Virtual Histology Lab
Using the Condenser Diaphragm to Control Contrast
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Contrast is the difference in intensity between the image and the background. Properly adjusted contrast is essential for good microscopy. If the contrast is too low, the image will appear washed out and you will not see important structural details of the specimen. If the contrast is too high, the image will appear harsh and out of focus because of optical distortions that become visible with high contrast. The appropriate contrast is that which allows you to visualize the structures you are interested in seeing. The contrast of an image in the light microscope is controlled by opening or closing the iris diaphragm of the condenser. As shown below on the left, the lowest contrast is achieved when the diaphragm is fully open, whereas the highest contrast, as shown in the image on the right, is achieved when the diaphragm is fully closed. Optimum contrast is usually somewhere in between the two extremes. Note that as you close the diaphragm, the image may get darker and you will need to increase the light by increasing the intensity of the illuminator. |
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Opening
the diaphragm => low contrast image
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Closing
the diaphragm => high contrast image
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Villus
of the small intestine; H&E stain; 40x objective lens.
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Demineralized
bone; H&E stain; 40x objective lens.
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Copyright by: Paul B. Bell, Jr. & Barbara Safiejko-MroczkaThe University of Oklahoma
Version: 010520