Kahles Helia 3 3-9x42

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

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

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

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

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.