

Diopter range in binoculars

The diopter ring is present in central focusing system on each of the barrels near the eyepiece, where you can correct the difference in the prescription of the left and right eye individually. Once you have set the right value, you can focus the image with using just the central focusing ring. If you’re wearing glasses, the diopter value should be set to 0, because the differences in your eyes are already corrected in your glasses.

To see a sharp image without wearing glasses you can easily set the diopter by looking with bare eye, turning the ring and adjusting sharpness. So when looking with both eyes your image should appear sharp. If you have astigmatism the diopter adjustment cannot correct it – you’ll still need your glasses and diopter set to 0 to see sharp images.

Source:Nikon

Optics Trade debate about Diopter settings on binoculars

Fast focus eyepiece

The eyepiece is that part of the rifle scope where you look in.

Fast focus eyepiece is a type of an eyepiece that enables fast diopter settings. With fast focus eyepiece you can adjust with rotation of the diopter setting to suite your eyes, reticle and even the target (image) at the same time.

All the newer rifle scopes have fast a focus eyepiece. There are not many rifle scopes without a fast focus eyepiece nowadays.

Older rifle scopes without fast focus had their whole eyepiece rotatable and a counter locking nut.

Source: oculus

Lens coating

Optical products have many lenses in their housing. With each lens about 5% of the light passing through is lost. This can be solved with an application of coatings on the glass surfaces. With years the technology of coatings changed. At first they used only one layer, where the reduction of the loss was to 2% per surface. Today they use multiple layers of coatings where there’s minimal loss of light - 0.1% per surface. The best binoculars have even 95% of the light transmitted to the eye, through all their lenses.

With increasing transmission of the light, the coating is also important as a protectant of the optical glass and to ensure the true color fidelity, so the colors when entering are the same when exiting binoculars/riflescope. Above all, coatings also increase the image quality because all the light bouncing around on the inside can cover up detail and blur colors.

Source: ZEISS

The process of applying coatings has to be precise, otherwise it can contribute to hazy and blurred image. They must be spread evenly and thinly to ensure the best quality. The better the coatings, the more expensive the optical product.

Lens coatings are as important as the quality of the lenses themselves. You can easily check whether your optical product has coatings – if you look at the reflection and it shows multiple colors such as purple, green or yellow the lenses are definitely coated. On the opposite, lenses with no coatings have a clear reflection without showing any colors.

There are many different ways of applying lens coatings:

• Coated: where one or more glass surfaces are coated with one thin anti-reflective layer.

• Fully coated: where all glass surfaces are coated in one thin anti-reflective layer.

• Multicoated: where one or more glass surfaces are coated in multiple layers. Light transmission is more than 75%.

• Fully multicoated: where all glass surfaces are coated in multiple layers. Light transmission is more than 85%.

• Outer surface coating: coating on the outer glass surface which protects the lens from external dew (especially in the winter), partially from dirt and other impurities. They can have different names, depending on the manufacturer (LotuTec, Swarodur, AquaDura)

Source: ZEISS

Waterproof

Waterproof feature is made to keep the optical products sealed and protected from water or dust. Such products are suitable for marine, hunting, hiking or in extreme humidity. Even if you’re not planning on using them in this kind of situations, it is a good feature to have in case of heavy rain or dust. Waterproof optical products are typically sealed with O-rings.

All optical products that are fogproof are also waterproof, because they have to be properly sealed to keep the dry gas inside. Yet not all waterproof products are fogproof as the air inside the barrels is not necessarily replaced with dry nitrogen or argon.

You should be careful not to confuse waterproof with weather-resistant as they’re designed to protect only against light rain and are not fully sealed.

Slightly better waterproofing of binoculars can also be ensured with an individual eye focusing mechanism, due to less moving parts than with the central focusing system.

Fogproof

Fogging in optical products can occur when you move them from the warm insides of your house to the cold outdoors. To prevent the formation of inside fogging they’re often filled with dry gas – either nitrogen or argon which contain no moisture.

To keep the gas intact on the inside, the optics have to be properly sealed, which is why all fogproof optical products are also waterproof.

It’s important to keep in mind that fogproof means that it’s to prevent fogging on the inside of the optics, not on the outside. If your outside surface of the lenses fogs up due to temperature differences or humidity just allow them to adjust back – do not wipe the condensation off as it can be damaging to the glass surface and its coatings.

Filled with

Optical products are often filled with dry gas to prevent the condensation on the inside of the housing when exposing them to temperature extremes. If there is even a slight sign of air inside, there is a certain % of moisture present. Usually they’re filled with either argon or nitrogen gas, which have the same effect – to prevent the moisture and internal fogging without affecting the optical properties. In addition, these gases also prevent the formation of fungus which would destroy the optics. Internal dewing was the biggest problem in older binoculars when exposed to lower temperatures, because they weren’t watertight and contained air. Newer binoculars are therefore all airtight and filled with dry nitrogen or argon.

Tube diameter

Tube diameter is that diameter, on the rifle scope, where mounts are clamped on (see the picture below).

This parameter is an important information when buying mount rings for your rifle scopes. Most common mounts have a diameter of 1 inch, 30 mm and 34 mm, but in the last years, manufacturers started to produce rifle scopes with a tube diameter of 35 mm, 36 mm and 40 mm to increase windage and elevation travel range.

Lower price ranking and older rifle scopes have a 1 inch (25.4 mm) main tube.

Common for hunting rifle scope is a tube diameter of 30 mm, which is enough tough for hunting use and all the inner parts are big enough.

Most tactical and some sport rifle scopes have a tube diameter 34 mm, because of a thicker wall (of tube) for a more reliable use in harsh environment or an extended elevation range against 30 mm main tube rifle scopes.

Some of new rifle scopes (for example Zeiss victory V8) have 36 mm tube diameter, also some IOR or the new Swarovski DS stands out with a 40 mm tube diameter. European hunting rifle scopes have rails, where mounts clamp on these rails and not around the tube.

Objective diameter

Objective diameter is outside (housing) diameter of front side (objective) on rifle scope.

On most rifle scopes objective diameter is bigger than tube diameter. Keep in mind, the last number in the description of rifle scope tells you the size of the objective lens diameter (example: 2 – 7 x 32). Objective diameter is always bigger then lens diameter.

Objective diameter is important to know, when buying ring mounts. The bigger the objective diameter the higher mount you will need.

This information is also important for all users of clip-on devices.

Source: Revija Lovec

Eyepiece diameter

Eyepiece (or ocular) diameter is outside (housing) diameter of rear side on rifle scope.

Maybe someone is thinking “ouh, so the bigger the eyepiece diameter the bigger image you get and you don’t need to look exactly in the middle to get a perfect picture…” Well, this is not correct. Eyepiece diameter can have different sizes - quality of the inner eyepiece parts and quality of rifle scope in general tells you how big the image will be and how far you can look from the middle point of eyepiece lens and you still get a perfect or good image.

Source: Revija Lovec

Mount length

Mount length is a length of rifle scope tube where ring mounts can be clamped on (see the picture below).

Mount lengths of the tube on rifle scopes are different due to the length of objective and ocular length and sizes. Wide angle type rifle scopes have longer mount length, due to the front side of the rifle scope has a tube till the end.

Source: Optics Trade