Lenses

No other factor determines image quality more than the lenses in the optic. There is a vast array of conditions that ultimately determines the overall performance. As the precision and quality of the mountings, lenses, coatings, and glass increases so does the cost. Light passes through and reflects on glass surfaces with multiple effects. The challenge is to utilize the light entering the optic and focus it into an image with minimal loss to obtain as bright and sharp an image as possible.

The different colors (or wavelengths) of light pass through the lenses and bend at slightly different angles, similar to a prism. Several lenses of different types of glass and designs are required to get each of the basic colors to focus correctly in the image. Uncorrected elements produce blurred images with distortions and muddied colors. Special types of glass are now commonly used to help alleviate some of the distortions and problems associated with the light passing through lenses. Optic manufacturers are progressively using more exotic, very dense (ED, HD, SD, etc.) glass and minerals such as fluorite (CaF₂) along with sophisticated designs to solve these problems.

Mirrors

The mirrors (or prisms) within an optic are just as important as the lenses. Some of the inherent problems associated with the mountings were addressed previously in the discussion (Basics I) of the differences between Porro and Roof Prism designs.

Like the lenses the mirrors must also be coated to prevent the scatter of light. Roof prism designs generally also include anti-phase shifting coatings that prevent an interference problem associated with this type of optic. Other special coatings are also applied to the mirrors of optics to improve light transmission. Since, as consumers, we do not get to choose the type of process the mirrors receive we do not address these in detail although it should be noted that they are important to image quality.

FIGURE #1

Lower priced optics will often use BK7 prisms where better optics use BAK4 prisms. BK7 prisms produce an exit pupil with shaded edges. The BAK4 prism projects a nice round exit pupil (see Figure #1). In bright daylight where your eye pupils are smaller than the exit pupil of the optic, you may not notice this distortion. As the light level drops and your eye pupils become larger, this aberration becomes more apparent around the edges of your view with BK7 prisms.

Coatings

As noted previously, whenever light is transmitted through a lens, some light reflects from the lens surface and is lost. Thin coatings are deposited on the lens faces to reduce reflective loss and improve light transmission. With the large number of lenses in a binocular or scope, these coatings can be as important as the quality of the lens itself. Without these coatings, each lens may lose up to 5% of the light. Lenses with multi-coatings may reduce this loss to tenths of a percent. Thus, a poor optic may loose as much as 35% of the light entering the objective where quality designs may lose less than 5% total.

The coatings also improve the image quality since the reflected light bouncing around in the interior of the optic washes out detail and blurs colors. Manufacturers of quality optics add several thin coatings (as many as 7) to optimize transmission of each of the basic colors. The coatings typically described in the literature are defined in the following ways:

Coated (C) Optics

A thin anti-reflective coating (usually of Magnesium Fluorite) is deposited on one or more of the lens surfaces.

Fully Coated (FC) Optics

At least one thin anti-reflective coating coating on both sides of the objective lens system, both sides of the ocular lens system, and the long side of the prism.

Multi-coated (MC) Optics

One or more of the lens surfaces have multiple coatings. Even some of the best optics have only a single coating on the outside lens surface. This is done under the theory that a single coating is harder, more durable and the light reflected from the outer surface does not affect image contrast.

Fully-multi-coated (FMC) Optics

All lens surfaces have multiple coatings. This is generally the case with the top-of-the-line optics. This does not guarantee the best quality, (quality is in the execution!) but it is an indicator that greater care and thought has gone into the design.

In conclusion, optical coatings are extremely important to delivering a sharp and bright image. Some coating schemes simply work better than others. An FMC 8 x 35 binocular can actually appear both sharper and brighter than a 8 x 42 binocular with poor coatings.

Distortions

Modern optics generally do a good job at presenting a normal image to the eye, although an absolutely perfect image is close to impossible, even with modern materials. Lenses have curved surfaces to focus the light. Thus, presenting a flat image to the eye can be a challenge. As you move from the center towards the edges, the image tends to stretch out... much as a map does. These distortions are called Curvature of Field and are probably the most common distortions.

Fortunately,Curvature of Field is probably the least damaging to the view since it is most obvious only very close to the edges. It is commonly noticed when you have a straight object, like a telephone pole, at the edge of the view. In this case, the image of the telephone pole will begin to curve slightly as it nears the edge of the view. Since most of us center what we are looking at, this is usually not a big problem.

The problem is more of a nuisance when the edges of the view also focus at a different point than the center. This is most common with wide-angle designs. Sometimes it is impossible to get the center of the field of view in focus at the same time as the edges. There are optics on the market, including wide-angle designs, that completely alleviate this distortion although they are both heavy and expensive. Most optics deal with this type of distortion quite well and it does not generally cause any great problems.

Two other types of distortions that should be mentioned here are pincushion and barrel distortions. These are not commonly noticed in modern optic designs. They are caused by the center and edge of the field of view being at different magnifications. The variations in magnification causes the whole view to slightly distort from center to edge and draw shapes out of true perspective. Specifically, barrel distortions magnify the center of an image more than the edges causing it to "bow out". Pincushiondistortions magnify the center of an image less than the edges causing it to "bow in".

Aberrations

Aberrations are similar to distortions and the terms are sometimes interchanged. Strictly speaking, aberrations are considered to be a result of an intrinsic defect prohibiting all of the information from an object to be focused orderly in the image. This results in reduced image sharpness and color-smear or loss of definition.

Chromatic aberration is most commonly mentioned since it is the most obvious. As was noted earlier, different colors of light bend at slightly different angles when they pass through a normal lens. Uncorrected, this produces an image with a muddy fringe of unfocused light. The overall damage to the true colors and contrast can be dramatic since this muddy fringe is actually happening throughout the whole image. Most optics deal with this problem by using a pair of achromatic lenses. These are lenses made of different glasses, each one keyed to bringing a different color into focus.

Low-dispersion (ED, HD, SD, etc.) glasses are becoming more popular. These high-density glasses reduce color separations dramatically. Contrary to achromatic lenses that key on focusing specific spectra of light, these new high-density glasses greatly reduce color separation altogether. They are not perfect nor do they completely eliminate chromatic aberration, but they produce an observable difference over standard achromatic lenses. These new materials combined with complex designs produce optimum color fidelity and contrast.

Another aberration receiving more attention is coma. The results of this are that light that passes through the center of a lens can be focused to a point. The light the passes through the lens off-axis (at an angle) will not focus to a point and look like a fuzzy circle. The further off-axis the more the light smears, giving objects a comet-like look. This is not to be confused with curvature of field and cannot be focused out. This is generally fairly well-controlled in modern optics.

Last is spherical aberration, that results from the actual curvature of the lens. A spherical lens surface focuses the light from the edge of the lens to a closer focal point than the light from the center of the lens. The result is an image lacking sharpness, detail and brightness. The solution to the problem is to add another lens in the path with a complex curve computed to correct the image.

Often you hear the term "residual aberration". This term is used as a "catch all" to describe combinations of aberrations that are not fully dealt with in the design. Residual aberrations lead to all the negative optical outcomes described above. In combination we get a sort of optical mud. The images lack sharpness, true colors, detail, contrast, etc. Optics manufacturers are getting better at resolving these problems through both design and materials but there is no optic that completely eliminates all distortions and aberrations.

Alignment

Modern optics may have as many as 10 pieces of glass serving different functions and most have at least 6. Proper alignment of all the elements, including the mirrors, will determine the final image quality. The position of all these elements must be precise in order for them to function and do what they are supposed to do. Each element must also be held and secured firmly. If the alignment shifts, the image quality suffers no matter how well the lenses were designed and processed.

In scopes, the distances are larger than with binoculars and slight misalignments can dramatically degrade image quality. With binoculars, the two barrels that must match so that the same image, focused to the same point and size, reaches our eyes. Our eyes may not be perfectly aligned and our brain will make some adjustments to differences between the images reaching our eyes. Improper alignment causes fatigue, eyestrain, inferior or double images and even nausea.

The term for the alignment between the barrels of a binocular is “collimation”. Collimation is a measure of how exactly parallel the barrels are mounted. The barrels are hinged so that they can be adjusted to suit the differentdistances between people’s pupils. This hinge must be precise to maintain collimation between the barrels and eliminate slop or play in their position. Low-priced optics often have trouble with consistently accurate barrel placement.

The layman test for proper barrel collimation or alignment is to look through the binoculars backwards (through the objective lenses) and try to sight on a horizontal line. If the alignment is not correct, the line will not be straight as seen through both objectives (the view through one side will be tilted).

Conclusions

• The lenses in optics are of critical importance. New materials are providing images with enhanced purity of colors and contrast.
• Lens coatings can be as important as the lenses themselves. The coatings improve light transmission and reduce internal light scatter that degrade image quality and brightness.
• Distortions are less damaging to image quality and are becoming better controlled with modern optics.
• Residual aberrations are mostly a combination of 3 common intrinsic defects. They cause poor image quality, contrast and reduce color fidelity.
• Precise and secure alignment of elements is critical to the optic performance. If the elements move, the image quality suffers.
• Poor barrel collimation causes fatigue and can be difficult to notice. The hinge must firmly secure the barrels in parallel position and eliminate sideways play.

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