First, consider the fundamental difference between a finite conjugate microscope objective and its infinitely corrected counterpart; a finite conjugate objective, such as an objective lens in a traditional laboratory or university microscope, focuses light directly onto the sensor or eyepiece. Typically, these objectives are achromatic and are only effective for chromatic aberrations of two wavelengths: red and blue.

On the other hand, an infinity corrected objective requires an additional optical component, a tube lens, to focus the light onto the sensor or eyepiece, but also allows access to a parallel optical plane. Optical devices for attenuating or filtering light, beam splitters for in-line illumination or additional optical components can be incorporated into the setup by accessing the optical plane.

Typically, these optics are apochromatic and the spherical and chromatic aberrations for the three wavelengths (blue, yellow, and red) are correct, and also have a higher numerical aperture to improve light level and contrast. For a more detailed description of the different types of targets, read Understanding Microscopes and Targets.