The diameter of the view field in an optical microscope is termed the field number and represents the diameter of the field measured in millimeters at the intermediate image plane. This interactive tutorial explores the effect of varying the field of view size on the viewable specimen area.
The tutorial initializes with a randomly selected specimen visible in the virtual microscope viewport and the field number set to a value of 20. Use the Field of View slider to adjust the field size between 10 and 28 millimeters, and note how the viewable specimen area increases or decreases with the field number. After viewing a specimen, use the Choose A Specimen pull-down menu to select another specimen.
In most cases, the eyepiece field diaphragm opening diameter determines the view field size. The field size in the specimen plane is then defined as the field number divided by the magnification of the objective:
If an auxiliary lens is inserted between the objective and eyepiece, the magnification factor of this lens should also be employed in the equation by multiplication with the objective magnification (prior to the division operation). Although the field number is usually limited by the magnification and field diaphragm (stop) size of the eyepiece, there is clearly a limit that is also imposed by the design of the objective lens system. In early microscope objectives, the maximum usable field diameter tended to be about 18 millimeters or considerably less, but with modern plan apochromats and other specialized flat-field objectives, the maximum usable field can sometimes exceed 26 millimeters.
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.