PURPOSE: A slightly more complicated but better way to show pincushion and barrel distortion.

DESCRIPTION: This demonstration provides a more diverse technique for producing barrel and picushion distortion. The optical system in the photograph includes the following: bright point source with condenser lens, 5cm focal length plano-convex lens with flat side toward light source, copper wire screen, and 10 cm diameter (approximately) 15 cm focal length focusing concavo-convex meniscus lens. The image of the wire mesh is cast on a screen about 10 feet from the optical cart. The photographs below show the detailed setups used to achieve either barrel or pincushion distortion (top row) along with the distorted image produced using each setup (bottom row).

In the pairs of photographs at the left and left center, the screen is positioned about 15 cm from the first lens, and the meniscus lens is positioned to focus the image of the wire mesh on the screen. To obtain barrel distortion the lens must be about 12 cm from the wire mesh and the convex side of the lens must be facing the screen (set at left). To obtain pincushion distortion the lens must be about 18 cm from the wire mesh and the convex side of the lens must be facing the light source (set at left center).

In the pairs of photographs at the right center and right, the screen is positioned close to the first lens, and the meniscus lens is again positioned to focus the image of the wire mesh on the screen. To obtain barrel distortion the lens must be about 18 cm from the wire mesh and the convex side of the lens must be facing the light source (set at right center). To obtain pincushion distortion the lens must be about 12 cm from the wire mesh and the convex side of the lens must be facing the screen (set at right).

This demonstration also shows "curvature of field," because the position for the 23 cm lens to focus central and peripheral rays is different.

View from within a few feet to see chromatic aberration.

SUGGESTIONS:

REFERENCES: (PIRA 6A65.30) See demonstration L6-40 for a slightly simpler version of this demonstration.

EQUIPMENT: Bright point source with condenser lens, 5 cm focal length plano-convex lens (flat side toward light source), copper wire mesh (1/16 inch grid), and 10 cm diameter (approximately) 15 cm focal length concavo-convex focusing meniscus lens, all on moving optical table.

SETUP TIME: 5 min.