Although the zoom ring is graduated with the nominal objective powers, it is impossible to return the zoom control to exactly the same position that would be necessary for precise measuring purposes. A microscope eyepiece is designed to further magnify the primary image formed by the microscope objective, and also limit the field of view. In Nikon's revolutionary new CF Optical System, chromatic aberration has been corrected in both the objectives and the eyepieces independently. The result is a dramatic reduction of chromatic aberration across the entire field of view. The orange coloring around the fringe of the field which was noticeable in the compensating system has been virtually eliminated.
Many microscopes still offer Huygenian-type eyepieces.
However, when eyepiece reticles are used, a Ramsden or Kellner eyepiece should not be used. The Huygenian eyepiece is of simple construction, consisting of two plano-convex lenses mounted with convex sides towards the objective. The lens nearest the eye is known as the field lens. This type of eyepiece is also uncorrected, giving a blue fringe to the edge of the field and is best suited for low-power achromatic objectives. The principal image is formed between the two lenses, making it inconvenient for use with a reticle whose accuracy is affected by the aberrations of the eye lens alone.