The Truth About Optics

Prisms:

Prisms are, as you might say, a necessary evil in optical design. It if weren’t for our unreasonable prejudice for seeing the world right side up and right way around we wouldn’t need prisms in binoculars and spotting scopes at all.

You might remember the experiments with chickens. Chickens were fitted with spectacles that inverted and reversed their view of the world. They stumbled around for a few days, but most of them quickly adapted to the new view, and were soon functioning normally (as normally as a chicken can.  Evidently the chicken brains “remapped” the visual space for the new conditions. If chickens can get used to an upside down, reversed view of the world, certainly we ought to be able to. Amateur astronomers (in a not often documented example of convergent evolution) demonstrate this same ability, though with the popularity of cat scopes, increasingly they only have to deal with an image that is reversed right for left.

The rest of us, though, continue to show a strong preference for “erect” images…which is another way of saying right side up, right way around…and all modern binoculars and almost all spotting scopes today employ a prism erector system.

Prisms accomplish this feat by “bending” or, more properly, reflecting light around a series of corners. When light strikes a glass-air boundary at an angle, some of it is always reflected back, while the rest passes through. Think of standing outside trying to look through a window into the room beyond. The amount that is reflected back is determined by the angle at which the light strikes the glass-air boundary (angle of incidence), and by the index of refraction of the glass. Index of refraction is a measure of how sharply glass bends light that enters and leaves its substance, and is closely related to the density of the glass. What is of interest in prisms is what happens when the light that is already inside hits a boundary. There is an angle, for any given glass, at which all of the light that strikes the surface from the inside is reflected back: producing, in the ideal, total internal reflection.

(Total internal reflection is useful not only in prisms. It is what makes light pipes and optical cable work as well.)

A porro prism erector system works, in theory, very efficiently because all of the light that will form the image strikes each air glass boundary at an angle that is shallow enough to produce total internal reflection. No light is lost to the image.

In fact, inexpensive porro prism erecting systems use a glass (Bk7) which has a index of refraction that is near the critical limit for what will work…so that light from the center of the prism is reflected back properly, while a small portion of the light from the edges of the prism leaks out through the glass-air boundary. If you look at the exit pupil (the small circle of light that floats in the center of the eyepiece when you hold the eyepieces away from your eyes) of a binocular that uses Bk7 glass prisms, you can see that edges are shadowed, with an arc of the perfect circle being cut off on two sides.

Increasing the density of the glass used (generally using a glass called BAK-4) cures that problem, and BAK-4 porro prism erecting systems are among the most efficient available, passing well over 90-95% of the light that enters them.

Porro prism erecting systems have other unique features, of course, primary among them the fact that the light entering the system is “offset” from the light leaving by the width of the system. This gives porro prism glasses, and most spotting scopes, their characteristic dog-leg, or “z” shape. Because the objectives of porro binoculars are always either further apart or closer together than the human eyes looking through them, they alter the perspective of the view, producing either an increased sensation of three dimensionality (further apart), or a decreased sense of dimensionality (closer together). Porros also alter our perception of the size of the object, and our distance from the object, for the same reasons.

Very early in the development of binoculars, roof-prisms, which keep the exiting light in line with the entering light, were developed. The barrels of roof prism binoculars are straight, and the sense of dimensionality, size, and distance relationships are much more natural.

(Besides the handling advantages that roof-prisms offer, about with more in a future article, this difference between in the apparent size of the object being observed for porros and roof binoculars of equal power probably did more to contribute the popularity of roofs among birders than any other factor. The bird simply looks bigger through roofs. It is not bigger. Projecting the images of an 8x roof and an 8x porro and measuring the size of the object shows that they are identical…however it is very difficult to convince our brains that we are seeing the same size bird through both glasses. I have a friend who has quantified the effect, associating the perceived difference in magnification with how far apart the objectives are. Inverted porros, by the way, with the objectives closer than the human eyes, show this effect at its most dramatic.)

A roof prism takes some special attention if it is going to work at all. In the most common roof prism, the Schmidt Pechan, one of the glass-air boundaries is at an angle such that it can not produce total internal refection for any of the light that strikes it. Most of the light would pass right through. This problem is solved by “coating” that surface of the prism with a mirror coating: a thin layer of reflective metal which turns all the light that strikes it (producing the “roof” in the “roof prism” design). Silver roofs were used until aluminum became readily available (aluminum has the advantage of not tarnishing or loosing its reflectivity as quickly as silver). Most high quality roof-prisms have returned to the use of silver because it reflects light more efficiently, and because the silver does not tarnish in a binocular that is waterproof and filled with inert nitrogen gas anyway.

Unfortunately, even the best silvered surface is not as efficient as a proper glass-air bounday at the correct angle. Some of the light that strikes it always leaks through: up to 15% for an aluminized prism. Therefore, until recently, no roof-prism binocular was as bright as a porro prism binocular of equal quality in side by side testing.

Then too, when light reflects back from a mirror, its phase is changed. One way of picturing this is to say that light waves come in bundles, with the waves arrayed in all directions, and crossing at the center of the bundle. When light reflects from a mirror, it becomes partially polarized, with more of the waves aligned, shall we say, horizontally than vertically. Some energy (brightness) and some information (resolution) is lost, and even more is lost when these partially polarized rays interact (interfere) as they are recombined in the image.

Untreated roof prisms, therefore, are slightly dim, and slightly soft, when compared to porros of equal quality.

As mentioned above though, many birders were willing to live with the difference in optical performance for the handling and perspective advantages of roofs. (And many more were convinced there could not be an actual difference in performance, or if there were one, it had to favor the more expensive roof prism design.)

There is another type of roof prism, called the Abbe-Konig, which can be used in binoculars. The Abbe-Konig is typical of designs from Zeiss, in particular, and, because of its size (primarily length) compared to a Schmidt Pechan, produces binoculars that are noticeably taller than other roofs (think of the 7x42 Zeiss Classics).

The Abbe-Konig has the advantage of not having a surface that requires mirroring: all the light is turned by total internal reflection. That means that the Abbe-Konig design is inherently brighter than any normal Schmidt Pechan system of equal quality in side by side comparisons.

Abbe-Konig systems still suffer from phase distortion however. The cause of the distortion and phase shift is slightly different than in the Schmidt Pechan, but, as the light from various paths in the prism recombines in the image, phase issues are no less troublesome, as they limit both brightness and resolution.

In the 1980s, Zeiss optical engineers developed a coating for prism surfaces. It is similar to the anti-reflective coatings used on lenses in that it consists of multiple thin layers of high index of refraction minerals, and it eliminates the phase distortion in the Abbe Konig design. Other manufacturers soon applied similar coatings to the Schmidt Pechan design to improve both the brightness and the resolution of images formed with their erector systems.

This is generally called “phase coating” and allows at least the Abbe-Konig design (which has, remember, no mirror leakage) to equal, for the first time, the efficiency and performance of the best porros (above 90%). The improvement in the Schmidt Pechan systems was more subtle, primarily, in my experience, consisting of increased contrast and sharpness.

Finally, within the past few years, high end makers have experimented with replacing the mirrored surface of the Schmidt Pechan system with very complex multi-layer coatings of high and low index of refraction minerals. In order to accomplish something approaching total internal reflection, the spectrum of visible light must be broken up into a high number of sub-spectrums, and coating layers developed for each division.

The first efforts used over 30 layers of coatings for a 2-3% increase in efficiency. More recent coating systems employ over 70 layers, in multiple applications, to raise the efficiency of Schmidt Pechan systems to equal or exceed that of the best Abbe-Konig and porro systems.

So there are now three choices for high efficiency prism erector systems: Porro prisms, Phase coated Abbe-Konig systems, and Phase coated Schmidt Pechan systems with dielectric “mirror” coatings.

Undoubtedly, just as multi-coating became a standard, and then Phase coating spread to cover the majority of roof prisms, dielectric mirror coatings for Schmidt Pechan prisms will proliferate, and will work their way down into less and less expensive binoculars over the next 5 years.

Remember though, that just as there are more and less efficient multi-coatings, and less and more efficient phase coatings, there will be, for some time to come, real differences in what manufacturers are calling dielectic mirror coatings. The key, as I understand it, is the number of segments the spectrum is broken down into, and the resulting number and tuning of the individual layers of coating. Not all dielectric mirrors are created equal nor will they be anytime soon. Performance will vary between manufacturers.

Which prism a designer uses depends on both price and performance goals. The best performance at the lowest cost can always be produced using simple, high-quality porro systems. Phase coated Abbe-Konig systems are appropriate for full sized binoculars in the upper price range (reaching down into the mid-range with the Zeiss Conquest 40s), and phase coated Schmidt Pechan systems with dielectric mirrors are currently appropriate for pricy full-sized, mid-sized, and even compact (pocket sized) binoculars.

In the end, the type of prism system is not as important today as its efficiency.

Or at least that's the way I see it....

The Truth About Quality

One reason Zeiss products are “better” than run-of-the-mill optics is the amount of hand labor involved in their manufacture and assembly.

Beginning at the design stage, experienced product managers and market specialists meet to determine, based on field experience and feed-back, what products will meet the needs of our current and future customers. Design specifications and criteria are developed.

Mathematical designs, both optical and mechanical, are produced, based on the specifications and criteria above, and then prototypes are built.

Prototyping is an art, and we have a separate group of technicians who take primary responsibility for hand-crafting the first working models of components and full optical systems. Prototypes are then individually tested by experienced engineers and market specialists, all of whom are end-users of the products being designed, and adjustments are made to the design until all design criteria are met, and the product specialists are satisfied.

As much time goes into designing the manufacturing and assembly process as goes into the design of the product.“Quality assurance” techniques are developed; jigs and testing equipment are built or adapted for each stage of assembly.

Our lenses are ground on precision diamond tip CNG machines. Each bank of two machines is run by an experienced master craftsman, generally assisted by an apprentice. The German apprentice tradition insures that no one operates a precision machine until they have had several years of direct experience working with the actual machine under a master craftsman. The CNG engineer is personally responsible for the operation and maintenance of his machine: and for tolerances and production goals being met.

Lenses are finished and cleaned by hand, and coated in the most up to date electron beam vapor chambers. The coating process can require several man-days per lens surface, and each layer is meticulously checked before the next layer is deposited.

Glass-fiber reinforced body parts are cast in computer controlled casting machines that are so sophisticated that engineers are able to specify the direction of the reinforcing fibers within the mold. The molds themselves require many man-hours of precision labor and are one of the most expensive single “components” in the production process. Mold making, like prototyping, is closer to an art than a science.

Metal parts are turned or milled on CNG lathes and milling machines, again, each bank tended by highly trained master mechanics and their apprentices.

Often the finish work on plastic molded parts is done on the CNG machines as well.

When the components are all finished, assembly begins. Assembly is done in “clean-rooms” by experienced technicians, most with years of experience. Our optics are essentially hand assembled. Each part is cleaned one more time, and then fit into the body or lens assembly by hand, and each component, as it is assembled, is tested for correct assembly and alignment. Testing procedures take close to half of the technicians’ time. A component or module only goes on to the next stage of assembly when it has passed its individual quality assurance tests.

Again, the design of the quality assurance tests, the testing jigs and apparatuses, requires as much attention as the design of the product itself.

Assembly of a binocular can require 8 hours, during which time it and its components are handled by 15-20 technicians.

Once a binocular is fully assembled it passes through a separate Quality Assurance Department, where, again, trained technicians test is overall performance to make sure it meets specifications and design criteria. We operate a veritable torture chamber for optics, exposing them to conditions and demands several times more severe than we expect our customers to ever experience in the field.

Of course, we use only the highest quality materials, and employ the most up-to-date machinery, but when building the finest optical instruments, there is no substitute for this kind of hands-on attention to detail and quality. Machines alone just don’t do the job: it requires a human eye and a human touch, and a human commitment to quality at every stage of the process, to produce the kind of optics that Zeiss is known for, worldwide.

We are confident that our customers experience and appreciate the difference in quality. Most, however, are not aware of the hundreds of individual hands and minds that make that quality possible.

So, next time someone asks you, “What makes Zeiss binoculars so good?” (Or, for that matter, “What makes Zeiss binoculars so expensive?”) remember the hundreds of hours of labor, the hundreds of careful hands and conscientious minds that are reflected in every Zeiss product.

We are Zeiss. We have been producing high-quality optics for 158 years, employing our hands and minds to push back the boundaries of optical excellence. We don’t know any other way to make the kind of optics we can put our name on.

Or at least that's the way I see it.

The Truth about Contrast

Full Natural Contrast Range

Image Contrast in Zeiss Victory FL Binoculars and the Competition

Stephen Ingraham: Zeiss Birding and Naturalist Product Specialist

Critical observers sometimes comment on the difference in “contrast” between our Victory FLs and competitive models. Some users see the image in the Leica Ultravids and the Nikon HD binoculars, especially, as having higher contrast than our binoculars.

Some users will describe the difference not in terms of “contrast” but in terms of “brightness”. They may say the Leicas or the Nikons appear “brighter”. Close questioning and side-by-side comparisons almost always reveals that what they are seeing is a difference in apparent contrast, not in brightness.

Highly sophisticated observers also note that there seems to be a brightness difference between the Zeiss Victory FLs and the Ultravids and Nikons, with the FLs appearing slightly brighter in all situations, but having “lower contrast”, which is the same conclusion those who originally commented on “brightness” (as above) come to after a more careful, side-by-side comparison.

By “contrast”, in all cases, users are referring to the apparent difference between the light shades and the dark shades in the image, or how distinct the differences are. They are saying, in effect, that blacks are blacker and whites whiter through the competing binoculars: reds are redder, blues are bluer, greens are greener, etc.

In reality, the Zeiss Victory FL binoculars have a higher contrast range than the competition, while being brighter at the same time. We call our contrast range “full natural contrast.”

Think of working with an image in Photoshop or a similar image editing program. To increase apparent contrast, you could, if you were comfortable with the “levels dialog”, go to the levels control and pull the sliders in from both ends, reducing the number of different shades of dark and light by clipping off the ends, and then you would move the center slider slightly toward the light end of the spectrum (emphasizing dark shades at the expense of light shades). Visually it looks like this.

Full natural contrast range, with 256 levels of gray and of each color.

Clipping off the ends of the contrast range reduces the number of intermediate shades of gray and each color, but makes the steps between the adjacent shades “higher” and, to the unstudied eye, easier to see. You move the blackest blacks down into the dark grey area, the reddest reds down toward lighter reds, etc. and the whitest whites up into the light gray area, the lightest reds up into the pinks, etc. Then you slide the center to the left, emphasizing the darker shades, because our eyes interpret “darker” scenes as having higher contrast anyway.

Note, however that while the adjusted image of the bird looks more “contrasty” you have actually lost detail or information in both the lightest and darkest areas of the image, some of the subtle shades of color, and some brightness.

Sound familiar? Look for yourself. The difference in detail, color discrimination, and brightness is relatively easy to see in the images above.

What the Zeiss Victory FLs provide is, as near as is technically possible, a completely “natural” contrast range, wide open to both ends, the blackest blacks and the whitest whites, the reddest reds, the bluest blues, the greenest greens. We manage this without compromising the brightness of our image.

We could adjust our contrast range, by playing with our coatings and glass types, to produce the “high apparent contrast” image common in other optics…but why would we do that? Why would we sacrifice image detail in the dark and light areas of the image, color discrimination, and brightness overall, to achieve an increase in contrast that is only apparent?

In fact, over time, our users come to appreciate and expect the full natural contrast image that the Zeiss Victory FLs provide…so much so that other optics appear “clipped” and “dim” by comparison.

“Full Natural Contrast Range”: it is a Victory FL difference you can see.

Or at least that's the way I see it.

The Truth about Housings

Okay, so there is just no glamorous way to say this. Housing materials matter. In the design phase of a binocular or spotting scope, as much consideration is given to the housing and materials as is given to the optical train. The fact is, the best glass in the world, and the most sophisticated optical designs, won’t do anyone any good if the lenses don’t stay were they need to be, if the optic breaks on its first contact with the hard edges of the real world, or if the optic weighs so much that it spends all it’s time on a shelf in the hallway closet.

Housing materials have to be

• Rigid enough to maintain alignment of the optical elements. This requires not only that the material hold a stable shape, but that it be possible to cast, machine, or mold very precise, incredibly precise, mounting surfaces into it for the various lenses, and that the assembly folks can somehow “lock” the lenses into position once they are there.
• Impact resistant. The material needs enough resilience or “bounce” to take a blow without breaking or denting. Also the more resilient the body material is, the less of the impact is transferred to the optics inside. Note the relationship between this and number one: rigid. As you might suspect this apparent contradiction leads to some interesting compromises.
• Durable: wear of moving parts can also cause misalignment of the optical train and any misalignment will affect the performance of the optics. Then too, you want a body that will stand up to the wear and tear of general field use.
• Thermally stable: yes, almost all materials stretch and contract with changes in temperature, and with the tight tolerances involved in optical design, that can be enough to throw alignments off. Thermal stability effects handling features like focus ease, hinge tension, etc. as well.
• Easy to work using modern manufacturing techniques. Cast, molded, stamped, turned, machined, and every combination of the above…time is money and the harder a material is to work with, the more separate steps required in its preparation or formation into a housing, but more it ads to the cost of the optics.
• Cost effective: there are realistic limits as the amount consumers are willing to pay for high quality optics. There are super materials out there that would make great optical housings…but no one is going to pay the price for optics housed in them. You can count the number of optics with titanium bodies on the fingers of one hand (on the finger of one hand as far as I know, and that was a compact, and a pricy special edition at that). Sometimes it is too difficult to justify the benefit to the customer that the extra cost would buy.
• Consistent in touch and texture, in feel, with the quality level of the optics incased. We all love the feel of quality, and recognize it, just as we recognize the feel of “cheapness.” You can’t put exemplary optics in a housing that feels like something off the bargain shelf…no matter how wonderfully it does all its other jobs. And, unfortunately, one part of the housing that doesn’t feel up to the standard we expect makes the whole optic, in many minds, suspect.
• Light enough to carry all day in the field. Contrary to some popular belief the glass in optics accounts for the vast majority of the weight. It is never a good idea to compromise on glass however, and the modern lead and arsenic free and fluoride ED glasses are already as light as they are going to get, so the body material and the weight of the body become items of intense scrutiny. In the birding market grams can make the difference in a person’s buying decision. Competition for the lightest binoculars is fierce. Here again, however, we run into the nature of things…light weight is a direct contradiction of all of the above, and the source of most of the compromises made in housing design.

So what do they use for housing materials.

The original materials were brass and steel. Brass for the body of the optic, steel where needed for durability or precision, generally in moving parts. Thin sheet brass can be rolled into simple tubes and rolled or stamped into relatively complex prism coverings with some ease. Brass can also be cast into complex shapes where needed, though it generally requires milling after casting to achieve the levels of precision needed for optics. Brass bar stock can be turned, drilled, milled, machined and threaded into precision mountings for lenses elements. Thin steel rings, machined and treaded, then hold the lenses in place. Add a steel hinge or tripod plate for durability, cover the whole thing with a thin layer of leather or a coat of baked on enamel or black lacquer and you have what for years was the state of the art in housing design. Brass is durable, inexpensive, relatively rigid, moderately impact resistant (it dents easily), and relatively heavy.

As aluminum technology developed for the aircraft industry, aluminum became available for optical housing design as well. The first aluminum bodied binoculars and spotting scopes appeared before WWII. Aluminum is mainly useful for parts that are cast and then milled to add precision surfaces. It is relatively rigid in casts of proper thickness, has poor impact resistance (it is strong, but not very resilient). In castings it can break before it deforms and it transfers most of the shock of impact to the optical train. In thin walled sheets…well think of an aluminum can...once it bends it is bent, period. It is not the most thermally stable material going. It requires very high temperatures and a great deal of energy to work or cast, and it is not, despite all those cans, all that inexpensive. Still, it is light, compared to brass in equal parts and forms. Aluminum castings are porous, and can contain air pockets. They require sealing by liquid immersion. Modern aluminum alloys can overcome some, if not all of the drawbacks of pure aluminum, but, of course, at additional cost.

Plastic. Oh yes. Plastic. (I am referring here to garden variety plastic…not Glass Fiber Reinforced Polyamide or Polycarbonate which we will cover below.) Plastic is easy to mold and work, not very durable, not very rigid, in some formulations it has relatively high impact resistance (the tough and bendable plastics) but is not rigid enough, in other formulations it is rigid enough (but lacks impact resistance hard and breakable, it seems to be very difficult to formulate a plastic that has all the needed properties), is very easy to work once the quantities get to up to the level where injection molding becomes a possibility. The difficulty is 1) it is not very light for its strength, and 2) it can not be used to form precision parts. Still, there are more binoculars sold with plastic bodies today than any other material…quite likely more than all other materials combined. (As a reference, the average optics sale in the US at one of our full line sporting goods retailers is still under $100. That is the AVERAGE sale, among all optics sold over a year’s time.)

Magnesium: Very similar to Aluminum, with many of the same advantages, lighter weight, better impact resistance, and higher cost. Like Aluminum, Magnesium castings are porous, and, the surfaces are highly reactive…they interact chemically with other materials used in the construction…and require many coatings of some kind of shielding lacquer where parts meet. Magnesium also does not hold up well under exposure to salt water. Still, given its weight advantage, some of the top-of-the-line optics from the major makers are in magnesium housings.

Glass Fiber Reinforced Polyamide: the other high tech alternative. GFRP is not, as above, your garden variety plastic. It is used in Formula One racing cars and performance aircraft for its combination of rigidity, impact resistance, stability, durability, workability (it can be precision injection molded) and weight. Its resilience is such that it is used in automobile bumpers. Using an example I can speak of with authority, Zeiss has been using GFRP in housing design for over 50 years, cooperating with major GFRP manufacturers to develop highly refined techniques, and I know that we are not the only maker with experience with the material. The technology has advanced to the point where designers can specify which parts have higher glass fiber content, for higher strength, and which parts have lower glass fiber content, for more resiliency and lighter weight. Anyone who uses GFRP is not using it to save money, anymore than those using magnesium are doing so. They are using GFRP because of its inherent strengths as a body material, and because it lends itself to a kind of holistic design philosophy where the housing is actually seen as part of optical train, and designed from the word go compliment and support the lens system.

So, what is the best housing material? Given the choices today, I am not sure, once you get beyond the plastic body, that there is much to choose between. Each of the high tech solutions has its strengths…and, honestly, once they get the armor on, who knows or cares what is underneath…as long as it works. Rigid, impact resistant, durable, light weight, that’s what we want, and that is what we get in any of today’s top designs.

Some data:

Specific weight (in grams / cubic centimeter):
Steel = 7.5 / Alu = 2,8  / Mag = 1.8 (but needs higher wall thickness compared to Alu or GFRP !) / GFRP = 1.7

Or at least that's the way I see it.

The Truth about Color Fringing

White light, ordinary daylight, is a mixture of all the individual colors in the rainbow. When light passes through glass, even the glass of the best lens, each color of light is bent at a slightly different angle, so that daylight is broken up into its individual colors. We call that “dispersion.” White light is “dispersed” into its individual colors. Because of the difference in how much each color is bent, no lens can bring all three primary colors to the same focus. Therefore you get fringes of out-of-focus color around every object in the image. They show up best at the high contrast edges of things. We call this effect: Chromatic Aberration.

Way back at the dawn of glass optics, designers discovered that using two different kinds of glass in lenses carefully ground and matched to each other, and then cemented together, could “cure” much of this aberration. In essence, the first part of the lens produces a normal “dispersion” of the light into its spectrum (rainbow), but then the second lens is ground to compliment the first (bowing in where the other bows out) and so to reverse the dispersion—to, in effect, pull the different colors of light back to the same focus. Carefully selecting the refractive index (the amount the glass bends the light) of the two lens elements produces a lens that is more or less color corrected for two of the three primary colors. We call such a cemented, color corrected lens an achromat, or an achromatic doublet.

The human eye is very forgiving. We tend to tune out the narrow greenish-yellow and reddish-purple lines on either side of light and dark edges that remain in even the best achromatic lens designs. I had to actually teach myself to see it in binoculars, after readers of BVD complained that I was not commenting on it. (Do yourself a favor. I you don’t see it, don’t teach yourself to. You will regret it.) However, the reality is that even at low powers, all that unfocused color tends to “muddy” the colors and reduce the visible detail in the image. It is not an effect you notice without comparing the achromat to a system with a better design, but it is visible when you do, primarily as an added “snap” or “vividness” to the whole image. The better corrected a system is, the “cleaner” the colors, and the whole image, will look.

As magnification and the focal length of the objective (the big lens) increase, however, as in a spotting scope, the color fringes themselves become objectionably noticeable. The longer the focal length of the objective, the more space the colors have to spread, and, of course, the fringes and the muddying effect are magnified right along with the image. Still, a properly designed and manufactured achromat can produce very satisfying images, especially at lower powers.

Spotting scope designers, and especially long camera lens designers, needed a better solution. Films (and now, digital sensors) are as unforgiving as the human eye is forgiving…every color fringe leaps out in a photograph (as many a beginning digiscoper has discovered to his or her grief), and the overall lack of vivid color is really noticeable in high power photos. And, as above, even the human eye and brain reaches the limits of its tolerance for aberration when the powers increase over about 40x. Designers began to experiment with “low dispersion” and “extra-low dispersion glass” ("ED”). ED is glass which has a high “heavy metal” or “rare earth” content, making it very dense, and keeping the various colors of light as close together as possible. Using ED in an achromatic doublet design, or, better yet, in an air spaced triplet design (where the light passes through three lenses before focus), can correct almost all of the chromatic aberration, and produce a very “clean” image.

Unfortunately, ED glass has it drawbacks. The primary one is weight. The same density that gives it its low dispersion also makes it quite a bit heavier than other glass. The heavy metals that were at one time used in its manufacture are environmentally dangerous, and it had, to my eye, a slight yellow cast . More modern formulations of ED have eliminated most of these problems, except for the weight.

Sometimes other “low dispersion” materials are used to the same effect. The most common is Calcium Fluoride or, as it is known, Fluorite. Fluorite crystals can be grown big enough in the lab to grind into lens elements of up to 100mm. It is not an easy material to work with, being both fragile and relatively toxic. It is not very stable with changes in temperature either. There are persistent rumors of fluorite elements cracking under field use, though I have to say I have heard of no confirmed reports.

Recently a third alternative has become available to optical designers. FL glass, a special optical glass which is enhanced with fluorine ions, has the advantages of both ED and Fluorite, but without the drawbacks of either. The introduction of FL glass allows the design of systems that are light weight, durable, and which offer superior color correction.

The use of any of these low dispersion materials, especially in multiple element objective designs can produce what we call Apochromatic performance, or a lens that brings all three of the primary colors of light to the same focus. One would think then that the Color Fringing demon is licked.

Unfortunately, to complicate matters, modern wide field eyepieces and optical systems that use them also produce some color fringing of their own, especially away from the center of the field. This off-axis color, or secondary spectrum, is related to spherical aberration: the failure of a spherical lens to focus light that passes through the center of the lens at the same point it focuses light that passes through the edges of the lens. As I understand it, spherical aberration effects each color of light slightly differently, so that, as you approach the edge of the field, the different colors of light spread out again, causing noticeable color fringes.

Therefore the best you can hope for in current modern optics is an elimination of “most” “noticeable” color fringing, at the center of the field. There will always be some visible at the edges. Still, a well designed modern system which uses ED, Fluorite, or FL glass produces an image, especially at higher powers, that is noticeably cleaner than conventional glass optics. The extra vividness increases your pleasure in the image, makes possible the differentiation of finer shades of colors (especially, for some reason, the blues), and, we are discovering, even at the lower powers common in binoculars, increases color perception in low light situations, enhancing the twilight performance of the optics.

Almost all of the high end makers produce a spotting scope that uses some kind of low dispersion material in the objective lens, often in conjunction with a multiple lens design (more than two elements). The use of ED glass in binocular objectives, while not as common, or as well advertised, is happening. Carl Zeiss Sports Optics has just introduced the first binoculars with FL glass, and a multiple lens design, in the objective, producing, what is to my (admittedly somewhat prejudiced) eye, the cleanest, clearest, most vivid binocular image currently available.

Are there improvements yet to be made? One of possible uses of “aspheric” elements in objective and eyepiece design (aspheric means a lens that is shaped to some complex curve that is not a simple section of a sphere) would be to “cure” the last of the secondary off-axis color and produce an image that is as close to color pure as possible. There are already camera lenses that have eliminated all visible color fringing, even in very long 800mm and greater telephotos, and in very complex 1-10x zooms on modern compact digital cameras. The question is: how many spotting scope and binocular customers would be willing to pay what one of those super-tele lenses cost ($8,000-$25,000) for their day-in, day-out optics, or, to put it another way, could any company hope to sell enough binoculars and scopes to equal the production run of your average compact digital camera? It is a matter of market.

Will an absolutely color perfect system be produced? It is only a matter of time, improved manufacturing techniques, new materials, and volume. It will be done. Who will do it? Well that will be the fun part to watch.

Try this:

• Choose any roof prism binocular.
• Look at a high contrast object: one with very dark horizontal or vertical bars against a light background is best. The bars of a window against the light from outside works very well, as do the sharp shadows thrown by architectural edges in buildings in full sun.
• As you focus on the vertical or horizontal edge you will see thin bands of color outlining the edge, similar to the fringes in the photos on the reverse side. If the dark object is thin enough you will see different color fringes on each side of it. If not, swing the binoculars to place the edge on either side of the center of the field and you will see the color change.
• Well corrected optics will show very thin color fringes near the center of the field, with the fringes becoming much wider and more visible as you move the high contrast edge toward the edge of the field, but all optics, especially roof prism binoculars, will show some color fringing.
• Note the brightness, and width of the color fringes near the center of the field and toward the edge of the field.
• Now try the same test with other roofs. Try ED glasses if you can find them. Definitely try the new Zeiss Victory FLs. Can you determine which ones have the best color correction? Does it, to your eye, correlate well with the overall cleanness and vividness of the colors. How does it correlate with the twilight performance of the optics.
• Try the same test with spotting scopes at powers over 40x. With spotting scopes it is relatively easy to find a variety of low dispersion designs.

Or at least that's the way I see it.