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.

