ED Glass

Extra-low dispersion glass is a specialized optical material that bends the different wavelengths of light by nearly equal amounts, instead of splitting them the way ordinary glass does. That splitting is what produces chromatic aberration, the faint colored halos along high-contrast edges that blur fine detail. By correcting it, ED elements in the objective-lens group deliver an image with cleaner edges, better contrast, and more faithful color, which is why it is marketed as HD on many premium optics.

The benefit grows as you turn up the magnification, since aberrations that are negligible at low power become obvious at high power when you are trying to resolve a small distant target or read mirage. A scope built with ED glass typically holds detail better toward the edges of the field-of-view as well. The result is less eye fatigue over a long session and more confidence in what the shooter is actually seeing downrange.