The labels at the top of the objective (i.e., Plan Apo) indicate correction.  Common abbreviations used are as follows:

  • Achro, Achromat: Achromatic aberration correction
  • Fluor, Fl, Fluar, Neofluar, Fluotar: Fluorite aberration correction
  • Apo: Apochromatic aberration correction
  • Plan, Pl, Achroplan, Plano: Flat Field optical correction
  • N, NPL: Normal field of view plan
  • Plan Apo: Apochromatic and Flat Field correction


Immersion medium is often noted on the lens near the correction.  If nothing is noted, the immersion medium is air.  Other immersion media are indicated as follows:

  • Oil: oil immersion
  • W, Water: water immersion
  • Mi: multi immersion


Lateral magnification is indicated by the number followed by X (i.e., 60X).


Numerical aperture (N.A.) is usually written after the lateral magnification (i.e., 60X / 0.95).  Numerical aperture indicates the light acceptance angle.  This angle determines light gathering power, resolving power, and depth of field of the objective.


Some objectives aredesigned for transmitted light fluorescence and darkfield imaging.  Such objectives are equipped with an internal iris diaphragm allowing for adjustment of the effective numerical aperture. The presence of an iris is denoted by abbreviations including I, Iris, and W/Iris.


Specialized optical properties are often written underneath the magnification and numerical aperture.  Common properties include the following:

  • P, Po, POL: polarized illumination
  • PH: phase contrast
  • DIC: differential interference contrast


Mechanical tube length is listed under the specialized optical properties.  This number denotes the length of the microscope body tube in millimeters between the nosepiece opening and the eyepieces.  An infinity symbol means the tube has been infinity-corrected.


After the tube length, the optimal thickness of cover glass to be used is noted.  Some objectives have a correction collar in the form of a movable ring with a graduated scale to compensate for different cover glass thicknesses.


Working distance is denoted by “W.D.”  This is the distance in millimeters between the objective front lens and the top of the cover glass.  Usually, working distance decreases as magnification increases.




Information and picture source: Nikon MicroscopyU

For more information and pictures, see MicroscopyU.