Objective Working Distance
Microscope objectives are generally designed with a short free working distance, which is defined as the distance from the front lens element of the objective to the closest surface of the coverslip when the specimen is in sharp focus. In the case of objectives designed to be used without coverslips, the working distance is determined by the linear measurement of the objective front lens to the specimen surface.
This interactive Java tutorial demonstrates the range of working distance values in modern infinity-corrected Nikon CFI60 objectives. The tutorial initializes with a 20x objective visible in the applet window adjusted for a working distance of 10 millimeters, which is considered to lie in the Long Working Distance range (LWD). To adjust the objective working distance, translate the slider back and forth to change from regular working distance values through Extra Long and Super Long working distances (ELWD and SLWD, respectively) utilized on reflected light and inverted microscopes. Actual objective working distance values vary over a wide range, depending upon the optical correction, parfocal length, manufacturer, and application.
In general, the objective working distance decreases as the magnification and numerical aperture both increase, as presented in Table 1 for a highly corrected series of Nikon plan fluorite and plan apochromatic objectives. The current trend is to produce dry objectives having working distances as long as possible, but the demand is somewhat limited by the need for high numerical apertures with their higher resolving power. This often leads manufacturers to a compromise between these two parameters.
Common Objective Working Distances
Immersion objectives, which operate with a liquid medium of defined refractive index between the front lens element and the coverslip, are more restricted in working distance lengths. If the working distance is too large, maintaining a confluent network of immersion liquid between the objective front lens and specimen can be a problem, leading to introduction of aberrations with subsequent deterioration of the image. Objectives that have extremely close working distances are spring loaded so the entire front lens assembly will retract when brought into contact with the coverslip. These mounts are often termed retraction stoppers and guarantee adequate protection against accidental damage to either the specimen or the objective front lens.
Long Working Distance Objectives
For many applications a long free working distance is highly desirable (and often necessary), and specialized objectives are designed for such use despite the difficulty involved in achieving large numerical apertures and the necessary degree of correction for optical aberrations. Long working distance objectives are particularly useful when examining specimens in vitro through thick glass walls and for chemical and metallurgical microscopy, where the objective front lens must be protected against environmental hazards such as heat, caustic vapors, and volatile chemicals by a thick coverslip. The working distance of these objectives often exceeds two to three times that of comparable objectives having the same or a slightly greater numerical aperture. Table 2 lists infinity-corrected Nikon objectives having extra long working distances (ELWD) and super long working distances (SLWD). Note that working distance decreases with magnification and numerical aperture, but not as dramatically as the objectives listed in Table 1. Also note that the SLWD objectives exhibit significantly longer working distances, but correspondingly lower numerical apertures, than the ELWD series of objectives.
Kenneth R. Spring - Scientific Consultant, Lusby, Maryland, 20657.
John C. Long and Michael W. Davidson - National High Magnetic Field Laboratory, 1800 East Paul Dirac Dr., The Florida State University, Tallahassee, Florida, 32310.