Magnification gets the headlines, but glass and coatings decide how bright, sharp and true-colored the image actually is. A little vocabulary helps you compare optics honestly across scopes, binoculars and spotting scopes.
The coating ladder
Every air-to-glass surface reflects a little light away; anti-reflective coatings recover it. The terms tell you how much is coated:
- Coated — at least one surface has a single coating layer.
- Fully coated — all air-to-glass surfaces have a single layer.
- Multi-coated — at least one surface has multiple layers.
- Fully multi-coated (FMC) — all surfaces have multiple layers. This is what you want: the brightest, highest-contrast image.
ED and HD glass
Extra-low-dispersion (ED), often marketed as HD, glass reduces chromatic aberration — the color fringing you see on high-contrast edges, especially at high magnification. It is the single upgrade that most improves resolution and color, which is why it matters more than a bigger magnification number.
Phase correction and prisms
On roof-prism binoculars, phase-correction coatings restore contrast and resolution lost in the prism — a meaningful quality marker. Combined with FMC and ED glass, it separates a good optic from a mediocre one.
The takeaway: prioritize FMC and ED glass over raw magnification. It shows most at dawn and dusk and at high power — see how to choose binoculars and how to choose a spotting scope.
