If you purchase just one piece of optical equipment for birding, binoculars are the right choice. But if you spend a lot of time viewing loons, waterfowl, shorebirds, gulls and other species that are either wary or in hard-to-penetrate habitats, a spotting scope can be extremely useful. Spotting scopes are also the instrument of choice when 'digiscoping' holding or securing a digital camera to the lens of optical viewing equipment to get a close-up photo of a distant bird.
What's the difference between a spotting scope and a regular telescope? Spotting scopes have much lower magnification - typically between 15x and 60x. You can view stars with a typical spotting scope (though you won't see as many as you could through an astronomical telescope), but you can't view birds with a typical astronomical telescope, which simply is not designed to have the light gathering ability or the field of view necessary for finding and viewing birds.
If you're just starting out birding and need to purchase binoculars and a spotting scope both on a limited budget, it's wiser to put most of your money into good binoculars, which you will use far more than your spotting scope. But if your budget allows, and especially if you plan to use your scope for digiscoping, you'll be happiest getting the best spotter scope you can afford, too.
What considerations should you think about when selecting spotting scopes?
* Size: As with binoculars, the size of the objective lens (the larger, outer lens) of a spotting scope determines how much light it gathers. If you're on a budget and buying a lower- or mid-price spotting scope, it's wiser to choose one with a larger spotting scope objective lens, because the brightness will compensate for some of the loss in optical quality. If you plan to digiscope, a larger objective lens will make your images brighter. Of course, the larger the objective lens, the heavier the scope. Most of the spotting scopes that serve best for birding are between 60 and 85 mm.
* Magnification: Most spotting scopes have interchangeable eyepieces, each one set to a single or zoom magnification. A single-power eyepiece will give you a superior image compared to the image you get through a comparable zoom eyepiece set to that power, so many birders buy just one single-power eyepiece, usually between 20x and 40x. Especially with lower and mid-price Simmons spotting scopes, where otical compromises are made to keep them affordable, this is usually wiser than getting a zoom eyepiece. If you plan to use your zoom spotting scope for digiscoping, the photos you take through a single-power eyepiece will be far superior to those taken through a zoom. In my personal experience, 30x or 40x can make a wonderful all-purpose eyepiece. I keep my 30x on my scope virtually all the time.
Of course, zoom eyepieces do have some advantages when a bird is very far away. But as you zoom in on a bird, you lose light and the scope's eye relief drops dramatically. Many birders who wear eyeglasses find that they simply cannot use the highest powers at all unless they take off their glasses and get their eye right next to the optical lens.
* Fluorite or ED glass: is far more expensive than regular glass, and provides a far superior image. You particularly see an advantage to better glass under low light conditions,or when digiscoping.
* Focus Adjustment: More and more waterproof spotting scopes are now providing two separate focus adjustments, one for quickly bringing the subject into rough focus, and the other for fine-tuning. This is an excellent feature.
* Angled or straight? I used to only use a straight spotting scope. It's intuitively easy to point it and get the bird in view, and I couldn't imagine switching. Then I bought an angled spotting scope, and quickly thought, "What took you so long??!!" There was definitely a learning curve to get good at pointing the hunting spotting scope in the right direction. But to offset that, it is SO comfortable to look through! If the bird watching spotting scope is set at a steep angle, you can raise the spotting scope tripod neck and look straight through, rather than crunching down on your knees and looking up. If the spotting scope is set across, it's easy to set the tripod fairly low and just bend your neck and upper back to view. Especially when birding with a group, the angled scope is SO much easier for people of different heights to share.