Abstract:
A zoom camera having enhanced focusing stability during high magnification zooming under environments of high temperatures or low temperatures. When the zoom camera performs zoom-in operations in an environment of high or low temperatures, aperture control is added as the zoom magnification becomes higher and the temperature becomes higher or lower whereby the depth of field is made deeper to assure the presence of a range with good focusability.

Description:
INCORPORATION BY REFERENCE 
     The present application claims priority from Japanese application JP 2007-282623 filed on Oct. 31, 2007, the content of which is hereby incorporated by reference into this application. 
     BACKGROUND OF THE INVENTION 
     The present invention relates to exposure control of high-magnification zoom cameras. 
     Zoom cameras have generally two major types of lenses. One of them is a zoom lens which determines the size of an image to be formed on a charge-coupled device (CCD) image pickup plane. The other is a focus lens which performs focus adjustment of an image being formed on the CCD image pickup plane. Regarding the focus adjustment of the focus lens during zoom lens magnification changing operations, there are two kinds of functions: automatic focus function, and manual focus function. 
     In the auto-focus function, at the time of an operation for changing the zoom lens magnification, a focus lens position is automatically determined in accordance with a target subject or object to be photographed in such a way that an image being formed on CCD image pickup plane comes into focus at all times. On the other hand, the manual focus function is such that trace curve information indicative of the relationship of a zoom magnification and a moved distance of the focus lens is stored and, based on such trace curve information, the focus lens position is adjusted. This trace curve has temperature dependency, and there are many portions which are dependent on the temperature characteristics of a housing of the zoom lens, called the lens barrel. 
     One example of prior known techniques for improving the trace curve&#39;s temperature dependency is disclosed in JP-A-2003-248171, which recites therein, as its objective, “in a variable magnification image sensing device, unwanted variation or fluctuation of an image formation position occurring due to a temperature change is lessened while permitting the use of a plastic lens, thereby retaining good focusing performance” and recites as the solving means “a variable magnification image sensor device having a four-group lens configuration, wherein the device has in a third group a plastic lens  3  which has positive refracting power with a negative temperature coefficient of refractivity and a predetermined focal distance, a first support lens barrel L 1  which holds together the third lens group and the first lens group, and a second support barrel L 2  which holds together the third lens group and an image pickup element  5 , characterized in that a variation of the image formation position due to temperature changes of the lens groups is countervailed or “cancelled” by a variation amount of imaging position due to an extension/shrink amount based on a temperature change of the support barrel having a prespecified linear expansion coefficient.” 
     In addition, one prior art concerning the improvement of focus adjustment method of the manual focus function is found in JP-A-2006-189571, which discloses as the object “if there is an error in distance information and actual focus position at the time of manual focusing, there is a case where accurate focusing becomes impossible, in particular, at the distance of a settable range end; however, even in this situation, the best possible focus state is obtained without bothering the user with troublesome works” and recites as the solving means “an image sensor device having manual focus means, which has auto-focus means for automatically obtaining a focus position from the periphery of a presently set focus position, wherein the autofocus means performs focus control when the focus position is set at a predetermined position.” 
     SUMMARY OF THE INVENTION 
     In recent years, a growing need is felt for advanced surveillance cameras with a built-in high-magnification zoom lens for visually monitoring far distant scenes by high quality video images, for the purpose of long-range monitoring or “watchdog” of airports or harbors or accident prevention surveillance of rivers or else. To meet the need, high magnification zoom lenses are used, such as a 24-fold (24×) zoom lens or a 35× zoom. 
     In the manual focus function using such high-power zoom lens, it sometimes happens that mere use of the traditionally used trace curve fails to prevent occurrence of out-of-focus or defocus at high temperatures or low temperatures. Theoretically, the defocus is preventable by measuring in advance the entire temperature characteristics of the trace curve and then letting the measured data be internally stored in a camera and next performing focus control. However, the technique for performing focus control by measuring for storage every temperature in accordance with variation of the individual of mass-production products is too costly and thus is not a realistic approach. 
     On the other hand, surveillance cameras of the type performing unmanned operations suffer from difficulties in setting the focus position periphery as suggested in JP-A-2006-189571. 
     It is therefore an object of this invention to stabilize the image quality by significantly alleviating temperature-caused defocus of a high-power zoom camera. To this end, the present invention utilizes the phenomenon that a focused state obtainable range (i.e., depth of field) becomes deeper by stopping down the diaphragm. Namely, instead of fully opening the diaphragm in a high-power zoom lens at high or low temperatures, the aperture value is enlarged or increased to some extent to thereby increase the gain magnification determined by an automatic gain control (AGC) circuit so as to significantly alleviate temperature-caused defocus of a high-power zoom camera. 
     To attain the foregoing object, the present invention employs, as one example, the configuration that is defined in the appended claims. More precisely, at the time of application of the high-power zoom using the manual focus function, diaphragm aperture control is added at high or low temperatures in such a way as to make the depth of field deeper, thereby letting it have a focus margin or allowance. 
     According to this invention, it is possible to reduce defocus of a sensed image to thereby achieve stabilization of the image quality. 
     Other objects, features and advantages of the invention will become apparent from the following description of the embodiments of the invention taken in conjunction with the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a diagram showing a configuration of a zoom camera in accordance with one embodiment of this invention. 
         FIG. 2  is a diagram graphically showing a relation of exposure value versus gain magnification of an amplifier in one prior known zoom camera. 
         FIG. 3A  is a graph showing a relation of exposure value versus gain magnification of an amplifier at room temperature in a zoom camera embodying the invention. 
         FIG. 3B  is a graph showing a relation of exposure value versus gain magnification of amplifier in the zoom camera embodying the invention in the case of using a 24-fold (24×) zoom lens at high temperatures. 
         FIG. 3C  is a graph showing a relation of exposure value versus gain magnification of amplifier in the zoom camera embodying the invention in the case of using a 35× zoom lens at high temperatures. 
         FIG. 4  is a graph showing a relation of exposure value versus gain magnification of an amplifier in a zoom camera also embodying the invention. 
         FIG. 5  is a diagram showing a flow chart of control procedure of exposure and gain magnification of an amplifier in accordance with an embodiment of this invention. 
     
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
     A currently preferred embodiment of this invention will be described with reference to  FIG. 1  below.  FIG. 1  is a block diagram showing an exemplary configuration of a zoom camera in accordance with one preferred embodiment of this invention. 
     In  FIG. 1 , a lens unit  101  is made up of a variator lens group  102  which performs magnification changing of light rays coming from a photographic subject or object, a diaphragm  103 , a focus lens group  104  which has focus adjustment function, a variator lens group  102 , an absolute position detector  111  which detects absolute positions of the variator lens group  102  and focus lens group  104 , such as a photo-interrupter or the like, a temperature detector  112  and others. 
     Detection information which indicates a detection result of the absolute position detector  111  of the lens unit  101  is stored in a data storage unit  128  of a microcomputer  121 . 
     After having photoelectrically converted the light rays from the target object which has passed through the lens unit  101  at an image pickup element  113 , such as a charge-coupled device (CCD) image sensor or like imagers, it is amplified by an automatic gain control (AGC) circuit  114  up to an optimal level and then is converted by an analog-to-digital converter (A/DC) circuit  115  into a digital signal, which is input to a camera signal processing circuit  116 . 
     At the camera signal processor circuit  116 , the signal is converted to a standard television (TV) signal  117  (video signal) and is then output therefrom. This circuit also outputs auto-focus (AF) information and auto-iris or auto-exposure (AE) information to the microcomputer  121 . 
     In the microcomputer  121 , the AE information and AF information are converted to AE evaluation value data and AF evaluation value data by an AE data processing program  123  and AF data processing program  124 , respectively. In a control unit  122  within the microcomputer  121 , based on the AE evaluation value data and AF evaluation value data as well as detection information of the absolute position detector  111  being stored in the data storage unit  128 , temperature information of the temperature detector  112 , zoom ratio information, and information of a trace curve data storage unit, a motor control unit  129  generates motor drive control information to thereby control electrical motors with respect to the variator lens group  102 , camera diaphragm  103  and focus lens group  104 ; simultaneously, the control unit performs control of an electronic shutter  130  and amplifier  114 . 
     The motor drive control information of the motor control unit  129  is supplied to a zoom motor driver  108  which drives a zoom motor  105 , a diaphragm motor driver  109  which drives a diaphragm motor  106 , and a focus motor driver  110  that drives a focus motor  107 , for performing driving of the variator lens group  102  toward a telephoto or wide-angle direction, setting of the optimum aperture value, and focus adjustment of the focus lens group  104  based on the trace curve. 
     By switching a shutter speed of the electronic shutter  130  and increasing or decreasing an exposure time period with respect to the image pickup element, such as CCD or else, light amount adjustment of an image being focused on a photosensitive plane of the image pickup element  113  is performed. Further, pseudo-light amount adjustment is also performed by setup of the AGC circuit  114 . 
       FIG. 2  shows a relationship of aperture value of diaphragm  103  versus gain of AGC circuit  114  in a traditional aperture control procedure. By letting the shutter speed be fixed at 1/60, the diaphragm is gradually opened with a decrease in light amount from the photographic object (in the direction of from Exposure F16 to F1.0 along the vertical axis of  FIG. 2 ) so that an output of the image pickup element  113  is controlled to be kept constant, with the AGC circuit  114  being fixed in gain magnification. After the aperture reaches its full-open state (F-number 1.0) as a result of a further decrease in amount of light from the target object, the output of image pickup element  113  decreases; so, the gain magnification of AGC circuit  114  is increased to thereby cause the output to the A/DC circuit  115  to stay constant. In reality, when performing zooming to higher level of magnification, the light amount from the object decreases as a narrow viewing field is expanded; thus, control of the diaphragm  103  and AGC circuit  114  is being performed as shown in  FIG. 2 . This becomes the optimum control in cases where a zoom magnification  127  is low magnification and where a lens temperature  126  is room temperature. 
       FIGS. 3A to 3C  are graphs each showing a relation of diaphragm  103  and AGC circuit  114  in this invention. At room temperature, the same exposure control as that of  FIG. 2  is performed. At high temperatures with a high level of magnification, the control unit  122  of the microcomputer  121  performs control in such a way as to satisfy the following equation:
 
 x×A×S×F   −2 =constant   (1)
 
where x is the AE information as read out of the AE data readout program  123 , A is the gain magnification of AGC circuit  114 , S is electronic shutter speed (sec.) and F is the F-number (aperture value).
 
       FIG. 3A  shows a relation of F-number (aperture) and gain magnification of AGC circuit  114  at room temperature, wherein when the photographic object is bright, exposure control is performed by setting the gain magnification A at 1 (A=1) and the F-number (aperture) in such a manner as to satisfy the above-noted Equation (1) at a shutter speed of 1/60 (sec). As the target object becomes darker, the F-number (aperture) is reduced by opening the diaphragm. When the brightness of the object is lowered to 30 or below, resulting in the F-number being equal to a value corresponding to the full-open state, the gain magnification A is increased to compensate for light amount deficiency to thereby cause the input to the A/DC circuit  115  to stay constant. 
       FIG. 3B  shows a relation of F-number (aperture) and gain magnification of AGC circuit  114  at a high temperature (60° C.) in the case of zoom magnification of 24-fold (24×), wherein even when the photographic object is bright, the gain magnification A is doubled to stay at A=2 with the shutter speed being kept at 1/60 (sec); then, the F-number (aperture) is adjusted in such a way as to be equal to half of the light amount of the object which reaches the image pickup element. As a result of this, aperture control becomes possible until the object brightness becomes equal to 15, resulting in the depth of field becoming deeper by stopping down the diaphragm. Thus, out-of-focus or defocus becomes rarely occurrable. 
       FIG. 3C  shows a relation of F-number (aperture value) and gain magnification of AGC circuit  114  at a high temperature of 60° C. in the case of zoom magnification of 35-fold (35×), wherein even when the shooting object is bright, the gain magnification A is set at 3 (A=3) while letting the shutter speed be kept at 1/60 (sec); then, the F-number (aperture) is adjusted to ensure that the light amount of the object which reaches the image pickup element becomes one third (⅓). As a result, aperture control becomes possible until the object brightness becomes 10, resulting in the depth of field becoming further deeper owing to stopping down of the diaphragm. Thus, defocus becomes hardly occurrable. 
     It should be noted that although in the above-stated case the value of AGC circuit  114  is set at AGC=1, 2, 3, the optimum value is different in a way depending upon various circumstances of noises applied to the zoom camera. 
       FIG. 4  shows a relation of diaphragm  103  and AGC circuit  114  in accordance with one embodiment of this invention. In the prior art of  FIG. 2 , the gain magnification of AGC circuit is increased after the aperture value becomes the full-open value (F=1.0). In contrast, in the embodiment of  FIG. 4 , the gain magnification of AGC circuit is increased before the aperture value becomes the full-open value, and the correlation of the aperture value and the gain magnification of AGC circuit is specifically controlled by forcing the x value (AE information as read from the AE data readout program  123 ) in the above-stated Equation (1) to stay constant, with the shutter speed S being set to a fixed value—here, S= 1/60 sec. In addition, as the zoom magnification increases from 24× to 36×, the aperture value is made larger to thereby enlarge the effect of the depth of field. 
       FIG. 5  shows a flowchart of a system procedure for setting up the F-number (aperture value) and the gain magnification of AGC circuit  114  shown in  FIGS. 3A-3C . 
     At a step  1 , the control unit of the microcomputer  121  acquires AE evaluation value data, temperature information of the temperature detector  112 , and zoom magnification information. 
     At a step  2 , an attempt is made to determine whether the temperature information of the temperature detector  112  falls within a predetermined temperature range (e.g., 0° C.≦T≦60° C.). If it is within the temperature range, the procedure goes to a step  5 . If it is out of this range, then proceed to a step  3 . 
     At the step  3 , a decision is made to determine whether the zoom magnification is 24× or greater. If it is less than 24×, then proceed to the step  5 . If it is more than or equal to 24× then go to a step  4 . 
     At the step  4 , the detected temperature and the zoom magnification are used to obtain the preset gain magnification of AGC circuit  114  and aperture value. Here, the camera diaphragm is stopped down more strongly as the lens becomes higher in temperature and the diaphragm is stopped down more strongly as the lens becomes larger in zoom magnification. Regarding the diaphragm squeezing rate, it becomes a trade-off with noises existing in electrical circuitry. If such noises are large in amount, it is impossible to stop down the diaphragm so significantly; however, if noises are less in amount, the diaphragm may be stopped down relatively strongly. 
     At the step  5 , in view of the fact that, as far as the zoom magnification range of from 1× (real image size) to 24× is concerned, appreciable defocusing does not take place even without intentional diaphragm stopping down, even where dark photographic object is subjected to image pickup in a similar way to the prior art control, there is employed the traditionally implemented technique with the use of the gain magnification of AGC circuit  114  and the aperture value for making the most of the light rays coming from the photographic object or subject, which reach the image pickup element. 
     According to this control scheme, even in the case of performing high-magnification zooming at high temperatures or low temperatures, it becomes possible to uniformly absorb unwanted variation or fluctuation of the temperature characteristics of the trace curve because of the fact that the depth of field is uniformly made deeper without having to depend upon irregularities of the trace curve&#39;s temperature properties. In other words, even where the focussing point is deviated from the target object or subject of interest to its near side or far side, it is possible to take the focus thereon although a temperature-caused change in positional relationship of the zoom lens and the focus lens exhibits random variability. 
     On the other hand, the control scheme has a drawback that strong use of the gain magnification of AGC circuit  114  would result in an increase in noise; however, this is improvable by improvement of electrical circuit parts or components of the zoom camera. 
     As apparent from the foregoing description, according to this invention, by stopping down the diaphragm  103  relatively strongly, the depth of field is made deeper to eliminate mismatch of the trace curve and the lens properties, thereby preventing defocus. This achieves increased stability of the image quality. 
     In the foregoing some preferred embodiments of this invention have been explained. According to this invention, in the event of high-power zooming, such as 24×, 35×, etc., it is possible to reduce defocus of a sensed image at high or low temperatures. This makes it possible to achieve enhanced stability of the image quality. 
     It should be further understood by those skilled in the art that although the foregoing description has been made on embodiments of the invention, the invention is not limited thereto and various changes and modifications may be made without departing from the spirit of the invention and the scope of the appended claims. For example, the embodiments are the ones that have been explained in order to explain in detail the principles of this invention, and the invention should not always be limited to those which comprise all of the arrangements as disclosed herein.