Binocular Basics - 2: numbers

All binocular models have a a brand and model name followed by a series of numbers to describe them. This "nomenclature" can give you some important information about the binocular. However, it requires a bit of knowledge to understand how these characteristics affect performance.

The binocular model above is a Leica 7x42 Ultravid HD. Leica of course is the manufacturer, and the model name is the "Ultravid HD". The model name is derived from the Latin root "vid" meaning "to see", and HD indicates that this model contains the highest level of flouride-infused glass, offering the best possible color rendition possible. Manufacturer & model numbers are of course random and vary between models, and don't tell you much about performance. The "nomenclature", however, (the series of numbers) are consistent throughout the industry and tell the consumer a lot about the product.

The product above is an older model, a Leica Trinovid 10x42 BN. Lets consider just the nomenclature on this product the "10x42" portion - "10x" describes the power of magnification. As you'd suspect, this binocular magnifies the subject 10 times. This means your subject will appear 10 times larger or 10 times closer than it does when viewed with your eyes alone.

The image above is a representation of a subject at 1x and 10x respectively. Unfortunately, at this size the graphic is not very effective because it is difficult to see well. The most popular powers of magnification in binoculars are generally 7x, 8x, and 10x. It is common for people to assume more magnification is better than less. However, it is important to remember that you are not only magnifying your subject, but you are also magnifying all the hand shake and movement from wind, etc.

Most people cannot hold a binocular with more than 10x magnification still enough to benefit from the increased power. As a matter of fact, many are surprised to realize that when comparing binocular models within the same manufacturer's line, that they resolve details better with a lower-powered binocular. When trying to decide which binocular is right for you, it is best to use a resolution chart (the optical equivalent of an eye chart). In lieu of a resolution chart, a dollar bill (or similar) usually offers suitable fine print for this test. Simply, hang one to the wall and compare which binocular power within a given line allows you to discern more detail. For me, I find I can discern more detail with a lower-powered binocular. I just can't hold a higher-powered glass steady enough to benefit from the increased power, and typically use a 7x42 binocular (7 times magnification) - I feel I see more detail on a smaller and brighter subject.

The second number describes the diameter of the objective lens in millimeters. The binocular above is an Ultravid 8x32. It magnifies 8x and the objective lens is 32 mm wide. In this binocular a 32 mm circle of light enters the binocular (the diameter of the objective), however for every 1x of magnification you must reduce the size of the circle of light entering your eye by the same power. Therefore, the circle of light seen coming out of the eyepiece & entering your eye is 32 mm divided by 8x, or 4 mm (see above). This circle of light is defined as the "Exit Pupil".

How does exit pupil affect binocular performance?.... Well again, all else being equal (assuming we are considering one particular line of a given manufacturer's products), a larger exit pupil means more light enters your eye.

In bright light, when your pupil is fully constricted, exit pupil is less important. However, in low lighting conditions your pupil dilates/expands. So an exit pupil of 4 mm may not fill every cone and rod in your pupil, where the 7x42 above with a 6 mm exit pupil will deliver a brighter image with superior resolution under these conditions. Also, as your hands shake, a smaller circle of light will be harder to stay over your pupil, while a larger exit pupil will continue to deliver consistent light to your eye. While your brain may well assimilate the image, over a full day's use your eye may feel more strain and discomfort with a smaller exit pupil, compared to the binocular with the larger exit pupil (once again assuming the same binocular quality/line).

Of course, this is but one consideration in binocular selection, and each individual will have to balance power & objective size, versus binocular size/weight & ease of use/ergonomics. This personal balance will obviously vary from user to user, but I think it safe to say that if "lightweight" and "compact" are two of your main criterion for binocular selection, it is true to suggest that within a given line, this cannot be achieved without sacrificing some optical performance. However, we will consider this fully in another post!

Binocular Basics 1

This is the first post in a series that will review binocular basics for those interested. As you'd expect I'll start at the very beginning with very basic information. Throughout the series though the information will get more complex. Feel free to comment or question in the comments section below though.

Generally speaking, binoculars are manufactured in two basic designs in regard to prism assembly and structure. The first is a "porro prism" design which is easily recognized by shape. In a porro prism binocular the eyepiece or ocular lens assembly, is offset from the distal objective lens assembly. In most cases the objective lens will sit outside the eyepiece, although some manufacturers offer compact "reverse porro prism" designs where very small objective lenses nearly touch and are much closer together than the ocular.

I've superimposed the typical shape of a porro prism assembly in red lines above while the green dashed line shows the light path as it travels through the binocular. Note how the light enters the objective lens cell, passes into the upper prism, is reflected 2 times, passes into the lower prism block, bounces two more times and finally exits through the eyepiece.

Roof prism binoculars are quite different in structure and are generally favored for their superior ergonomics. In a roof prism binocular the ocular and objective lenses are lined up in a straight line (not off-set as in porro prism designs). Once again, I've drawn a rough roof prism assembly in red and the dashed, green line represents the path of light travel through the system.

As always, the light enters through the (typically larger) objective lens and travels up to the prism assembly. The assembly (in this case) is compromised of two glass blocks just as in the porro design at top. Unlike the porro however, a roof prism assembly reflects the light 5x. The area where the two blocks meet is called the "interface mirror", and this surface has to both reflect light at points and allow light to pass in others. It is very difficult to accomplish this without losing light through back reflection and/or refraction though, and the angles are more crucial than in simpler porro designs. As such, roof prism binoculars average more expensive than a comparable porro prism.

This higher price adds some very real advantages though to include: superior ergonomics, better waterproofing, and a higher impact resistance (again assuming a comparable level of manufacture and design). Roof prism binoculars generally offer better ergonomics through their slimmer design, as porro prism binoculars average bulkier due to the offset design.

Most porro designs utilize a moving external bridge (see above) that slides the oculars in and out to achieve focus. Once again, this is a simpler and cheaper manufacturing design, however it suffers from one predictable flaw. As these tubes slide over top of one another you are continually wearing the seals that keep moisture out and inert gases in. Over time these will wear to the point of failure leading to moisture problems and internal fogging. In addition this external bridge is very susceptible to impact damage, leading to alignment problems as well.

In most roof prism binoculars the focusing system is internal. In the case of the Leica Ultravid above, internal "field collector lenses" move back and forth between the prism assembly and objective lens cell. Since this system is completely internal it is much more impact resistant, and since you are not constantly wearing the seals through abrasion the roof prism design maintains its water proofing & fog proofing.

Summary

As a final note, I need to qualify implications above. I've used words like generally & typically above and also used qualifying statements like, "assuming comparable quality". In most cases in optics the old adage, "You get what you pay for" is true to its word. In looking at binocular prices quickly online, I note full-sized porro prism binoculars (compacts excluded) range from just over $25 up to about $1,200 US. By comparison roof prism prices ranged from about $50 on up to nearly $3,000 US. Assuming less expensive binoculars are created by cutting costs on raw materials (quality of components), manufacturing costs (quality of construction), & labor costs, I certainly expect that a $1,200 dollar porro prism would out-perform a $100 roof prism in every way. However, if we compared the highest quality premium roof prism versus the highest priced porro prism, I'm certain most users would prefer and gravitate toward the roof prism. I say this from my own personal preferences, my experiences of meeting and birding with thousands of birders annually (and noting what binoculars they are using), and from my knowledge of the many independent reviews that have appeared in industry articles in varying popular publications. All suggest roof prism designs are favored across the board by most users.

Canopy Critters from Panama

male Brown-throated Three-toed Sloth resting in a tree near Canopy Tower

In an earlier post I'd mentioned how Canopy Tower was an ideal location for digiscoping, but I didn't explain fully why that is. I will try to explain this more thoroughly and share some of the many pictures I was able to capture from the tower as well! Let me backtrack and review a bit for those that are perhaps just joining us. Digiscoping (at least as I practice it) is coupling a digital point & shoot camera like the Leica C-Lux 2 that I used behind the Leica APO Televid spotting scope for these pictures. Again my primary aim here (pun intended) is to capture wildlife images with the spotting scope that I carry most of the time in my pursuit of birds and wildlife anyway. As such, by simply adding a tiny camera and digital adapter to a hip pouch or shoulder strap, I'm ready to take stunning images at a moment's notice!

Hoffman's Two-toed Sloth in trees at the Canopy Tower

The Canopy Tower offers an ideal situation for digiscoping not only because you are in the tree tops in a safe, stable structure with the creatures that utilize the canopy, but because many of these creatures are anywhere from 50 to 150 feet away. With patience most will come close enough for standard SLR and telephoto lenses, but I tend to be impatient and love the fact that this digiscoping set up allows me to achieve the equivalent of a 6,000 mm lens (120x magnification)!

Geoffrey's Tamarin digiscoped from Canopy Tower, July 2007

This allows me to get up close and personal with the wildlife even when it is farther away. The Geoffrey's Tamarin above was actually much closer but I still shot this at a 35 mm equivalent of a 1720 mm lens because I wanted to see it's whiskers! ;)

EXIF data:
35 mm equivalent on camera = 86 mm
scope zoom eyepice at 20 power
or 86 mm x 20x = 1720 mm equivalent lens.

Sloth hangs out at Canopy Tower

To put that into perspective, the longest telephoto lens readily available on the commercial market is an 800 mm lens. So even at nominal camera zoom I'm far exceeding the capabilities of most telephoto systems. Plus, an 800 mm lens can reach over 2 foot in length, weigh over 20 pounds, and force you to have to re-mortgage your house. ;p The Sloth above was actually scratching but I've entitled this shot "the Thinker!"

Mantled Howler Monkey (male) hangs down to get to Cecropia fruit

There's always a good assortment of mammals "hanging out" in the trees surrounding the Canopy Tower. Both of the aforementioned sloth species, preferred the loner approach but the troops of Monkeys always seemed to come through en masse. The small Tamarins were less numerous and came and went quickly during our stay, but the Howlers were regular and numerous throughout our stay. One guest told me that the large wound on the side of the male above was a Bot Fly larvae, but don't know this for certain. Seems plausible. He explained that Vampire Bats can often make the initial wound and allow the Bot Fly to get under the flesh. Makes for a good story but I can't say for certain if this is the case.

Does anyone reading know more about this relationship or hypothesis?... would love some feedback.

male Howler Monkey stares intently at some distant subject

The ever vociferous and social Howlers were a treat to watch. Even though they are a common species throughout much of the Neotropics, I don't see these in the wild here in Florida so it was a great treat.

female Howlers with tail at the ready feeds on tree top fruit

The female Howlers were notably slimmer and smaller than the mature males lacking the distinct "mane" shown by the latter. The animal above was taking full advantage of fruiting Cecropia that surrounds the Tower. It is also worthy of note that this is not a tower in the typical sense (a narrow spire sticking up) but more of a very wide 4 story building with large observation deck ringing the top. This allows easy panoramic viewing options in any direction which is also advantageous for viewing the birds and wildlife moving through the tree-tops. It's a very unique and beautiful perspective for observing the "Canopy Express".

Mantled Howler Monkey digiscoped July 2007, Canopy Tower

These are merely a few of the many species that could be viewed by the tower and to see and digiscope them all will undoubtedly take more trips! I will share more from our 8 day trip, never fear, but for now I'll wrap it up by saying the trip was more fun than a Barrel full of Monkeys (or better yet a wild troop in their natural habitat), and we are already planning on offering this workshop again next year.

I can't wait!