What’s the difference between a regular scope and an ED scope?

ED means "extra-low dispersion" and refers to the composition and optical properties of the glass used for the lenses.

ED glass is specially formulated, including rare-earth compounds to greatly reduce chromatic aberration (colors not all coming to the same focal plane) compared to standard crown and flint glasses.

Besides getting rid of color fringes, ED glass scopes also yield sharper images because the fringes are no longer present to soften the focus.

EDF glass scopes have an ED glass element using fluorite, giving it even better performance and a color-fringe free across the visible spectrum.
 

Updated 12/18/13