The objective lens is the lens that is closest to the object or specimen. It is essentially the information-gathering lens of an optical system. Therefore it is regard as the most important lens of the microscope. There are many different types of objective lenses. The most common and inexpensive is the achromat.  It is corrected for spherical aberration for only green light. Chromatic aberration is corrected in only two colors. The apochromat objective is far superior and generally very expensive. Chromatic aberration is corrected for all three colors and it spherically corrected for two colors. These objectives quite often will require a special compensating eyepiece. Semiapochromat objectives have correction in between the apochromat and achromat. Flat field or plano objectives compensate for curvature of field and are excellent for histology work. The flat field objectives can be optically constructed to be also an achromat, semiapochromat or apochromat. In the latter case the lens would be called a plano apochromat which is generally regarded as the finest lens available. The price of a single plano apochromat will run into the many thousands of dollars.

  Each objective has information critical for the maximum resolution possible written on the side of the barrel. Generally the magnification is print in the largest text with the manufacturer type designation. The second value is the numerical aperture. Beneath that, in a smaller font the tube length and the cover glass thickness is given. Any special information will also be added such as if it is an oil lens, infinity etc. The tube length usually 160 refers to the distance between the objective and the eyepiece in millimeters. It must be maintain if the aberrations are to be corrected. You can recognize a superior microscope if when adjusting the interpupillary distance you can see the eyepiece extend. This happens to maintain the proper tube length. The coverslip thickness usually around .17mm is also critical. This corresponds to a cover glass of No. 1.5. The more sophisticated objectives even have a coverglass compensation control that you dial in the thickness of the coverglass.