Whether you’re considering the purchase of a binocular or a spotting scope, you’ll want to know the optical device’s magnification. Magnification, or “power” as it’s sometimes referred to, is a number indicating how much larger an object appears when viewing it through the optic versus the naked eye. Thus, an 8x42 binocular magnifies your view eight times.
Magnification is the first number indicated in a binocular specification, e.g., 6x32, 8x42, and 10x50. These are considered fixed magnification because they have a single power. However, nearly all spotting scopes (but only a minority of binoculars) have variable magnification, more commonly called zoom. Zoom magnification is expressed as two numbers from lowest to highest power. Examples would be 15-45x60 and 20-60x80, the two most popular spotting scope configurations.
It’s somewhat regrettable in optical jargon that “power” is synonymous with magnification. In everyday vernacular, adding power to something is generally deemed a desirable thing. However, a binocular with higher magnification doesn’t necessarily translate to a better view. Sure, objects will appear larger (closer to you), but high magnification binoculars are generally less easy to hold steady. This can have a critical impact on your ability to discern detail. Most individuals are fine with 6x and 8x, fewer can hold a 10x steady, but binoculars over 12x should probably be mounted on a tripod. This is why spotting scopes must always be mounted on a tripod; higher magnification makes them impractical to hold in your hands.
Increasing magnification can also render an image that’s less bright. At the same aperture size (42mm, 50mm, etc.), as you increase magnification the image becomes less bright, and this is more obvious at dawn and dusk. Thus, a 10x42 binocular, technically having more “power” than an 8x42, will be less bright under low light conditions. More power isn’t always a good thing!
Many spotting scope manufacturers sell eyepieces separately from their spotting scopes, offering the user a choice between fixed and zoom magnification. While zoom eyepieces are the most popular due to the versatility of being able to change the magnification for situations that demand either high or low power, fixed magnification eyepieces offer a wider field of view.