Birding FAQ

Birding Tips








Overview of the
Birding Scopes Market

Parts of a telescope
Focal length and magnifying power
Correcting chromatic aberration
Erecting prisms
Straight-through vs. angled scopes
Eyepieces, fixed vs. zoom
If you wear eyeglasses
Should you get the special glass?
Overview of the market
Small prismatic spotting scopes
Large prismatic spotting scopes
Small, portable astronomical scopes
Catadioptric mirror scopes
A last word

Table of birding scope specifications


What you need to know before you purchase
Once a birder has obtained good binoculars, the next optical purchase is usually a spotting scope. Compared to the 7-to-10 power you get with binoculars, the 20-plus power that birding scopes deliver will open up a whole new world. Suddenly you can pick out the horned grebe in the group of eared grebes across the marsh. And up close you can study crucial diagnostic details you may never have seen before, such as the degree of feather wear on a shorebird's tertial edges. A good scope expands the joy of sight. And honestly now, ever since you first heard about birds, haven't you secretly aspired to have the eyes of an eagle?

The question is how to choose the best scope. It's not enough merely to rely on reviews of a scope's optical prowess, because you could end up with one that's too heavy, or too fragile for your purposes, or one that doesn't work with your eyeglasses.

Also, to make an informed choice, you need to understand how telescopes work and what the words mean that are used to describe them. But that will be easy, because telescopes are really very simple devices.

Parts of a telescope
All telescopes have two essential parts. An objective lens focuses an image of an object inside a tube. A second lens, called an eyepiece, magnifies the image and presents it to the eye. (For more about this see the section from the Birding Binoculars article on how binoculars work.)

The quality of a telescope depends upon that first image, the one created by the objective lens. If it's a highly detailed, sharply focused, and bright image, it can be magnified many times by the eyepiece and still keep revealing more information. At a certain point, however, increasing the magnification does not show you any more detail. Beyond that point, you only see more clearly the fuzziness of the image. The telescope's limit has been reached. And, almost always, that limit has been set by the properties of the telescope's objective lens.

So the first thing to understand about telescopes is what goes into making a good objective lens.

Aperture is the diameter of the objective lens. Aperture is literally how big an eye the telescope has, how broad is the stream of light and information it starts with. The size of the aperture will determine the brightness of the image and set the limit for how much detail can be seen in the image. Other things being equal, a bigger aperture will always mean an image with higher resolution.

When people properly describe a scope, the aperture is the first specification given. It's usually expressed in millimeters. For example, the classic Bushnell Spacemaster is called a 60mm scope.

Focal length and magnifying power
Every lens has a focal length, usually expressed in millimeters. It's the distance from the lens to where its image is formed. When we speak of a telescope's focal length, we're referring to the focal length of its objective lens. That measurement is what accounts for most of the physical length of a telescope.

Focal length is important. An objective lens with a longer focal length forms a bigger image. The bigger the image, the less you have to magnify it with the eyepiece, and the better the final picture.

For eyepieces, the rule is different. Eyepieces work like ordinary magnifying glasses: the shorter the focal length, the greater the magnifying power. More powerful eyepieces are actually shorter and smaller in the hand.

The magnifying power of a telescope is determined both by the objective lens and by the eyepiece. To find the magnifying power, divide the focal length of the objective lens by the focal length of the eyepiece. For example, a spotting scope with a 640mm objective lens and a 32mm eyepiece would be a 20 power scope (640 ÷ 32 = 20). See why? The formula takes into account both how big that first image is and how much the eyepiece is magnifying it.

You could give a short scope the same magnifying power as a long scope simply by putting on a more powerful eyepiece. But the image in the longer scope would better. Therefore, if more magnification were your main consideration, you'd be better off to choose the bigger scope, the one with the longer focal length.

Note that, by itself, the magnifying power tells you nothing about the quality of a telescope. An unscrupulous manufacturer could advertise a 100 power scope, but the image quality at that magnification could be unusable. A telescope's aperture and focal length are much more informative.

Correcting chromatic aberration
A glass lens works by bending light. This phenomenon is called refraction. But there's an inherent problem with color and refraction. Glass bends each color in a beam of light to a different degree. This is why a prism will spread a ray of sunshine so that it becomes a rainbow on the wall: each color in the sunlight comes out of the prism at a slightly different angle.

The same thing happens with a telescope lens. If a telescope's image were focused by a single ordinary lens, no two colors in the image would be in focus at the same time. The red feathers of an elegant trogon might be sharp, but all the green feathers would be out of focus.

The problem is called chromatic aberration, and it affects all lenses. Understanding how telescope designers try to correct chromatic aberration will help you understand what you're getting when you're buying a scope.

The basic fix for chromatic aberration is to use a double lens. Two lenses, cemented together, each made from a different kind of glass, can make the chromatic errors tend to cancel each other out. The correction is not perfect, but it's a great improvement. The resulting two-part, color-corrected lens is called an achromatic doublet.

Apochromatic lenses (sometimes called APO's) go further than standard achromatic doublets in bringing the whole spectrum of colors into focus. An apochromatic lens is defined as a lens that can bring three different colors of light into focus at the same point, while an ordinary achromatic doublet does this for only two colors. APO's can be expected to show sharper images and better color, especially at higher magnifications. Apochromatic lenses are expensive: they're found only in high-end scopes, such as the new Leica Televid 77 APO.

Another important development is the use of exotic materials, such as fluorite crystal, ED glass, or SD glass, in the objective lens. These are low-dispersion materials, which don't spread the color spectrum so much as ordinary glass. They create less chromatic aberration to begin with. They are costly, but they can make a difference at high magnifications.

A completely different solution to the problem of chromatic aberration is the reflecting telescope, which uses a mirror instead of a lens to focus the image. Because the light bounces off a curved reflecting surface instead of being bent by passing through glass, chromatic aberration is never created. As a result, the best reflecting telescopes are noted for their razor sharpness and purity of color.

Erecting prisms
Another factor besides the objective lens that affects the quality of the image is the erecting prisms. Let's look at what they do.

An objective lens naturally forms an upside-down-and-backwards image. Therefore prismatic spotting scopes add a set of erecting prisms, which work like mirrors to flip the image before it gets to the eyepiece. That big bump near a scope's eyepiece is what houses the bulky prisms.

After the image gets flipped twice, once to invert it, and again to reverse it left to right, the picture looks normal. However, each time the light path passes through a prism, the image loses some contrast and resolution.

Some scopes, such as the Tel Vue Ranger, don't have the erecting prisms built in. You add your choice of image-erecting system to the back of the scope. One common choice is merely to invert the image but not to reverse it left to right. Then, because there is only one process performed on the light instead of two, the image will lose less information.

The problem with such an arrangement is that until you get used to it you will tend to move the scope to the left when you mean to move it to the right. This configuration also requires you to look down at a 90° angle into the eyepiece, which is not so comfortable as the straight-through or 45° angle eyepiece designs.

However, this is the kind of choice you must make when you buy a scope. Should you go for maximum image quality, or for more usability? Only you can decide. Here are some other choices you will face.

Straight-through vs. angled scopes
Many prismatic spotting scopes are available in two versions, depending on how the eyepiece is mounted. Straight-through models have an eyepiece in line with the front lens: you look straight ahead at the bird. Angled-eyepiece models have the eyepiece pointing upward: you look down into the eyepiece at a 45° angle.

While binoculars are personal, and all serious birders will have their own, scopes are often shared. A birder focuses the scope on a bird, takes a satisfying look, and then steps aside and offers the experience to a friend. The 45° angle models work better for sharing, because people of different heights can look through them without having to readjust the height of the scope and lose the carefully composed bird in the process. If you plan to share your scope, I recommend the angled eyepiece design.

Another advantage of the angled scope is that it's easier to look overhead. You can look higher without having to bend your neck back as you must with a straight-through scope. And the angled scope can be used with a shorter, hence lighter, tripod.

Some 45° angle designs, such as those made by Kowa, Swarovski, and Nikon, have a mount that lets the whole instrument turn to the side. This design allows you to use the eyepiece from the side as well as from above. When these scopes are used on a window mount in a car, you don't have to twist your body so far as you would with a straight-through scope. Viewing is more comfortable. Furthermore, the viewing angle ahead of and behind the car is extended.

Some people say it's harder and less intuitive to aim an angled scope, but I have not found this to be true. I think it's a matter of what you get used to. Most angled scopes have a sighting line somewhere on the scope. With practice, you'll find it works quite well.

Eyepieces, fixed vs. zoom
With binoculars, you're stuck with a fixed level of magnification. But most telescopes allow you to change their power by replacing the eyepiece. Put on a low-power eyepiece to scan a wide area. Or switch to a super-high-power eyepiece to zero in on the peeps across the lake. You can customize your scope for a particular purpose.

Zoom eyepieces let you vary the power simply by turning the eyepiece, which is much easier than juggling eyepieces in the field. However, because zoom eyepieces are more complex than fixed-power eyepieces, good ones are very costly to create. Until recently, the price of most birding scopes did not support the expense of a high-quality zoom eyepiece. As a result, zooms were notoriously inferior in optical quality.

But things have changed. When birders showed they were willing to open their wallets for good optics, manufacturers responded with high-end scopes and improved zoom eyepieces. The result is that zoom eyepieces of excellent quality are now available. For example, the latest Nikon, Leica, and Swarowski zooms have received high marks from reviewers.

Nevertheless, as good as zooms have become, it's optically inevitable that the highest quality image is still produced by a fixed eyepiece. But the difference is small enough now that most people prefer the zooms' convenience. Besides, you can always buy both.

If you wear eyeglasses
If you wear glasses, you can't get your eye as close to the eyepiece as everybody else can. As a result, you may not be able to see the whole field of view, but just the center section of it. If you're merely nearsighted or farsighted, you can take off your glasses. The scope's focusing action will replace the role of your glasses, and you will see a nice sharp image.

If you have astigmatism, however, you need to keep your glasses on. So you need an eyepiece that allows your eye to stay farther back from the lens. Such eyepieces are said to have long eye relief. Most manufacturers make at least one eyepiece of this kind specifically for glasses wearers. Fortunately, the newer zoom eyepieces also tend to have better eye relief than previous zoom designs. But you'll have to actually test the eyepiece with your particular glasses to see if its eye relief is suitable for you.

A scope without a tripod is missing its feet. All the benefits of the most expensive optics come to naught if the image isn't steady. So consider the tripod as part of the optical system, and get a good one.

A birding tripod involves a compromise between two opposing needs. It needs to be sturdy, to prevent vibration in the image. And it needs to be light, so it will be easy to carry. The limiting factor is usually what you can carry comfortably.

In evaluating a tripod for weight, be sure to try it with your scope on it. Put the whole rig on your shoulder. Does it balance? How far would you be happy to carry it? I own several tripods, and I find that I always choose the lightest one unless I'm planning to set up in a windy situation.

You have to match the tripod to your scope. For example, a straight-through scope design will require a tripod that will extend taller than your head, so that you can look up at a bird. Or you may need a heavier tripod to use your scope at its highest magnification.

Get a video tripod rather than a photography tripod. The difference is in the head, the mechanism that lets you point the scope. Video tripods have fluid heads that are made for smooth panning and tilting of camcorders. They allow you to aim your scope in any direction with only one handle. A video head's gliding motion lets you scan the edge of a lake or follow a bird in flight with a continuity of image. It's a perfect match for a scope.

Do not buy a cheap tripod. It's going to get a lot of hard use, and your scope won't work at all without it. I've seen too many people fight a half-working tripod and miss the bird.

Should you get the special glass?
One of the hardest decisions in buying a scope is whether or not to get a model made with fluorite, ED glass or SD glass. It can double the price of the scope. The agonizing choice for the birder is whether or not to cough up the extra bucks for those last few degrees of quality.

There have been many side-by-side comparisons of equivalent scopes from the same manufacturer with and without special glass. Interestingly, at lower magnification, where scopes get most of their day-to-day use, many birders cannot see any difference in the image. However, at high magnification, most people see an improved image through a scope with special glass.

My advice? For psychological comfort, get the special glass unless it's really going to break the bank. Just so that when you can't quite make out the bird you don't go, "Ohh, if only I had bought the better scope." Unless you're talking real financial hardship, you'll never be sorry you bought the best tool. The experience of birding is priceless, the cost of the tools trivial by comparison.

On the other hand, the cheaper scope will give you 95%. And 95% of infinite beauty ought to be enough for anybody. So don't fret if you really can't afford the special glass.

Overview of the market
Here is an overview of the kinds of birding scopes on the market today. Like the accompanying table, it is divided into four categories.

Small prismatic spotting scopes
This category includes the classic prismatic spotting scopes, such as the Bushnell Spacemaster. They tend to be compact, tough little telescopes, light and easy to carry in the field. The aperture is usually 60mm. They present a normal image to the eye, one that is not reversed left to right.

Most manufacturers make a scope in this category. It is the best choice if you want maximum portability. Their compact size makes them ideal for grabbing out of the back seat of the car, maneuvering past the steering wheel, clicking onto your window mount, and focusing on the bird in record time. You can usually get them in either straight-through or 45°-angle-eyepiece design.

You do sacrifice something for light weight and compactness. Although the average 60mm scope is fine in the 15 to 40 power range, it simply can't work as well as an 80mm in dim light or under high magnification. However, some of the new, high-end 60mm scopes, such as the Nikon Fieldscope II ED or the Kowa TS 613, both of which use special glass, can come close to the performance of their 80mm big brothers.

Large prismatic spotting scopes
This category contains the bigger, more expensive prismatic spotting scopes with apertures around 80mm. Because of their larger aperture, these scopes can reach higher magnifications (20 to 60 power) and will work better in dim light. But they are longer, fatter, and heavier than the 60mm scopes. Like their little brothers, they are usually available in either straight-through or 45°-angle-eyepiece design.

In this category you can often get the same model with the special low-dispersion fluorite or special glass. Other high end features to look for are rugged construction and waterproofing. For example, the Swarovski AT80 can actually be submersed in water without damage.

The downside to these scopes is their weight and size. They may require a sturdier, heavier tripod. They are also more awkward than their 60mm comrades to maneuver on and off the birding bus.

Small, portable astronomical scopes
In this category, which contains refracting scopes such as the Tel Vue Ranger, are the cross-over scopes from the astronomy market. These scopes are modular. You can choose to add a full erecting prism system for ease of use or to go with a simpler diagonal for maximum image quality. A great variety of fixed eyepieces are available, from super-wide angle to the highest usable power. This design is tops for versatility. It's an especially good choice if someone in the family has an interest in astronomy.

The tradeoff for all this flexibility is that a lot of the mechanism is not housed inside the scope. The prism, or star diagonal, which erects the image, is added onto the back. Then the eyepieces go onto that. The rig seem a bit fragile when compared to a spotting scope whose prisms are inside the scope. Like a traditional astronomical telescope, it's better for transporting it to a specific spot, setting up, and staying in one place. It's not a good knockabout design for getting in and out of the car 20 times an hour or carrying over your shoulder in the rain and dust. On the other hand, if you want the highest optical quality for the money, you can't beat it. These scopes can be used at very high powers.

Catadioptric mirror scopes
This last category contains scopes of a radically different optical design, reflecting telescopes. In a reflecting telescope, the image is brought to focus by a mirror instead of a refracting objective lens.

The kind of reflecting telescopes that are used for birding are of an unusually compact design. They are catadioptric scopes, often called Cats. Cats are a hybrid design that uses both a lens and a mirror to focus the image. The mirror does the main focusing work, while the refracting lens makes only a slight correction for the curvature of the mirror.

In a Cat, the light first passes through the correcting lens, gets reflected by the primary mirror at the rear of the scope, bounces off a tiny secondary mirror on the back of the front lens, and passes back through a hole in the primary mirror and then on to the eyepiece. All the bouncing back and forth lets you have a long focal length in a compact package. Compact is good, because not only are short telescopes more portable, but also they vibrate less when mounted on a tripod.

Replacing the lens with a mirror avoids the main problem of lenses ­ chromatic aberration.

When you look through a Cat, the birds' colors seem incredibly alive and vivid, and the details snap into place. What you are seeing is the absence of chromatic aberration. The reflecting telescope never creates chromatic aberration in the first place.

Cats are sometimes used as long-distance microscopes, because they can focus closer than conventional scopes. These little telescopes can show you a bird at close range better than if it were in your hand. You can see every fine feather detail, every mite, the texture of the skin around the eye, the reflected image of the day bouncing off the bird's eye.

The 3.5 inch aperture Questar is the quality leader in this field. Its optical performance has historically been the reference standard against which other scopes have been compared. It can resolve details beyond the theoretical limits of a scope of its size. Questar builds its instruments to order, one at a time and by hand. The fit and finish of everything about the scope is museum grade. It's top dollar, but you get what you pay for.

Much less expensive but good quality Cats are available, such as the rubber-armored Celestron C-90. Celestron also makes the 5 inch aperture C-5, which is about as large as a scope can get and remain portable enough for birding. The C-5's aperture should allow it to reach higher usable magnifications than the Questar, assuming ideal viewing conditions. In the real world, however, the turbulence created by heat waves in the atmosphere can limit the ability of large-aperture scopes to perform at their best. At such times, a smaller aperture will be better at piercing the turbulence. Sometimes a needle will do where a sword will not.

Catadioptric mirror scopes will let you use very high magnifications. Over 100 power is possible.

A last word
Buy the scope you really want. Buy the best scope you can afford. Optical instruments are not used up by looking through them: a well-built scope can last many years and will return an unparalleled amount of enjoyment. The daily cost of a $1000 scope, amortized over 20 years, is less than 14 cents. What a deal! Especially since nature provides the birds for free.

--Michael Porter

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