There are 3 basic focusing mechanism designs: single knob, double knob, and helical. If you can, try different mechanisms and see which one works best for you. If you can't, then choose a knob focuser which is generally preferred for bird watching and nature viewing.
There are also rack & pinion designs which are commonly found on astronomical telescopes and usually offer fairly fast, smooth focusing, but most components are external and subject to potential deterioration from dust and moisture over time.
Color aberration is sometimes noticeable with refractor scopes. Also light transmission can be increased by reducing reflections. This can be mostly eliminated with the right kind of glass and coatings.
Look for scopes with ED (extra-low dispersion) FL (Fluorite) HD (High Density) and/or APO (apochromatic) glass. These elements will provide you with an image of higher clarity, detail, and sharpness which in turn will reduce eyestrain.
Of coarse scopes with these extras add extra cost. Another factor to weigh, but you'll be paying the cost in frustration when you can't see those details on that special bird on an overcast (low-light) day.
Spend the extra money on your spotting scope, save by making coffee at home, renting movies, skip the fast-food, make your own x-mas or birthday presents. Not eating french-fries for a month may buy you the feathered look of a lifetime!
Unless your scope is going to reside inside your home, weather proofing is not an option. You never know when that rain cloud will just pop-up out of nowhere.
Look for scopes with waterproof and fogproof (nitrogen/dry gas filled) designations. Rubber armoring is also a nice addition. It provides protection against abrasive materials, corrosion and helps cushion the scope against unexpected impacts (oops!). It also makes it more easier and comfortable to handle in cold, wet weather.
Eye relief is an important feature on for eyeglass wearers. It is the distance in millimeters between the eyepiece of the spotting scope and your eye that still allows you to see a complete field of view image in focus.
As an eyeglass wearer your eye is further away from the spotting scope eyepiece, which means a longer eye relief is needed in order to see the entire field of view.? For most eyeglass wearers, an eye relief between 12-15mm will be adequate. Without adequate eye relief, you won't get a complete field of view and find you'll need to remove your glasses in order to see the image properly.
So long eye relief promises full field viewing with eyeglasses. As mentioned previously in the eyepiece section, there are eyepieces specifically made with long eye relief. You may also want to select eyepieces that have folding or rollback eyecups so you can get your eyes closer.
Close or near focus is the distance between the scope and the nearest object you can focus on, while maintaining a good image and sharp focus.
In general, as magnification increases, the minimum close focal distance also increases. So scopes will typically have longer close focus ranges than binoculars.
For bird watching, a short close focus is beneficial for seeing details of a bird that has landed up-close to your scope. It is also better for taking photographs (see digiscoping). So if this is important to you, selecting a scope with a close distance of 15ft or less would be optimal.
So as with anything else, weigh the cost against the benefits, and get the best spotting cope you can afford. If you can't afford a Swarovski, Leica or Zeiss (top-end, most expensive manufacturers) then look at others such as Kowa, Nikon or Pentax which can have just as good designs, but be significantly lower priced. And don't forget to leave money in your budget for a sturdy tripod to support your scope, otherwise it's like putting a Mercedes on golf cart tires.
Hope you've enjoyed this review of spotting scope basics and we wish you the best in finding that perfect scope.
Digiscoping - Photography with spotting scopes
How to Choose Bird Watching Binoculars
Types of Binoculars
Binocular Repair and Care