An Achromat lens system compensates the wavelength differences of two colors to diminish chromatic aberration.
For observation the two colors usually compensated are blue and red, and for photography, purple and red.
An achromat diminishes chromatic aberration so that only a little remains.
* Apochromat
An Apochromat lens system compensates the wavelength differences of three colors or more. For example, an apochromat system is designed to make purple, green and red focus at the same point so that chromatic aberration is significantly reduced.
The extensive edit in post #2 is still valid but completely out of date. Slow focal ratio achromatic doublets are hardly made anymore and with modern optical glass it is posible to make a 100 mm f/8 apochromatic doublet.
Even if might be seen outdated because there exist apochromatic lenses not only achromatic lenses, that post is still relevant for our day because the majority of producers can’t afford to include apochromatic lens elements because of their very high cost.
Most of binoculars advertised as ED pretending as reducing chromatic aberrations are achromat in order to maintain the price low.
You cat see from that quote:
"Although APO lenses are more effective against chromatic aberration, achromatic doublets are more common as they are cheaper, lighter and more compact (Best Binocular Reviews, n.d.)."
Ed glass doesn’t mean apochromat because ED refers to a range of glasses with varying levels of Fluorite content. Nearly all apochromatic refractors use one or more elements of ED glass so both terms correctly apply to many telescopes. Some use ED glass but fail to achieve apochromatic performance - to those, only the ED term applies.
Most ED binoculars are just with achromat lenses which mean compensate for only 2 Colors from the 3 that should be compensated. I mean they have chromatic aberations from they 3rd color that is not compensated, but those chromatic aberrations from the uncompensated 3rd color are just reduced by the use of ED glass.
In this case the binoculars with achromat instead of apochromat do not compensate to solve the problem, they just reduce the problem when they reduce the CA by use of ED glass. And like this are the majority of binoculars advertised as ED.
Is explained here
Lenspire
and here (see the section
Achromatic Doublet Lens Correcting for Primary Longitudinal Chromatic Aberration but not correcting for the second one because the achromat doublet can correct only for 2 chromatic aberations, the secondary chromatic aberations represents the third color which is not corrected, only reduced with ED glass):
https://www.edmundoptics.com/knowledge-center/application-notes/optics/chromatic-and-monochromatic-optical-aberrations/
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