Spherical optical surfaces introduce optical distortion at large numerical apertures, and optical dispersion in refractive materials produces chromatic aberration (i.e., the focal length is dependent on wavelength of light). Refractive objectives combine positive and negative elements to correct for these spherical and chromatic distortions. A good rule of thumb is that the higher the magnification and resolution, the larger the number of optical elements are required.
A multi-element refractive objective
Refractive microscope objectives suitable for UV excimer laser beam delivery provide resolution on the order of one micron at 10 to 30X demagnification, using five to seven elements. Due to the number of elements, optics losses through refractive objectives tend to be high (between 30% and 50%). Because of the high resolution, short optical path length and ease of on-target viewing, refractive objectives are often used for low-duty-cycle applications, such as IC lead deletion.