An important term in microscopy, "working distance" describes the distance between the lens of a microscope and the top of the sample being observed. In other words, the working distance of a microscope is a measurement of the free area available to the viewer to work and manipulate the sample, from the very bottom of the microscope to the top of the observed object on the microscope platform or stage.
Relationship With Magnification
Generally speaking, the working distance of any given microscope is reduced as magnification increases. As in most compound microscopes, the lens is moved physically closer to the specimen to increase magnification; the working distance available between the lens and the specimen is reduced considerably as magnification increases.
Working With Large or Live Samples
Working distance is seldom a consideration when working with flat, static or very small samples that allow the lens to be completely lowered to achieve full magnification. In the case of larger, fragile or moving samples, however, a microscope user should take working distance into account when designing an experimental protocol and realize that the need to place and manipulate the sample may restrict the possible magnification with any given microscope.
Contamination and Distortion
When working with a large or moving sample under close magnification, working distance becomes critically important to avoid contact with the lens, which can cause both contamination of the sample or distortion of the image. Thus, always make sure the lens and the microscope stage have been properly cleaned and disinfected, and monitor the working distance to keep an appropriate distance for the size, sensitivity and possible movement of the sample.
Alternatives for Reduced Working Distance
Of course, the easiest solution to the problem of reduced working distance is to use a microscope with more potential magnification, thus increasing the physical distance between stage and lens to achieve the desired magnification. Specially designed manipulation tools are also available in most laboratories for observations with reduced working distance.
To address the inconvenience of reduced working distances with large samples, several laboratories also have zoom microscopes, which magnify the image by zooming the lens but without drawing the tip of the lens closer to the specimen. Zoom microscopy is often used when observing living or easily contaminated specimens.