Abstract:
A viewfinder system for a camera, in particular a movie camera, in which there runs a viewfinder beam path that is diverted from a recording beam path for film exposure is provided. The viewfinder system comprising an eyepiece, an eye closure, a shutter for masking out stray light penetrating into the camera via the viewfinder system, and a control device that triggers the shutter and is connected to a sensor. The viewfinder system further comprising a sensor device for contactless detection of an eye or head, located at the eye closure of the viewfinder system, of a person viewing the picture to be recorded through the eyepiece.

Description:
CROSS-REFERENCE TO A RELATED APPLICATION 
     This application is a National Phase Patent Application of International Patent Application Number PCT/EP2007/003357, filed on Apr. 11, 2007, which claims priority of German Patent Application Number 10 2006 018 865.9, filed on Apr. 13, 2006. 
     BACKGROUND 
     The invention relates to a viewfinder system for a camera, in particular for a movie camera. 
     In order to view a picture or a picture sequence to be recorded by a still camera, movie camera or videocamera, a viewfinder beam path is diverted from the recording beam path running through the camera objective by means of a beam splitter or a mirror reflex device and is imaged as viewfinder picture by a transmitting optics as a real image, for example on a focusing screen or fiber plate or as an air image, where it can be viewed through an eyepiece and assessed. The viewfinder closure of the camera is formed by an eye closure made from a soft plastic that is applied around the eye to the head of a cameraman and prevents laterally penetrating stray light from degrading the viewfinder image viewed. The eye closure can optionally be designed as an eyepiece cup anatomically adapted to the field of view of the cameraman, or in the manner of a bellows. Usually, a leather cloth is pulled over the anatomically shaped eyepiece cup or over the bellows in order to give a more pleasant contact with the skin. 
     If the eye of the cameraman is not located at the eye closure, stray light can penetrate into the camera interior via the viewfinder system of the camera in a direction opposite to the viewfinder rays diverted from the recording beam path, be reflected into the recording beam path at the rear side of the camera objective, or be scattered in the viewfinder beam path at the focusing screen or fiber plate, and thereby lead to faulty exposures of film or video pictures, and thus render the film or video recording unusable. 
     In order to avoid the penetration of stray light into the viewfinder system when a head is not applied at the eye closure, it is known to arrange between the eye closure and the eyepiece a shutter that is connected to a lever guided to the outside of the camera housing, such that the cameraman is able to open the shutter by manual actuation of the lever in order to view pictures with his head applied to the eye closure, and to close it again after viewing pictures but before moving his head from the eye closure. However, this known apparatus for preventing the penetration of stray light into the viewfinder system of a camera presupposes extreme concentration on the part of the cameraman, since premature opening or delayed closing of the shutter when no head is located at the eye closure allows stray light to penetrate into the viewfinder system of the camera and can lead to faulty exposures of the film or video. 
     For this reason, control apparatuses have been developed for a shutter arranged in the viewfinder system of a camera; with these apparatuses, the pressure of the head against the eye closure opens the shutter via a mechanism, and a spring connected to the mechanism closes the shutter when the cameraman no longer presses against the eye closure with his head. This apparatus for automatically opening and closing the shutter has the disadvantage, however, that pressure is continuously exerted on the field of view about the eye of the cameraman while images are being viewed, and this is felt as being unpleasant. 
     DE 31 35 950 A1 discloses an apparatus for reducing or masking out the quantity of parasitic light incident in a monocular mirror reflex viewfinder and which includes a light sensitive element that is arranged in the camera viewfinder and outputs a voltage as a function of the parasitic light entering through the eyepiece of the viewfinder. A comparator receives the output signal of the light sensitive element and generates a control signal as soon as the intensity of the parasitic light exceeds a predetermined value. A shutter is closed as a function of the control signal such that parasitic light does not scatter into the optical system of the camera through the viewfinder system and cannot degrade the image quality. Instead of the shutter, an aperture stop or a filter can be used in the known apparatus so that the cameraman can observe the object to be recorded through the viewfinder even when the parasitic light passing into the optical system of the camera is reduced. 
     So that in the case of the known apparatus for triggering a shutter arranged in the viewfinder beam path the viewfinder rays diverted from the recording beams are not received by the light sensitive element and therefore also lead to a closure of the shutter even when the eye of the cameraman is located at the camera viewfinder, there is provided in the eyepiece arrangement of the viewfinder system a small wall which occludes the viewfinder rays diverted from the recording beam path such that the light sensitive element in the form of a photodiode or of a phototransistor picks up only the parasitic light incident through the eyepiece. Despite this measure, given an appropriately set sensitivity of the device for triggering the shutter it is impossible to prevent the viewfinder rays from the end termination of the viewfinder from being reflected at the eye or the visual aid of the cameraman or at a lens, inserted into the viewfinder, for correcting vision defects, falling onto the light sensitive element and initiating a closure of the shutter even though the eye of the cameraman is located at the eye closure of the camera viewfinder. 
     A further disadvantage of the known apparatus consists in that given low illumination the shutter is open because of a lack of parasitic light incident in the camera viewfinder, and in the event of suddenly incident parasitic light, for example upon an interchange of light and shade, parasitic light is incident in the viewfinder system for a short time up to the closure of the shutter by a control signal output by the light sensitive element, and leads to faulty exposures. 
     Furthermore, given a minimum distance of the field of view, surrounding the eye of the cameraman, from the eye closure of the camera viewfinder, parasitic light can penetrate into the viewfinder system of the camera and lead to closure of the shutter given an appropriate setting of the sensitivity of the control device, even though the small quantity of parasitic light would not lead to faulty exposures. In order to enable images to be viewed even in such cases, the known apparatus requires a complicated adjustment of the response threshold of the device for triggering the shutter. 
     SUMMARY 
     It is an object of the present invention to specify a viewfinder system of the type mentioned at the beginning that does not exert pressure on the field of view during viewing of the image, prevents the penetration of stray light into the viewfinder system of a camera even when light is incident suddenly in the viewfinder system, does not respond to the viewfinder rays diverted from the recording beam path, and is of simple design and easy to adjust. 
     Since the shutter is always closed in the case of the inventive viewfinder system when the eye of the cameraman is not located at a prescribed distance from the eyepiece, viewfinder or eye closure, even a sudden incident of stray light in the viewfinder system cannot lead to faulty exposures. Since, furthermore, the control device for triggering the shutter is oriented solely to detecting the eye, the viewfinder rays diverted from the recording beam path do not influence the triggering of the shutter, and the response threshold of the control device can be adjusted exclusively to the interocular distance such that a quantity of stray light still just permissible can be set exactly and with simple means, and thus the entire apparatus for preventing the penetration of stray light into the viewfinder system of the camera can be of simple design. 
     Since the control device cooperates with a sensor device operating in a contactless fashion, not only is it possible to permit the penetration of stray light that does not lead to faulty exposures by means of a short distance of the field of view of the viewer from the eye closure, but there is also no pressure of any kind exerted on the field of view of the cameraman while he is viewing pictures. 
     In a first exemplary variant of the inventive solution, a capacitive or inductive sensor is provided for detecting the capacitance or inductance changed by the approach of the eye to the eye closure, and for outputting a sensor signal to the control device. 
     This sensor device operating in a contactless fashion detects changes in capacitance between the two electrodes of a capacitor, or changes in inductance in the magnetic field of a coil when a body part is located at the eye closure, and can, for example, be set such that the shutter is opened only when the field of view of the cameraman that surrounds the eye is applied in a lightproof fashion at the eye closure. 
     In a second exemplary variant of the inventive solution, the sensor device consists of a transmitter that outputs signals, directed into the eye closure, with wavelengths that lie outside the wavelength of visible light, and a receiver that is tuned to the signals output by the transmitter, receives the signals reflected by the eye or the field of view surrounding the eye, and outputs sensor signals to the control device as a function of the signal reception. 
     Configuring the sensor device as a transceiving device firstly enables a miniaturized design of the sensor device and its arrangement in a region of the viewfinder system, or the eye closure, that does not disturb viewing of the pictures and, secondly, ensures reliable detection of the eye of a picture viewer at, or at a prescribable distance from, the eye closure. Furthermore, this sensor device operating in a contactless fashion can be adjusted very sensitively such that it is possible to class as still just tolerable even small quantities of stray light that still do not lead to closure of the shutter, in particular if the user wishes to avoid touching the eyepiece cup and prefers to keep a minimum gap between head and eyepiece cup. 
     Since the signals output by the transmitter have wavelengths that lie outside the wavelengths of visible light, the measuring beams retroreflected by the eye of the picture viewer into the viewfinder system cannot lead to faulty exposures of the film or videotape, or to irritation of the eye of the user. 
     In an exemplary embodiment, the transceiving device of the sensor device is designed as a reflected light barrier and, in particular, as an infrared reflected light barrier, that emits and receives very short light flashes. 
     With reference to the wavelength, the measurement signals lying in the infrared region differ substantially from the wavelengths of visible light, and thereby enable simple tuning. Moreover, they are invisible to the picture viewer and can therefore not disturb or influence the viewing of the pictures. Owing to the restriction of the measurement signals output by the transmission part to very short light flashes with a frequency in the kHz region, the eye of the viewer is, in addition, loaded only in a fashion that is negligible and far below permissible limiting values. Moreover, the electronic evaluation of the control device pays heed solely to a receiver signal that is correlated temporally with the emission of the light flashes so as to effectively exclude the shutter from responding to stray light. 
     A number of sensors arranged distributed on the circumference of the viewfinder closure ensure a uniform and complete detection of a cameraman looking into the viewfinder, it being preferred to arrange two sensors diametrically relative to one another on the circumference of the viewfinder closure. 
     In one exemplary configuration of the inventive solution, the sensors have at least one light emitting diode as transmitter, and at least one photodiode or one phototransistor as receiver; in each case two light emitting diodes and a photodiode or a phototransistor preferably form a sensor, the photodiode or the phototransistor being arranged between the light emitting diodes. 
     Owing to the redundant arrangement of in each case two light emitting diodes, the fault tolerance is raised and a uniform distribution is ensured for the measurement signals output by the sensors and preferably lying in the infrared region. Additionally, the presence of the user is reliably detected even when he looks into the eye closure from an unusual angle. 
     The control device is connected to a driving device, which actuates the shutter, and to a device for setting the response threshold for activating the driving device. 
     The drive for actuating the shutter can be of any desired design and consists, for example, of a miniature motor with a gear or lifting mechanism for opening and closing the shutter. The connection of the control device to a device for setting the response threshold for activating the driving device enables the setting of the desired switching distance at which the shutter is intended to operate as a function of the film sensitivity and ambient brightness, since some users avoid skin contact with the eyepiece cup and, instead of this, keep their head at a minimum distance from the latter. 
     The driving device exemplary consists of a lifting magnet whose armature is connected to the shutter, and whose winding or windings is or are connected to a control output of the control device. 
     This configuration of the driving device enables extremely short reaction times for opening and closing the shutter upon outputting of an appropriate control signal by the control device, is of very simple design and has an only very slight energy demand. 
     In a further exemplary refinement of the invention, there are integrated into the eye closure heating elements that prevent misting of the eyepiece when the camera is located in cold surroundings and the picture viewer or cameraman is looking into the camera viewfinder. 
     It is additionally possible to provide a multistage switch with an automatic position, in which the shutter is opened or closed as a function of the detection of an eye or head of a user, with an open position, in which the shutter is permanently open independently of the detection of an eye or head of a user, and with a closed position, in which the shutter is permanently closed independently of the detection of an eye or head of a user. 
     Such a multistage switch enables a deactivation of the automatic function for special applications of the camera, and can preferably be combined with that for switching in the heating elements integrated into the eye closure. 
     A method for preventing the penetration of stray light into the viewfinder system of a camera, in particular a movie camera, in the case of which a viewfinder beam path diverted from a recording beam path for film exposure runs in the viewfinder system, and that includes an eyepiece, an eye closure and a control device that triggers the shutter and is connected to a sensor is characterized in that the presence of an eye or head of a person at the eye closure is detected in a contactless fashion, and the shutter is opened given a prescribed measure of covering of the eye closure. 
     The contactless detection of the presence of an eye or head of a person at the eye closure can be performed by detecting the capacitance or inductance in the region of the eye closure, and the shutter is opened or closed in the event of a changing capacitance or inductance as the eye or head of a person approaches the eye closure. 
     Alternatively, measurement signals directed into the open region of the eye closure can be output with wavelengths that lie outside the wavelength of visible light, the signals reflected by the eye or head of a person being received and evaluated, and the shutter being opened or closed as a function of the evaluation of the reflected signals. 
     In order to prevent faulty opening of the shutter because of the penetration of stray light into the eye closure, it is preferred to carry out a measuring cycle in which the reflected signals are measured before, during and after the output of the measurement signals, and the received reflected signals of the measuring cycle are compared with one another and the shutter is opened or closed as a function of the comparison. 
     If the middle measurement, performed during the output of a measurement signal, of the reflected signal does not differ significantly from the two measurements before and after the output of the measurement signal, this indicates that stray light has been detected in the measurement before and after the output of the measurement signal, since no measurement signal has been output at these instances. Only if the middle measurement of the measurements before and after the output of the measurement signal differs significantly from the two further measurements are the preconditions for opening the shutter present. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The aim is to explain the fundamental idea of the invention in more detail with the aid of an exemplary embodiment illustrated in the figures, in which: 
         FIG. 1  shows a schematic of the film recording beam path of a movie camera, and of a viewfinder beam path diverted therefrom into the viewfinder system of the movie camera. 
         FIG. 2  shows a longitudinal section through the viewfinder closure of the viewfinder system having an eye closure, designed as an eyepiece cup, and a shutter. 
         FIG. 3  shows a plan view of the viewfinder closure with a lifting magnet for actuating the shutter, and sensors for detecting a user. 
         FIG. 4  shows an individual illustration of an eye closure designed as a bellows. 
         FIG. 5  shows a block diagram of the sensor and control device for triggering the shutter. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  shows a partially cutaway side view of a movie camera, with a schematic of the beam paths of the movie camera. The movie camera includes a partially cutaway camera housing  1  with an objective carrier  11  for fixing an objective  10 , and a film gate  12  in the interior of the camera housing  1 , past which a movie film F is intermittently moved. 
     A rotating mirror shutter  13  periodically releases the film recording beam path S 1 , which is incident through the objective  10 , as film exposure beam path S 2  for the purpose of exposing a film image, at rest in the film gate  12 , of the movie film F, or reflects the film recording beam path S 1  as viewfinder beam path S 3  into a viewfinder system  2  during the film transport. Arranged in the viewfinder beam path S 3  at the plane which is optically identical to the image plane at the film gate  12  is an apparatus for viewing images or assessing images that consists of a focusing screen or fiber plate  3  onto which an image of the object to be recorded is reflected when the reflecting surface of the rotating mirror shutter  13  is located in the film recording beam path S 1 , while the film transport mechanism transports the movie film F further in the film gate  12  by a film image. 
     The viewfinder beam path S 3  is guided by means of a deviating prism  20  in a direction suitable for viewing images and assessing images, and imaged by a transmitting optics  21  as a real viewfinder image in the plane of a field stop  22 , where it can be viewed through an eyepiece  23 . 
     The viewfinder closure  25 , illustrated in  FIGS. 2 and 3  in longitudinal section and in a plan view from the user side, of the viewfinder system  2  includes an eye closure  4  that consists in  FIG. 2  of an eyepiece cup  41  that is anatomically adapted to the field of view around the eye of a picture viewer or cameraman and can be replaced in accordance with the individual illustration in  FIG. 4  by an eye closure  4  designed as a bellows  42 . The viewfinder closure  25  further has a coupling,  26  for connecting the viewfinder closure  25  to the camera viewfinder including the deviating prism  20 , the transmitting optics  2 , the field stop  22  and the eyepiece  23 , an optical lens holder  27  for holding a correction lens that compensates vision defects of the picture viewer, and a heater  9  for heating the viewfinder closure  25  and/or the lens holder  27  that prevents misting of the eyepiece  23  when the movie camera is located in cold surroundings and the picture viewer or cameraman is looking into the viewfinder. 
     The heater  9  has a number of heating elements  93  that are let into the lens holder  27  or the viewfinder closure  25 , or are mounted on the surface thereof, and which are connected via an electric line  92  and a switch  94  to a plug  91  that serves to switch the heater  9  on and off and can be connected to the power supply of the movie camera. 
     When the cameraman removes his eye from the eye closure  4 , it is possible for there to enter into the viewfinder system  2  of the movie camera light that, as stray light, pass in the direction opposite to that of the viewfinder beam path S 3  into the film recording beam path S 1  and thus, via the film exposure beam path S 2 , onto the film F to be exposed, and thereby make film images unusable. In this case, depending on the sensitivity of the film F and the brightness of the object to be recorded even small quantities of stray light suffice to make the corresponding film section unusable. 
     Provided in the viewfinder closure  25  for the purpose of blocking such incidence of stray light is a shutter  5  whose shutter blades  51 ,  52  are connected in accordance with  FIG. 3  to a driving device  8  that is triggered by a control device  6  in accordance with  FIG. 5 . The control device  6  is, in turn, connected to a sensor device that is aligned with the eye or the head of a picture viewer or cameraman, and detects the presence or, if appropriate, the distance of an eye or head from the eye closure  4 , and outputs a sensor signal to the control device  6  that, upon detection of the eye of the picture viewer or cameraman, triggers the driving device  8  to open the shutter  5 , or closes the shutter  5  when the eye or the head of the picture viewer or cameraman is removed from the eye closure by a prescribed measure. 
     In accordance with the plan view of the viewfinder closure  25  illustrated in  FIG. 3 , the sensor device includes, seen from the viewer side, two sensors arranged diametrically relative to one another on the circumference of the viewfinder opening and respectively having two light emitting diodes  721 ,  722  and  723 ,  724  respectively, and a photodiode or a phototransistor  731 ,  732  arranged between the light emitting diodes  721 ,  722 ,  723 ,  724 . In accordance with the block diagram illustrated in  FIG. 5 , the sensor device has a clock generator  71  that reduces a clock frequency of 10 kHz in the ratio of 10:1 and drives the light emitting diodes  721 ,  722 ,  723 ,  724 , which output short light flashes  74 , preferably in the infrared region, into the eye closure  4 , that is to say in the direction of an eye adjacent to the eye closure  4 . The photodiodes or the phototransistor  731 ,  732  receive the light flashes  74  output by the light emitting diodes  721 ,  722 ,  723 ,  724  and, upon reception, output a sensor signal to an input of the control device  6 . 
     The light emitting diodes  721 ,  722  and  723 ,  724 , respectively, and the photodiodes or phototransistors  731 ,  732  are preferably arranged and designed as infrared reflected light barriers that output very short infrared light flashes and are tuned to reception of these very short infrared light flashes. This results neither in an impairment of picture viewing nor in the corruption of the measurement by reflections in the region of the eye closure, and has the effect that the loading of the eye of the viewer is negligibly small and is a factor of 10 below the permissible eye loading. Moreover, a reliable distinction is made during the evaluation between desired light flashes and undesired stray light owing to clocked light signals. The evaluation circuit measures the signal of the sensor before the emission of the light flashes, during the emission of the light flashes and after the emission of the light flashes. If no significant brightness differences occur at the sensor during the three measurements, it is possible to conclude unambiguously that stray light is present in the region of the eye closure. If, by contrast, the detected measured value of the reflected signal is significantly higher during the emission of the light flashes than therebefore and thereafter, the condition for opening the shutter is fulfilled. 
     The control device  6  is connected on the input side to a potentiometer  60  for setting a response threshold. On the output side, the control device  6  is connected to the driving device  8  that, in the preferred embodiment illustrated in  FIG. 3 , consists of a lifting magnet whose windings  81 ,  82  are connected to the trigger output of the control device  6 , and whose armature  80  is coupled via a pivoted lever  50  to the blades  51 ,  52  of the shutter  5  such that, given a control voltage output by the output of the control device  6 , the armature  80  of the lifting magnet is picked up and the shutter  5  is closed or opened in the opposite sense. 
     A contact coupled to the lifting magnet or the shutter  5  sends feedback to an input of the control device  6  in order to specify the open or closed state of the shutter  5 . 
     It is possible in addition to provide a multistage switch (not illustrated in the drawing) that renders it possible to switch off the automatic shutter and to keep the shutter permanently open or permanently closed. For this purpose, the multistage switch has an “ON” switch position for the permanently open position of the shutter, an “OFF” switch position for the permanently closed position of the shutter, and an “AUTOMATIC” switch position for activating the automatic shutter. In addition, the multistage switch can be combined with the switch  94  for the eyepiece cup heater  9  so that only one switch is provided at the viewfinder closure  25 .