Automatic cameras for photomicrography are equipped with a light-sensitive metering system that determines exposure time. These devices work by averaging light in the central 30 percent of the viewfield, and many are equipped with a spot metering option that allows light to be measured in a very small portion of the field.
This tutorial simulates an automatic exposure metering system on a modern microscope. The rectangular area delineated in the viewfield represents the portion that will be captured onto 35 millimeter film. Crosshairs in the center are used for aligning focus between the focus finder and the film plane. Use the Metering radio buttons to select Average (over 10 percent of the viewfield) or Spot (1 percent of the viewfield) metering. When Average is selected, a yellow circle appears in the viewfield and a yellow dot approximating the size of the circle appears in the small window below. Use the sliders or mouse to move the circle around the viewfield to gauge light intensity at different locations on the specimen. As the metering circle is moved, the Exposure Time digital readout displays the calculated exposure time in seconds. Translate the circle around the viewfield to see how illumination varies at different parts of the field. Select the Spot radio button for a more accurate light measurement. The Reticle Color radio button allows the visitor to select a reticle display color from a palette that includes red, yellow, and black. Specimens can be selected from the Select a Specimen pull-down menu.