The present invention relates to an electric contact arrangement for individual objectives (lens assemblies) in a photographic camera of the interchangeable lens type.
Information processing in photographic cameras has recently been developed to higher and higher degrees so that information on the maximum diaphragm value and the minimum diaphragm value, information on selection of automatic diaphragm control or manual diaphragm adjustment, and information on the shortest distance as well as focusing detection signal or the like in an auto-focus system can be processed by an electronic circuit contained within a camera body. For transmission of such information to the electronic circuit contained within the camera body, it has already been proposed to embed electrical contacts in the objective mount and the body mount at mutually confronting positions (e.g., U.S. Pat. No. 4,357,089, incorporated herein by reference). Such a structure is well known also from Japanese Patent Application Unexamined Disclosure Gazette No. 54-88122, Japanese Patent Application Unexamined Disclosure Gazette No. 50-67650, Japanese Patent No. 51-44652 and Japanese Patent Application Unexamined Disclosure Gazette No. 52-56926, the disclosures of which are incorporated herein by reference.
However, in these paired, aligned electric contacts of the prior art, one contact of each pair is biased to project in the perpendicular direction from the surface of the associated mount and the other contact of each pair is fixed on or in the associated mount so that only a point contact is established between the aligned contacts upon coupling the objective to the camera body. In consequence, there is a problem in the reliability of electric conduction established between these contacts. More specifically, there will often occur unreliable contact when insulative substance from the environment clings between the paired aligned contacts or insulative film is formed on the contact surface.
The insulative substances from the environment such as dust, sand or fiber will not be a serious problem insofar as such a substance has a size substantially smaller than the irregularity present on each contact surface. It is also possible to visually recognize such extraneous matter when the electric contacts are exposed and said extraneous matter is relatively large in size. Furthermore, such problem can be avoided by previously activating a member for removal of such extraneous matter (see U.S. Patent application Ser. No. 692,370, filed on January 18, 1985, the disclosure of which is incorporated herein by reference.
To the contrary, the problem of the surface film is more serious. Specifically, formation of film such as oxide film and sulfide film on the electric contact surface increases the contact resistance between the paired, aligned electric contacts, particularly when the contact pressure is low between these electric contacts. With the contact resistance thus increased, the signal to be transmitted will be accompanied by a transmission error at a minute voltage or current, or sometimes the transmission itself will be impossible. With surface film as thin as less than several tens of angstroms, the tunnel effect maintains conductivity. However, the surface film will exhibit an insulative property or the behavior of a high-resistance semiconductor, as the film grows beyond 100 angstroms.
As a means to solve such a problem, it is well known to plate the electric contacts with material which is resistant to air pollution to prevent development or growth of such surface film. Nevertheless, the desired pollution resistance is often not achieved because of pin holes in the plating layer.
This problem of surface film has a significant influence not only upon the pair of photographing mode indication contacts to transmit whether a photographer has selected the automatic diaphragm control mode or the manual diaphragm adjustment mode, as a desired photographing mode to the electronic circuit contained within the camera body, but also upon the pairs of diaphragm value information contacts serving to transmit specific diaphragm values, such as the maximum diaphragm value and the minimum diaphragm value to the electronic circuit.
Concerning the changeover member arranged within the objective for selection of the photographing modes, Japanese Utility Model Application Unexamined Disclosure Gazette No. 56-l26627 discloses a changeover contact strip which is movable into contact with the electrically conductive objective mount. The changeover member proposed by this prior application has a problem with respect to reliable electric conduction, since grease used for smooth movement of rotational members within the lens barrel may cling to the contact area and serve as an insulator. According to this unexamined application, the changeover member is contained within the lens barrel and, accordingly, it would be difficult for a user to clean the surface of this changeover member unless there is provided suitable means for removal of clinging grease.