Iris control

Abstract: A video camera device comprises: image signal level varying means for varying the signal level of an image signal; image signal level detecting means for detecting the signal level of the image signal; zoom lens position detecting means for detecting the position of a zoom lens; and brightness correcting means supplied with an output from the zoom lens position detecting means for correcting brightness attenuating characteristic of the zoom lens responsive to the position of the zoom lens. The gain of the image signal level varying means is varied by an output of the image signal level detecting means and an output of the brightness correcting means.

Description: This invention relates to a video camera device and, in particular, to a video camera device having an exposure control circuit. There is known a video camera device using an inner focus lens assembly (lens block) as its lens system whose front lens is fixed and whose zoom lens and focus lenses are supported for movements in the lens assembly. In the accompanying drawings Fig. 1 shows a block diagram of a video camera device using such an inner focus lens assembly. In Fig. 1, an image of a subject entering through the inner focus lens assembly 51 is focused on a light detecting surface of a CCD image pickup device 53 through an iris 52. Opening and closing of the iris 52 are controlled in response to an output from an iris driving circuit referred to later. An output signal from the CCD image pickup device 53 is supplied to an AGC amplifier 54 whose gain is controlled in response to an output signal from a D/A converter described later. An output signal from the AGC amplifier 54 is converted from an analog form to a digital signal by an A/D converter 55. The digital signal is supplied to a signal processing circuit 56 and to a detecting circuit 57. The signal processing circuit 56 performs a predetermined signal processing for the digital signal supplied, and then outputs it from an output terminal 58. The detecting circuit 57 detects the digital signal, and then supplies it to a comparator 59. A reference level setting circuit 60 is connected to the comparator 59 to set a reference level. An output signal from the comparator 59 is supplied to a control amount computing circuit 62 through a loop filter 61. The control amount computing circuit 62 calculates a desired opening amount for the iris 52 and a desired gain for the AGC amplifier 54 based on the image signal level. Thereafter, an output from the control amount computing circuit 62 is supplied to an iris driving circuit 63 and to a D/A converter 64. Output from the iris driving circuit 63 is supplied to the iris 52 to control the opening amount thereof. The output signal released from the control amount computing circuit 62 and converted to an analog signal by a D/A converter 64 is supplied to the AGC amplifier 54 to control the gain thereof. Fig. 2 shows an arrangement for the inner focus lens assembly 51. In Fig. 2, a first group lens F1 and a third group lens F3 of the inner focus lens assembly 51 are fixed while a second group lens F2 and a fourth group lens F4 can move. The second group lens F2 behaves as a zoom lens, and the fourth group lens F4 as a focus lens. The iris 52 is located between the zoom lens and the third group lens F3, and a color separating prism P is located between the focus lens and the CCD image pickup device 53. Existing video camera devices involve the problem of F drop phenomenon. With reference to Fig. 3, F drop phenomenon is explained. In Fig. 3, incident light through the first group lens F1 is diverged by the zoom lens, and travels through the iris 52. After corrected in direction by the third group lens F3, the light is focused by the focus lens F4 to make an image on the CCD image pickup device 53. Fig. 3A shows a beam of light travelling when the zoom lens is located at its wide end. Among the incident beam of light d, the beam of light that passes through the iris 52 has a diameter as large as the maximum open area of the iris 52. That is, the quantity of light passing through the iris 52 can be determined by changing the open area of the iris 52. Fig. 3B shows the beam of light travelling when the zoom lens is located midway between its wide and telephoto ends. The quantity of light passing through the iris 52 can be determined by changing the open area of the iris 52 as in Fig. 3A. Fig. 3C shows the beam of light travelling when the zoom lens is located at its telephoto end. In Fig. 3C, the diameter of the beam of light travelling through the iris 52 is smaller than the diameter of the maximum open area of the iris 52. Note that the diameter of the beam of light passing through the iris 52 is determined by the diameter of the first group lens F1. The F drop phenomenon is that, because of shading of the beam of light caused by the first group lens F1, the diameter of the beam of light passing through the iris 52 becomes smaller than the maximum open area of the iris 52, and makes it impossible to vary the quantity of light even by changing the open area of the iris 52 (J in Fig. 3C). Fig. 4 shows a gain curve A for the iris and a gain curve B for the AGC amplifier relative to the brightness of a subject in the existing video camera device. The abscissa indicates the quantity of light of the subject while the ordinate indicates the gain. These gain curves are obtained by: Q x R x S x T = U where Q is the brightness of the subject, R is the gain of the iris, S is the gain of the AGC amplifier, T is the photoelectric conversion ratio of the CCD image pickup device, and U is an output of the detecting circuit. In equation (1), if the brightness of the subject is in a range in which exposure can be controlled, then the output of the detecting circuit is constant. However, if the F drop phenomenon occurs because of the position of the zoom lens in the inner focus lens assembly 51, it causes an iris blind zone (shown by E) which is a difference (F drop amount F) between the gain of the iris at the wide end (shown at position C) and the gain of the iris at the telephoto end (shown at position D), and the brightness is not changed even by opening and closing the iris. Moreover, equation (1) is not effective in the iris blind zone E, and the responsibility of the iris decreases. Further, an unnaturalness is produced at the changeover between control by the iris and control by the AGC amplifier (shown at position G). The foregoing statement is indicated by: Q x V x S x T = U where V is the actual gain of the iris, and by: R - W = V where R is the same as that of equation (1) and W is a gain corresponding to the F drop amount F. If the first group lens has a larger diameter, then the F drop phenomenon would be removed. However, the use of larger lenses as the first group lens increases the scale and weight of the lens assembly itself. It does not meet the requirement of a decrease in scale and weight of video camera devices and invites an increase in the cost. In addition, in case of the aforementioned video camera device in which the exposure control circuit controls the opening amount of the iris and the gain of the AGC amplifier in response to the image pickup signal level, an increase in the amount of F drop increases the region in which the image signal level cannot be changed even by controlling the open amount of the iris. As a result, unnaturalness is produced in the responsibility of the iris and at the changeover between the iris and the AGC amplifier, and the performance of the circuit as an automatic exposure control circuit is significantly lowered. JP-A-03222586 discloses a video camera in accordance with the pre-characterising portion of claim 1. It discloses providing a parabolic brightness correction signal, where level is larger towards the circumference, in response to the zoom lens position. This corrects for a level decrease around the circumference caused by iris opening and zoom position. It is therefore an object of the-invention to provide a video camera device which removes blind zone of its iris, and realizes natural exposure control by correcting the gain in response to the amount of an F drop. According to the present invention, there is provided a camera comprising: image signal level varying means comprising an iris and an automatic gain control circuit for varying the signal level of an image signal; zoom lens position detecting means for detecting the position of a zoom lens; brightness correcting means supplied with an output from said zoom lens position detecting means for correcting the brightness attenuating characteristic of said zoom lens by varying the gain of said image signal level varying means in response to the position of said zoom lens; characterised in that image signal level detecting means are provided for detecting said signal level of the image signal and providing an output to said brightness correction means; in that the gain of said image signal level varying means is varied according to the output of said image signal level detecting means; and in that said brightness correcting means is adapted to vary the gain characteristic of said automatic gain control circuit in response to the detected position of said zoom lens to compensate for F-drop. According to the video camera device mentioned above, blind zone of the iris caused by an F drop of the zoom lens is removed by controlling the open amount of the iris on the basis of an output from the zoom lens position detecting means, and the gain is corrected in, for example, an AGC amplifier by an amount corresponding to the blind zone of the iris. The invention will be further described by way of example, with reference to the accompanying drawings, in which:- Figure 1 is a block diagram of a video camera device for use in explanation of a prior art; Figure 2 is a schematic diagram showing an arrangement for an inner focus lens assembly; Figures 3A to 3C are schematic diagrams for use in explanation of F drop phenomenon; Figure 4 is a graph showing a gain curve for an iris and a gain curve for an AGC amplifier in the prior art; Fig. 5 is a block diagram of a video camera device to which the invention is applied; Fig. 6 is a graph showing relation between the amount of movement of a zoom lens and the amount of F drop; Fig. 7 is a diagram showing a gain curve for an iris and gain curves for an AGC amplifier; and Fig. 8 is a block diagram showing an arrangement of a zoom lens position detecting circuit. An embodiment of the invention is explained below with reference to the drawings. Fig. 5 shows a block diagram of a video camera device to which the invention is applied. In Fig. 5, rays from a subject incident through the inner focus lens assembly 1 are focused into an image on a light detecting surface of a CCD image pickup device 3 through an iris 2. The open amount of the iris 2 is controlled by an output from an iris driving circuit referred to later. An output signal from the CCD image pickup device 3 is supplied to an AGC amplifier 4. The gain of the AGC amplifier 4 is controlled in response to an output signal from a D/A converter referred to later. An output signal from the AGC amplifier 4 is converted from an analog form into a digital signal by an A/D converter 5. The digital signal is supplied to a signal processing circuit 6 and to a detecting circuit 7. The signal processing circuit 6 performs predetermined signal processing for the digital signal supplied, and outputs it from an output terminal 8. The detecting circuit 7 detects the digital signal and supplies it to the comparator 9. A reference level setting circuit 10 is connected to the comparator 9 to set a reference level. An output signal from the comparator 9 is supplied to a control amount computing circuit 12 via a loop filter 11. A zoom lens position detecting circuit 15 detects the amount of movement of the zoom lens in the inner focus lens assembly 1, and the detected amount is supplied to a ROM 16. The ROM 16 stores amounts of F drop F corresponding to moving amounts of the zoom lens as shown in Fig. 6. In Fig. 6, the abscissa indicates the position of the zoom lens, and the ordinate indicates the amount of F drop. An amount of the F drop corresponding to a moving amount of the zoom lens is supplied from the ROM 16 to the control amount computing circuit 12. The control amount computing circuit 12 calculates an open amount for the iris 2 and a gain for the AGC amplifier 4 on the basis of outputs from the loop filter 11 and the ROM 16. Thereafter, an output from the control amount computing circuit 12 is sent to the iris driving circuit 13 and the D/A converter 14. An output from the iris driving circuit 13 is sent to the iris 2 to control the open amount thereof. The output signal released from the control amount computing circuit 12 and converted to an analog signal by the D/A converter 14 is sent to the AGC amplifier 4 to control the gain thereof. Fig. 7 shows a gain curve A for the iris relative to the brightness of a subject and gain curves B and B' for the AGC amplifier. The abscissa indicates the quantity of light from the subject, and the ordinate indicates the gain. These gain curves are defined by: Q x V x (S + W) x T = U In comparison of equation (4) with equation (2), although the third term of equation (2) is "S" alone, the third term of equation (4) is "(S + W)". That is, by adding to the gain of the AGC amplifier the gain "W" corresponding to the amount of F drop, the gain curve B of the AGC amplifier is corrected to the gain curve B' shown by the broken line. As a result, the gain is corrected by the amount corresponding to the iris blind zone E, and the iris blind zone E is removed. Fig. 8 shows an arrangement for the zoom lens position detecting circuit 15. In Fig. 8, the zoom lens position detecting circuit 15 includes a variable resistor 15A and an A/D converter 15B. An arm of the variable resistor 15A is coupled to a zoom lens 1A and to the A/D converter 15B. Fixed bias voltages are applied to electrodes at opposite ends of the variable resistor 15A, and the voltage at the variable contact between the variable resistor 15A and the arm varies in response to movements of the zoom lens 1A. The amount of a change in the voltage is converted to a digital value by the A/D converter 15B. The digital value from the A/D converter 15B is supplied to the ROM 16 as a signal indicating the position of the zoom lens. Having described a specific preferred embodiment of the present invention with reference to the accompanying drawings, it is to be understood that the invention is not limited to that precise embodiment, and that various changes and modifications may be effected therein by one skilled in the art without departing from the scope of the invention as defined in the appended claims. For example, although the embodiment has been described as using the inner focus lens assembly as its lens system, the invention is not limited to this. Additionally, the invention may be employed also in still cameras. The invention allows the iris to be controlled in its open amount in response to the amount of F drop, and the gain of the AGC amplifier to to be corrected by an amount corresponding to the amount of F drop. Therefore, blind zone of the iris is removed, and changeover between control by the iris and control by the AGC amplifier is smoothed. As a result, natural exposure control in a camera is possible.

Claims: A camera comprising: image signal level varying means (2,4) comprising an iris (2) and an automatic gain control circuit (4) for varying the signal level of an image signal; zoom lens position detecting means (15) for detecting the position of a zoom lens (1); brightness correcting means (12) supplied with an output from said zoom lens position detecting means (15) for correcting the brightness attenuating characteristic of said zoom lens (1) by varying the gain of said image signal level varying means (2,4) in response to the position of said zoom lens; characterised in that image signal level detecting means (7) are provided for detecting said signal level of the image signal and providing an output to said brightness correction means (12); in that the gain of said image signal level varying means (2,4) is varied according to the output of said image signal level detecting means (7); and in that said brightness correcting means (12) is adapted to vary the gain characteristic of said automatic gain control circuit (4) in response to the detected position of said zoom lens (1) to compensate for F-drop. A camera according to claim 1, wherein the gain attenuating characteristics of said iris (2) and said automatic gain control circuit (4) of said image signal level varying means are substantially the same. A camera according to claim 2, comprising: a lens block (1) for receiving light from a subject to be photographed and including the zoom lens, the iris (4) being positioned to limit the quantity of light from said lens block (1); an image pickup device (3) for converting the light passing through said iris (2) into an electric signal, the automatic gain control circuit (4) being connected to vary the signal level of an output signal from said image pickup device (3); an analog-to-digital converter (5) for converting an output signal from said automatic gain control circuit (4) into a digital signal; a video signal processing circuit (6) for converting a digital signal from said analog-to-digital converter (5) into a predetermined video signal; the image signal level detecting means comprising a signal level detecting circuit (7) for detecting the signal level of an output signal from said analog-to-digital converter (5); a comparator (9) for comparing an output signal of said signal level detecting circuit (7) and a reference level; and wherein the brightness correcting means (12) comprises a control amount computing circuit for determining a gain of said iris (2) and a gain of said automatic gain control circuit (4) on the basis of an output signal of said comparator (9) and an output signal of said zoom lens position detecting circuit (15). A camera according to claim 3, wherein after the gain control of said iris (2) is performed, the gain control of said automatic gain control circuit (4) is performed. A method of controlling the camera of any one of the preceding claims, wherein the iris (2) is controlled to control exposure at image brightness levels above a predetermined value and the automatic gain control circuit (4) is controlled to control exposure at image brightness levels below a second predetermined value, the second predetermined value being varied according to the detected position of the zoom lens (1).

Inventors: SHUJI SHIMIZU, SHUJI, SHIMIZU, Shuji, Shimizu, c/o Sony Corporation
Applicants: SONY CORP, SONY CORPORATION
Application year: 1993
Main classification: H04N5
IPC codes: H04N5