PURPOSE: To show pincushion and barrel distortion.

DESCRIPTION: The optical system in the photograph includes the following: bright point source with condenser lens, 10cm focal length plano-convex lens with flat side toward light source, iris, fine wire screen, large diameter 23 cm focal length convex lens, and screen at about 12 foot distance (not shown in photograph). This arrangement produces barrel distortion as shown in the photograph at the right. To produce pincushion distortion it is only necessary to move the iris from before the lens to after it and to slightly refocus by moving the 23 cm lens.

This demonstration also shows "curvature of field," because the focal position of the 23 cm lens is different to focus the center of the mesh than to focus the periphery. This effect is most pronounced with the iris wide open or removed.

View from within a few feet to observe chromatic aberration.

SUGGESTIONS: To facilitate efficient changing from one demonstration to the other a component positioning template is included (the meter stick lying in front of the optical rail).


EQUIPMENT: Bright point source with condenser lens, 10 cm focal length plano-convex lens, iris, wire mesh,and 23 cm focal length convex lens in baffle, all on moving optical table, component positioning strip.

SETUP TIME: 5 min.

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