The beginning student, in using a microscope, often has difficulty in understanding depth of field and relating to the size of objects. The depth of field refers to the thickness of the plane of focus. Depth of field and magnification are related because as magnification increases, the depth of field decreases. A mite was chosen as the initial subject for viewing because many students are able to relate to the size of this small fauna. This undecomposed mite was located on a slide buried in soil and stained with rose bengal solution. Although the mite is too small to see in any detail with the unaided eye, hairs on the legs are clearly visible at 1,000x magnification. At 1,000x magnification, however, the field of view (portion of the mite seen) is small and the depth of field is shallow. The shallow depth of field indicates a narrow plane of focus, which is defined as the plane (layer) perpendicular to the principal axis of transmitted light that has a sharp image. Adjusting the plane of focus is controlled by the fine adjustment of the microscope. The video shows moving the fine adjustment at different magnifications and seeing different images moving into and out of focus. The latter portion of the video allows a student to relate to size of magnification and depth of field by taking a familiar object (a period produced by a laser printer) and magnifying it at 4, 10, 100, 500, and 1,000x magnification. The images across the field of view appear sharp at 4, 10, and 100x magnification. At 500 and 1,000x magnification, however, the video shows continuous fine adjustment to view different planes where items below and above are out of focus. The individual ink particles at 1,000x magnification are larger than a single bacterium. A downloadable, high-resolution version of this movie in RealMedia format is available at http://www.agron.iastate.edu/~loynachan/mov/.
2. Sylvia, D. M., J. J. Fuhrmann, P. G. Hartel, and D. A. Zuberer. 2005. Principles and applications of soil microbiology. Pearson Prentice Hall, Upper Saddle River, N.J.