Objective Lenses

Objective lenses are the lenses of a microscope closest to the object and are the most important single part of your microscope.  The quality of objective lenses varies widely from manufacturer to manufacturer and the only way to evaluate objective lenses is to physically compare one with another.  Having performed evaluations like this, we have come to the conclusion that the optics offered by National Optical, Motic and Meiji are superior in quality to other instruments found in their price range.  

At left are a variety of objective lenses:  Achromat, Semi-Plan, Super High Contrast, and Plan (all are 100x objectives).

Achromatic Lenses

Some of the problems encountered with the optical characteristics of a lens are caused by the refraction of light as it passes through the lens system.  White light is really a combination of many different colors and each color refracts or "bends" at different angles when passing through a lens.  The inability of a lens to bring all of the colors into a common focus results in a slightly different image size and focal point for each predominant wavelength group. This leads to chromatic aberrations or colored fringes surrounding the image.

Special techniques must be employed to compensate for these chromatic aberrations.  First, by using two different types of glass (sandwiched into a "lens doublet") with different refractive indices, this effect can be minimized.  Lenses that employ these techniques are called "achromatic lenses ".   This simply means that they refract light with little spectral color separation.  Achromatic lenses are the most common type sold with microscopes.

Super High Contrast (ASC) Lenses
National Optical has a series of objective lenses called the Super High Contrast objectives.  They are standard achromats with an internal field stop which offers a higher contrast image.  For image quality, these lenses are excellent for the price.  Like the laundry detergent commercials on TV, they produce whiter whites and darker darks to offer a very nice sharp high contrast image.  The plan achromatic lenses are top of the line, and if you are doing a lot of video or photo work and need a flat field, then plan objective lenses would be the best for your work.
Plan Lenses

Not to be confused with the term "achromatic" is the correction induced in the objective lens to adjust for focusing errors caused by field curvature.  Since a lens is curved, the sharpness of the image may be a bit off at the outer edges of your field of view.  You can, of course, refocus slightly to bring it in, but then the center part of the image will be a bit out of focus (and this is all assuming that the specimen is perfectly flat, which is not usually the case!)  Field curvature effects are most noticeable at the higher powers.  Think of the field of view as a big pizza.  At the outer edges, some of the pepperoni may look a bit fuzzy.  

The standard achromatic objectives guarantee flat focus quality for 60% of your field of view.  This is actually quite a bit as the outer 40% of the area of the "pizza" has a relatively larger circumference (draw a circle on paper and see what I mean).  If you are shooting pictures through your microscope or need more than 60% of a flat field, you might choose the semi-plan or plan lenses.  Semi-plan lenses guarantee a flat field for 80% of the field of view and plan lenses (the best) are a full 100%.  These lenses are also achromats, correcting for the color dispersion effects.

Phase Contrast Objectives
Phase contrast objectives are only useful on specimens that do not absorb light (they are called "phase objects") and are very useful in showing details in certain specimens such as cell parts in protozoans, bacteria, sperm tails and other types of unstained cells. For more information visit our page dedicated to phase contrast microscope information.
Other Lens Information
We recently tested several of the different types of lenses in our Great Lens Shootout! You may want to take a look at what we discovered and see the images we captured with different lenses.