Submarine periscopes often provide a field change mechanism but there is a distinct preference for fixed magnifications for immediate awareness of range. However for a variety of reasons, the standard periscope is being succeeded by television. Even high definition TV is restricted in displayed resolution points compared with a direct vision periscope, therefore as apparent size is now dependent on the display, a continuous zoom is suggested to ensure relevance of the captured image. The need for steering and stabilization of the sight line and viewing through a window, within the bounds of an unobtrusive and seaworthy package present special optical problems. A stable entrance pupil must be projected onto the window beyond the line of sight prism. It follows that the optical system incorporates a relay stage to provide a real aperture stop and that configuration changes on both sides of the intermediate image are necessary to control the entrance pupil. Compact zoom lenses are usually based on Galilean telescopes and the moving group diverging; in contrast this zoom periscope is based on the Kepler type and all the lens groups are converging. The system provides an 8:1 zoom range, is unconstrained in overall length but fits a standard 6 multiplied by 40 ocular within a tube of 50 mm in diameter. A disadvantage is the motion of moving lens groups through a focus with a risk of blemishes becoming visible but the image quality is hardly inferior to a traditional layout using interchangeable lenses.
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