When you have a rare bird in the eyepiece of the spotting scope and are trying your best to thumb through a field guide to confirm the identity, all before the bird flies away, the last thing you need is to spend time changing the eyepiece on the spotting scope when you need more magnification. Remember, that bird in the eyepiece may be one you will not see, again, for years or perhaps never see again in your life and the sooner you can confirm an identification, the better. Seconds can count. A zoom eyepiece allows you to quickly zoom in as needed before the bird leaves.
Zoom Eyepiece Quality
That's the good news about zoom eyepieces. The bad news about zoom eyepieces is the cost. It is very expensive to make a quality zoom eyepiece. Why is this important? Contrary to what many beginners think, there is more to a spotting scope than the size or type of glass in the front (objective) lens. Truth is, the eyepiece on a spotting scope is as big or bigger a factor in image quality than the objective (front) lens of a spotting scope. After all, the eyepiece is half of the spotting scope! Cheap zoom eyepieces are notoriously poor in terms of image quality and ease of use with eyeglasses. If tempted to get by cheap and just upgrade to a better eyepiece, later, be warned that you do not have that option on inexpensive and even most mid-priced spotting scopes. Spotting scope eyepieces are NOT interchangeable between brands or even models within a brand. You may wish to read that, again. In other words, if the manufacturer does not list optional eyepieces for the bird scope model you select, you are stuck with the eyepiece supplied and, on inexpensive spotting scopes, that is almost universally an inexpensive zoom eyepiece. This means that, if you want a better eyepiece, you must buy a better spotting scope. Getting a different eyepiece is not an option until you get into more expensive spotting scopes.
When we move into more expensive spotting scopes, usually around $500, we begin to see optional eyepieces and also better zoom eyepieces as standard equipment. When given a choice between a fixed power eyepiece and a zoom eyepiece for birding, I still recommend the zoom eyepiece - a zoom is still the most versatile eyepiece. If you plan to take pics of birds by digiscoping (Digiscoping Update), though, a fixed power eyepiece is a better choice. Since I do both on every trip to the marsh, I carry both types of eyepieces for my spotting scope. If you cannot afford both, go with the zoom, since you can still use a zoom for digiscoping. Add the fixed power eyepiece as a second eyepiece.
As a last note on eyepieces, be careful when pricing. Many premium grade spotting scopes are sold as "body only". When you see "body only", it means just that. You still need to purchase the eyepiece as a separate item and add it to the price.
To ED or not ED?
ED, HD, APO, fluorite are special optical glasses that improve performance on a spotting scope by reducing or eliminating an optical defect called chromatic aberration (color fringing). As a result, an ED or other special glass spotting scope, will offer better image quality than the same model without ED. However, just adding an ED element to a less expensive spotting scope (a current market trend) will not turn it into a premium spotting scope. A poorly ground and polished lens is still a poorly ground and polished lens, regardless of the glass used. Then, too, placing an ED objective in front of a mediocre eyepiece is going to be a questionable improvement. Remember, the eyepiece is at least half the scope in terms of performance and image quality. When in doubt, price is always the best indicator of quality and performance in a spotting scope, not the presence of ED, HD and so on. A premium spotting scope with a standard glass lens and great eyepiece will easily outperform a less expensive model with ED.
Is going to an ED option on a given model of spotting scope worth it? That depends. You will see a difference with ED, HD and so on, visually, but only at higher magnifications above 40x or so. If you plan to use your spotting scope above 40x on a continuous basis and/or for tough bird groups like shorebirds, I recommend an ED option. For lower magnifications and/or easier bird groups, a standard lens is fine. On the other hand, if you plan to take pics (digiscope) with your spotting scope, going ED, HD and so on will make a difference in pic quality at any magnification. ED, HD and other special glass spotting scopes are highly recommended if you will be using your spotting scope with a camera.
Angled or Straight?
Angled or straight on a spotting scope is a matter of body design, not the eyepiece, as some believe. It is a feature built into the body. You cannot convert an angled body scope into a straight or vice versa by changing eyepieces or adding adapters. You must choose this feature up front when you buy a spotting scope. That said, there is no wrong choice, here, just a matter of some advantages and disadvantages. In the end, it as much a matter of personal preference, as any. The majority of serious birders, by a slim margin, use angled but go with what works for you. I have both.
Straight bodied spotting scopes are easier to line up on target for beginners and also balance nicer if you attach a heavy SLR camera on the back for photography. Straight bodied spotting scopes are also a better choice for looking down at objects, as you might from an elevated blind of observation tower. Straight bodied spotting scopes are also much easier to handle and use when viewing from a vehicle with a window mount.
Angled bodied scopes are more comfortable to look through for longer periods of time, since you are not squirming down to look through the eyepiece. Angled bodies spotting scopes are also more stable, because they can sit lower on the tripod. Angled scopes are also easier on the neck when looking at objects at an elevation, as in birds in distant trees or mountainside. Lastly, angled bodies scopes, if supplied with a tripod collar (a ring around the center of the scope that allows the scope to rotate while on the tripod) and are a better choice if you are sharing the scope with people of different heights, since you can adjust the eyepiece to the side for shorter observers.
For the same reason as using a zoom eyepiece - speed - a spotting scope that focuses, quickly, can be a real advantage for birding, especially if you are viewing birds at varying distances. Most bird scopes are, in fact, small refractor telescopes and focusing speed is rarely an issue with this design. On the other hand, there are other telescope designs sometimes offered as spotting scopes, namely Maksutov-Cassegrains (Maks) and Schmidt-Cassegrains (SCTs). Be warned that these designs are very slow to focus and are thus marginal choice for birding. These 'telescopes' type spotting scopes do offer the advantage of easily changed eyepieces, but the telescope zoom eyepieces that fit in these models invariably produce too much magnification to be practical for a spotting scope.
Birders are less likely to use spotting scopes in severe weather than, say, hunters, but a waterproof spotting scope is still recommended, since a waterproof model is also better sealed against dust and dirt. This is actually a non-issue, though, since nearly all spotting scopes, these days, with the exception of a few inexpensive models, re waterproof, anyway. Check the specs to be sure.
Many spotting scopes include or offer as an option, a "view thru" or "field" case that allows you to use the spotting scope with case installed, usually by means of flaps that fold back on front and back of the scope to expose the objective and eyepiece. The idea, here, is added protection. Note that these are very model specific accessories and must be an exact match for model of spotting scope, even to the point of specifying angled or straight. In short, there is no interchangeability between models when it comes to a field case. However, I don't regard a field case as an essential item. Many birders are religious about using one, but I, personally, find them to be annoying and use the case, instead, for protection during transport. Mine usually stays in the car. Each to their own.
As always, any spotting scope can attach to any photo tripod, but that does not make just any photo tripod a good match in terms of performance. The best spotting scope in the world is only as good as the tripod under it, so do not be tempted to cut costs with this important accessory. Bottom line is that there is no such thing as a good tripod under $100 and it only takes one session with a rickety, cheap tripod to learn this truth. Keep in mind that the larger the spotting scope, the heavier the tripod needed to support it, adequately. For 60mm class spotting scopes, the Nikon 848 or a Bogen kit (must say kit) with the 700RC2 head will work. For 80mm class spotting scopes, I strongly recommend a Bogen tripod kit with either the 128RC head or the 710RC2 head.
Most birders use a two way "video head" on their tripod, which offers simple right-left and up-down adjustments. These offer maximum stability and balance, since the head sits lower on the tripod. Some birders like ball heads and a ball head does allow you to move the scope, quickly, in any direction, but that comes at the price of stability and weight, since a ball head places the spotting scope higher off the tripod and to be effective with a large spotting, you must use a heavy, large ball head. A small ball head is a disaster with an 80mm spotting scope.