Stephen Ingraham (who?)

the unofficial site for birders and digiscopers using Zeiss equipment.

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.