Top report - The 56mm myth in riflescopes from waffenkultur.com

Excellently researched report by waffenkultur on the subject of riflescopes - myths and truths about them 56 mm optics

By Henning Hoffmann - Source, also all other online magazine issues: http://waffenkultur.com

Download the report from issue 48 as pdf

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The Image quality of a riflescope depends on several factors. The Lens diameterThe larger the lens diameter, the more light can be absorbed by the lens of the optical system. Glass purity plays a major role. " More info is not the decisive criterion. In the end, it is only a "side effect" and not a quality feature per se

"Telescopic sights with a 56-mm LensWhen we talk about the objective lens of binoculars, we mean the front lens. The front lens is the foremost lens of the objective. " More info are especially brightFor night glasses, which are ideal for sitting game and as sow glasses, the light intensity is the most important feature. " More info and therefore offer excellent visibility even in the most unfavourable lighting conditions", these or very similar I'm sure everyone has heard this nonsense from a reputable retailer. heard. It is of course easy to Sales talk to an obvious Design feature of a telescopic sight.

For the buyer, the matter is therefore crystal clear and also so beautifully logical:

• Larger aperture equals more light. Physics can be that simple. And you are already Sufficient expertise for long range shooting or twilight hunting.
• At the end, the duped customer leaves the shop with a overpriced riflescope in his shopping bag that probably won't even fulfil its actual purpose.

Metaphor: When the first turbo petrol engines with unprecedented engine performance appeared on the roads in the 1980s, fuel consumption skyrocketed. To ensure an adequate range, manufacturers had to install larger petrol tanks. Alpina BMW even enlarged the fuel tank to over one hundred litres with a 30-litre additional tank.
Here comes the bad news: just because a car has a large petrol tank doesn't make it an Alpina BMW. The larger fuel tank was merely a side effect of the high engine output; not a quality feature for high-performance cars per se.

In a figurative sense, this means, A 56 mm lens does not make a fast long-range riflescope. As shown below using the criteria Exit pupilIn addition to the twilight number, the binocular exit pupil at the eyepiece of the binoculars is also very important for seeing in twilight (twilight vision). " More info, Twilight numberIn order to be able to recognise details over long distances even in twilight, binoculars with high magnification and corresponding twilight performance (e.g. 10x56 or 8x56) are recommended. " More info, Light transmissionLight transmission is the ability of optical glass or optical systems to "let through" as many light rays as possible. " More info and focal length and with the aid of physical laws.

Criterion 1: Exit pupil (AP) for riflescopes

As Exit pupil is the name given to the Diameter of the beamwho has the EyepieceThe eyepiece is the lens group of an optical system that faces the eye. The eyepieces consist of several separate lenses - achromats (3-6 individual lenses).  " More info leaves the lens. The exit pupil corresponds to the quotient of lens diameter and MagnificationThe magnification of a binocular model is perhaps the most misunderstood feature. High magnification can be useful! " More info. With an 8×56 riflescope, the AP is therefore seven millimetres; with an IF dimension of 10×50, it is still five millimetres.

However, the exit pupil is only the image on the image side. The other side of the equation is the human EyeThe retina of our eyes has two different types of sensory cells or receptors, the cones (for day vision) and the rods (for night vision). " More info itself and decisive for the perceptible image brightness. Only the part of the luminous fluxwhich is also used by the eye can be absorbedcontributes to the brightness of the image.

The average Maximum pupil width of a human being depends on the Age. If it is still 1.5 millimetres (day vision) and a maximum of eight millimetres (night vision) for a young person, the Maximum opening width for adults to four to six millimetres.

One 80-year-old old man only has about two millimetres of pupil width left for night vision. The 7 mm exit pupil of the 8×56 riflescope can therefore hardly be seen by an adult. The 10×50 riflescope therefore has no perceptible disadvantage for them.

Two really useful Rules of thumb here: The The exit pupil should always be at least as large as the pupil of the observer or shooters. And an exit pupil of less than four millimetres is fundamentally unsuitable in diminishing light. No matter how high the twilight index is.

The twilight number is not meaningful

Quality of an exit pupil
The quality of exit pupils can be quickly assessed: Hold the binoculars with an outstretched arm against a bright surface and look at the eyepieces. This makes the exit pupils visible. With a good binoculars, these are absolutely circular, sharp and evenly bright at the edge.

Angular shapes and shadowing (vignetting) are an indicator of small PrismsLenses have the property of imaging objects upside down and laterally inverted - therefore the image must be rotated again. " More info or GlassIn addition to the coating of optics, the glass quality or type of glass is one of the most important quality factors for binoculars and riflescopes " More info with a low refractive index and therefore inferior quality.

Criterion 2: Twilight number for riflescopes

The twilight factor is the square root of the product of magnification and lens diameter. It is therefore a purely mathematical value and does not take into account performance data such as image sharpness, contrast, light transmission and colour rendering.

The poor informative value of the twilight number, which serves as a guide for hunters in German-speaking countries in particular, is shown in a sample calculation:

All binoculars in the dimension 8×56 have a Twilight factor of 21.2independent of their quality or number of installed LensesThe use of aspherical lenses leads to a reduction in the overall length of the binoculars and in some cases to a significant reduction in weight. " More info and their coating.

Even binoculars in 56×8 would have an identical twilight factor of 21.2. Binoculars in 56×8 are obvious nonsense and are not available anywhere. The twilight factor is therefore meaningless.

The calculation of the twilight factor for riflescopes differs from that for binoculars. In this case, only the lens aperture is taken into account, which results in a maximum exit pupil of eight millimetres. This is because even a very young shooter would not be able to utilise more light.

Here is a concrete example:
The hunting all-rounder 3-12×56 has a theoretical exit pupil of 18.7 millimetres at 3x magnification (56 millimetre objective lens diameter divided by three). No human eye is capable of this, such a large lens aperture and effectively utilise the resulting exit pupil.

Therefore, the lens aperture is calculated first, which would result in an 8 mm exit pupil at 3x magnification. That is 24 millimetres. In the 3x position, this results in a purely mathematical twilight factor of 8.5 (root of the product of three and 24).

Criterion 3: Light transmission for riflescopes

The Light transmission is the "light transmittance". The Transmission is the percentage of incident light that emerges in the eyepiece. Light losses always occur in a target lens. The decisive factor is how large these losses are.

Calculating this transmission is under laboratory conditions with great effort feasible. The average values for the spectral colours are identified using several hundred measurements and the result is evaluated with the sensitivity of the human eye with regard to colours. A distinction is made between day and night transmission.

Premium binoculars today are generally above 90 per cent transmission. A ZEISS Victory 8×56 T* FL, for example, delivers 94 per cent light transmission, while the HT series riflescopes from ZEISS, which are popular for night hunting, achieve 95 per cent according to the manufacturer.

The Quality and type of anti-reflective coating of the glass used plays a role, as the light transmission is influenced by reflections or special anti-reflective coatings on the surface of the glass.

One example is the ZEISS T* multi-coating on all glass-air surfaces. This multi-layer process with its 6-layer StructureThe different types of DDoptics binoculars Binoculars can be divided into two main types according to their basic construction: Construction with roof prism (different types of construction) or with Porro prism. Both designs have their unique advantages and disadvantages. In principle, Porro and roof prism binoculars can offer the same performance depending on the type of glass (BAK 4, crown glass, etc.), the amount of coating required and the manufacturing quality. The details decide whether they are good binoculars or not. " More info is customised to individual lenses and glass materials. Over one hundred layers vapour-deposited in a high vacuum on each side of the glass are common. Such a coating also takes into account the Increased blue sensitivity of the eye at dusk and thus helps to get as much light out as possible.

The lens diameter is irrelevant for the light transmission

As an end customer, you cannot know which standards a manufacturer has when determining light transmission and how the measurement results are obtained. 95 per cent light transmission on average in a very high and therefore representative number of measurements under identical laboratory conditions and precisely defined lighting conditions in advance (typical for twilight or a moonlit night) will also fulfil their promise in practice.

If a resourceful (or desperate) marketing department picks a lone top result of a daily measurement and then states this as light transmission without further explanation, the user will be disappointed.

The criterion of light transmission therefore becomes a null argument, at least for end users, due to insufficient information.

Criterion 4: Focal length for riflescopes

The Focal length here means the distance between the objective lens and the lenses in the first image plane. This Distance has a significant influence on image quality of a riflescope. The greater the distance, the better the image quality because the objective lens does not have to focus the light rays so aggressively.

The Abbe number, named after the German physicist Ernst Abbe, is a dimensionless quantity used to characterise the refractive properties of optical glass. A high Abbe number stands for low dispersion and indicates a high refractive index. Lens coatingIf light hits a glass-air surface (e.g. with a roof prism), the reflection causes a loss of light. " More info there.

If aberrations occur when light waves are refracted, this is called chromatic aberration. One possible countermeasure for chromatic aberration is to combine several lenses to form a lens group.

One Short focal length in the ZF enables the (ultra) short riflescopes that have been available on the market for several years. One High image quality must here, however with a bundle of several objective lenses be bought. Premium suppliers often install two or even three objective lenses, to minimise chromatic aberration. In addition, the lenses are anti-dispersion coated. Both design features increase the manufacturing costs enormously, making the ZF correspondingly expensive.

Riflescopes with a long or normal focal length, on the other hand, deliver a correspondingly high image quality even without this extra optical effort.

The larger the lens, the more difficult it is to minimise the aberration. Inexpensive but large lenses will always produce significantly lower image quality than inexpensive small lenses. If the budget really plays a major role in the purchase of a riflescope (which it never should), the end user is better advised to purchase a riflescope with an objective lens diameter of less than 45 millimetres and a long focal length.

Once again: The image quality of an IF does not increase proportionally with its lens diameter.

Criterion 5: Lens diameter

Until now, the lens diameter has only served as one of two basic variables in the calculation of exit pupil and twilight index. Criteria that without significance for quality, grade or performance of a riflescope.

The lens diameter is irrelevant for the light transmission. This is because light transmission is significantly influenced by the goods and the number of lenses used.

Guide to buying a riflescope

Above all, before Purchase the intended use clearly defined be. There is no such thing as an "all-round" riflescope. Every application is special and requires a suitable scope. You don't automatically buy more usefulness just because the scope is offered on the market at an above-average price. Extreme criteria such as extreme zoom factors, extremely short designs or extreme objective lens diameters are generally not recommended.

Conclusion

In hardly any other market segment is used with so much half-knowledge and charlatanism, as with riflescopes. As soon as a consultation about riflescopes is based on the twilight number or the phrase "fast ZF due to 56 mm lens" the conversation should be cancelled and the salesroom left as quickly as possible.

Remark
All the riflescopes presented in this article are recommended purchases. Some of the ZFs have been used successfully in practical tests for several years.