Optics for Hunting
The optical systems of binoculars, spotting scopes and riflescopes are similar. Binoculars use objective lenses, erecting prisms, and eyepieces. Best spotting scopes use objective lenses, erecting prisms or erecting lenses and eyepieces. Riflescopes use an objective lens, erecting lenses, and an eyepiece although some high-end units use prisms in lieu of erecting lenses.
The complete optical system is important and not only parts of it. If you have the very best type and grade of optical glass along with a great optical design, it is meaningless if you have a subpar eyepiece or poorly designed erector lenses or prisms. Therefore, all the components of the optics must be designed and built with good materials, precise and accurate manufacturing of the materials, the best technology for fully multi-coated optics and applying them properly, very careful assembly and the detailed and exhaustive end product quality control to ensure performance of the optical system is as advertised and expected.
The optical systems rely on well-made mechanical parts that house the optics and ensure that alignment is good and all parts function smoothly and repeatedly without problems. On very high-end products (Leica, Swarovski, and Zeiss – which I call the Big 3), you should see an image that “pops” out and is super bright with brilliant color fidelity from edge-to-edge. I have experienced this with all three brands many times and it is incredible. Recently a disturbing thing is that one of these brands is offering much lower cost products with lower quality and I am not sure this is good for their brand image.
About 15 to 20 years ago, there was quite an optical performance gap between the Big 3 and the next level of competitors but that gap has been closing rapidly the last few years from several brands. I will mention several brands (the higher end models of each brand) that to me are surprisingly very good optically on a consistent basis for the cost they sell at – a great value! In no particular order they are – Meopta, Vortex Optics, Nikon, Minox, Celestron, Bushnell, Kowa, and Steiner. There are many more brands just a tad under the quality brands mentioned. I have looked through some brands costing about 50% to 70% or so as much as the Big 3, and the views have been very good with only subtle difference in optics but they were very disappointing and I won’t mention them here to avoid any potential legal issues. For the very best optical systems, manufacturers are using a fluorite element or ED (or similar low dispersion) glass in the
objective lens. Both are of very low dispersion (the variation of the index of refraction of a transparent substance with the wavelength of light) and both benefit from reductions to chromatic aberration. Some writers and reviewers note no difference between ED glass optical systems and non-ED glass optical systems as far as chromatic aberration is concerned. However, something is wrong in what they are seeing, as there will be considerable chromatic aberration with any standard achromatic objective lenses versus one with ED glass objective lenses. ED objective lenses should virtually eliminate chromatic aberration unless there is something wrong with the optical design, the glass itself, assembly, or something else. Fluorite (fluorspar) is a halide mineral made of calcium fluoride (CaF ₂) in the form of crystals. Natural fluorite crystals without flaws are microscopic in size. Thus, artificially grown calcium fluoride crystals were developed. The crystals are useful in place of optical glass in very high-end optical systems. Fluorite eliminates all chromatic (color) aberration. Fluorite is very fragile and difficult to manufacture into optical components and these factors are part of why the material is so expensive. When used as an element of an
achromatic objective lens, it is normally the inside element (positive) and the other element is of a high dispersion glass as the outer element (negative) facing the outside environment. However, products using fluorite have excellent color fidelity and razor sharp images free of any color aberration. Only a few brands have or are using fluorite in some of their optical systems.
ED is an optical glass of very low dispersion. Over the last several years there have appeared quite a large number of optical products from various brands using this glass in mid to high end prices where the main benefit is (when designed properly as part of the optical system) to help reduce or virtually eliminate chromatic (color) aberration. ED glass, as well as fluorite, can help extend the usability of optical products in low light conditions when normal glass products cannot. Most ED optical glass has traditionally come from Japanese factories such as Ohara and Hoya. In recent years, manufacturing of ED optical glass in China started. A few European countries also manufacture some ED (or equivalent in dispersion) glass.
There are different grades of ED glass that give different levels of optical performance. In general, using better grades of ED glass, in a well-designed optical system, will give you excellent color fidelity and razor sharp images virtually free of color aberration. I say in general because I have seen many binoculars and spotting scopes with ED glass that had quite noticeable color aberration and were not as sharp as they should be due to a lower grade of ED glass or a poorly designed overall optical system. In regards to ED, there are some names associated with this – LD, XD, SD, SLD, and UD among others. Fluoride glass is ED glass and not Fluorite (a crystal) which is generally better for most optical systems but much more expensive than ED glass. Some companies call their products HD or UHD and other names meaning high definition or ultra-high definition but these, in general, are not ED glass as high definition doesn’t really have a meaning and does not compare to fluorite or ED optical glass in low
dispersion – don’t be fooled! I might add that you could also use an APO (apochromatic) lens design for the objective lens. An APO optical design traditionally has three elements (triplet) of different types of glass to greatly reduce or eliminate chromatic (color) aberration. However, due to optical design technology advances and lower density glass availability, two element APO designs can be about as good as triplets or fluorite if the very best grades of ED glass are used.