A simple, single- element lens magnifies, but it also acts to some extent like a prism. Thus a small point of white light would be slightly spread out into different colors. This effect is called chromatic aberration. Such lenses are used only in toy microscopes. Achromatic lenses correct for this by using two or more elements.
Achromatic lenses vary in quality and are adequate for most purposes.
Better correction is achieved by apochromatic lenses. It is capable of either dry or oil-immersion use. Objectives are expensive. It should be easy to see why. Since the eye can compensate for the very slight aberration left by an achromatic lens, apochromats are normally needed only for very critical applications and for photography.