Not to be confused with the term "achromatic" is the correction induced in the objective lens to adjust for focusing errors caused by field curvature. Since a lens is curved, the sharpness of the image may be a bit off at the outer edges of your field of view. You can, of course, refocus slightly to bring it in, but then the center part of the image will be a bit out of focus (and this is all assuming that the specimen is perfectly flat, which is not usually the case!) Field curvature effects are most noticeable at the higher powers. Think of the field of view as a big pizza. At the outer edges, some of the pepperoni may look a bit fuzzy.
The standard achromatic objectives guarantee flat focus quality for 60% of your field of view. This is actually quite a bit as the outer 40% of the area of the "pizza" has a relatively larger circumference (draw a circle on paper and see what I mean). If you are shooting pictures through your microscope or need more than 60% of a flat field, you might choose the semi-plan or plan lenses. Semi-plan lenses guarantee a flat field for 80% of the field of view and plan lenses (the best) are a full 100%. These lenses are also achromats, correcting for the color dispersion effects.