Editing Objective (optics) (section)

===Lens design===
Basic glass lenses will typically result in significant and unacceptable [[chromatic aberration]]. Therefore, most objectives have some kind of correction to allow multiple colors to focus at the same point. The easiest correction is an [[achromatic lens]], which uses a combination of [[Crown glass (optics)|crown glass]] and [[flint glass]] to bring two colors into focus. Achromatic objectives are a typical standard design.

In addition to oxide glasses, [[fluorite lens]]es are often used in specialty applications. These fluorite or semi-apochromat objectives deal with color better than achromatic objectives. To reduce aberration even further, more complex designs such as [[apochromat]] and [[superachromat]] objectives are also used.

All these types of objectives will exhibit some [[spherical aberration]]. While the center of the image will be in focus, the edges will be slightly blurry. When this aberration is corrected, the objective is called a  "plan" objective, and has a flat image across the field of view.