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Light Microscopy: Instrumentation and Principles

Author(s): David R. Caprette, PhD

Field of View and Light Intensity

When you look into a microscope, the true field of view is the actual circular area of specimen magnified by an objective lens. With increased magnification, the area diminishes, and so does the amount of light entering an objective. An ocular lens magnifies an image so that the view we see has the same apparent diameter, regardless of magnification. It follows, then, that each time we raise magnification the image becomes dimmer.

The loss of light at high magnifications is dramatic. The area of the true field and amount of light it transmits to an objective are proportional to the square of its radius. When we increase magnification by a factor of ten, we reduce the radius of the true field of view tenfold. The amount of light entering the objective lens and eventually reaching the eye is reduced a hundredfold.

With a sufficiently intense light source, it is necessary to reduce the light intensity at low magnifications to avoid hurting one's eyes. Enough light should be available to view a specimen at the highest available magnification without having to look at a dim image. With source intensity at maximum, if an image is still too dim it may be time to change the bulb or re-align the light path.

Only the source intensity control should be used for adjusting image brightness. The aperture diaphragm in the condenser also will affect brightness, but it should be used strictly for adjusting contrast and resolution.