Etienne, forgive my pedantry but, as I read them, neither of these sources supports your statement that these terms have the same meaning. 'ED', 'HD' etc refers to the composition of the glass from which the lenses are made. Apochromat describes the construction of the lens itself. The Leica scopes used cemented triplets in the objectives as I recall. This means that the lens is actually a composite of three separate lenses cemented together, one or more of which is made from flourite (ie 'ED'/'HD") glass. This serves to bring the so-called 'secondary spectrum' to a common focus resulting in a sharper, more accurate image with reduced chromatic aberration.