| 摘要 |
PROBLEM TO BE SOLVED: To prevent dew condensation on the surface of a lens by constituting a lens frame of material having large heat conductivity and providing an extension area part for heat exchange from the lens frame to a space part between lenses. SOLUTION: The 2nd lens frame 5 is constituted of the material having the large heat conductivity, for example, aluminum, copper or silver. It is the most desirable to set aluminum alloy from the viewpoint of production and cost in practical use. Plural annular grooves 5a are formed as the extension area part for heat exchange facing to a front space part on the front end face of the frame 5, and their surfaces are formed to be rugged so as to function as a projection part for heat absorption. As for the shape of the projection part for heat absorption as the extension area part for heat exchange, it is important to make surface area as large as possible to such an extent that it does not interfere in other lens barrel parts. By making the surface area large, heat transfer between the space part between a 1st lens group L1 and a 2nd lens group L2 and the frame 5 is improved.
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