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Choosing Scopes


Next to binoculars and field guides, birders use scopes more than any other tool. The main reason is that scopes give much more magnification than binoculars. This allows birders to see detail on birds that are too distant to be safely identified through binoculars. They are designed to be mounted on tripods so they can be kept steady under normal conditions

Scope design and terminology is discussed in our All About Optics - Scopes section. If you are not well versed in this topic, we suggest you read it before continuing.

Design

There are three basic scope designs: refractive, Newtonian, and catadioptric. Nearly all scopes sold for birding are refractive. A few are catadioptric, and the only Newtonian scope is difficult to find, both in stores and in the field.

The two main factors responsible for the low use of catadioptric scopes are the cost (3 to 5 times the cost of top end refractive scopes) and the body construction does not stand up to the rigors of birding. Even though catadioptric scopes produce cleaner images at higher magnification, few birders are willing to pay the higher costs. Therefore, in the rest of this discussion we will stick to refractive scopes.

There are two body designs used in refractor scopes: angled and straight. Even here at Optics4Birding, both have their proponents.

Angled scopes are preferred because:

  • Easier to share with a group
  • Higher eye point means you can use a shorter tripod
  • Better for birds soaring or in tall trees

Straight scopes are preferred because:

  • Less strain on your neck for level viewing
  • Less chance of eyepiece collecting rain or dust
  • Easier for viewing birds on the ground or water or below a cliff
  • Easier to aim when inexperienced

To decide between angled and straight scopes, you will have to consider how you will be using yours.

Magnification

Birders use scopes because there are many instances where binoculars are not powerful enough. Scope magnifications go from a low of 15x to a high of 75x. While higher magnification is theoretically possible, the resultant exit pupil would limit successful use to only the brightest lighting situations. When the exit pupil falls below 1.33mm, image quality begins to suffer. Also, very high magnification increases the likelihood of distortion from heat waves and scope movement, as these effects are magnified along with the image.

Objective Lenses


Size

Objective lens diameters fall into four size ranges. 50mm, 60-65mm, 77-85mm, and above 85mm. In the latter range, only the 100mm Pentax ED scope is currently available. Objective lens size is the main indicator of scope brightness, as that is dependent upon the area of the objective. There have been some new advances in scope quality in the 65-66mm objective range, but these scopes are still not up to the challenge of the 80mm+ scopes in the most adverse conditions. As with binoculars, larger objectives mean more weight, but they also allow for larger magnification. When deciding on the objective size for your scope, get the largest objective you're willing to carry. Unless you really need to save weight, you will be rewarded with a superior image, other parameters being equal.

Glass

The best scopes have objective lenses with special glass that corrects certain optical problems. Labeled Extra-low Dispersion (ED), Fluorite (FL), or High Density (HD), this glass makes it possible for all the wavelengths of light to focus at (APO = apochromatic) or very near (achromatic) the same point. This has an immense affect on sharpness, shows better detail on the bird, and reduces eye strain. Many scopes come in both ED/FL/HD and non ED/FL/HD versions, with the better glass adding 50% or more to the cost, but few active birders would say the price isn't worth the improvement in view.

Eyepieces

The part of the scope that determines the magnification is the eyepiece. For a given focal length objective, shorter focal length eyepieces give greater magnification. You don't have to worry about the math, however, as most eyepieces are labeled with the magnification they provide rather than their focal length.

Advances in zoom lens technology over the last decade have taken the zoom eyepiece from the lens to be avoided to the one most favored by birders. Zoom eyepieces allow birders to scan at low power, then crank up the magnification to get the most detail. There is nothing like a 60x image that is so sharp you worry about cutting your fingers on it. Other eyepieces have their place, however. 30x and 50x (give or take a few x) wide angle eyepieces are often favored by sea watchers and hawk watchers. A 27x eyepiece with long eye relief can be a boon to eyeglass wearers and digiscopers. If you're getting a scope that can handle its full range of zoom, go with the zoom eyepiece. If you plan to spend most of your scope time at Hawk Mountain or Pigeon Point, you might want a fixed wide angle, or you might want to consider getting both.

Some manufacturers use the same eyepiece mount for both their 60-66mm scopes and their 77-85mm scopes. Because of variations in body design, some scope brands will have the same magnification for a given eyepiece with both scopes while others will have different magnifications. In the latter case, you may see a designation that says:

Zoom eyepiece 15x-45x/20x-60x for 65mm/80mm scope

This will mean that with the 65mm scope this eyepiece has a zoom range from 15x to 45x, but with the 80mm scope the range is 20x to 60x.

Focusing

There are three focusing mechanisms used in refractor scopes: single knob, double knob, and helical. Single knob is the most common. It tends to be a bit slow, but very precise. Double knob, available on Leica, consists of two knobs with different drive ratios for the same internal focusing gear. The fast knob allows for quick focusing travel while the slow knob gives precision once you're close to the proper focus. Helical focus, available on Swarovski and some Nikon scopes, consists of a collar around the scope body to drive the focusing gear. They are good for rapid focus changes. Now that variable speed focus is available on Brunton binoculars, expect to see this type of focusing on future scopes.

Photography using scopes

There are two ways to take photos through a scope. You can get a camera adapter that allows you to attach a 35mm SLR to the scope. This system will have a fixed focal length and you will need to use the camera's viewfinder as your eyepiece, and there will be no way to change the aperture. Or, you can put a digital camera up to the back of your scope. This is covered extensively in our Digiscoping section. In either case, you will get the best results by using a scope with the largest possible objective lens, as it will gather the most light.

Previous Article - Binoculars for Kids | Next Article - Choosing Tripods

Learn About Optics


Day Optics

Designs - Quality, compacts, porro and roof prism designs for binoculars and scopes...
Designations and Considerations - Designation values, eye relief, weight & cups, exit pupil, and twilight factor...
Additional Consideration - Focusing, field of view, depth of field, weather proofing and nitrogen fill...
Optic Components & Image Quality - Lenses, mirrors, coatings, aberrations, distortions, and alignments...
Spotting Scopes - Construction, Objective lens, eyepieces, angled or straight, and focusing...
Tripods - Heads, legs, monopods, shoulder stocks, and window mounts...
Digiscoping - About, power, editing, considerations, cameras, techniques, and effects...
Care & Tricks - Holding techniques, cleaning, carrying, and protecting your optics...

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Spotting Scopes - All the factors to consider when buying a spotting scope.
Tripods - Selecting the best tripod for your scope.
 
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