Camera

A camera a has an image sensor; an exposure controller that conducts a main exposure and a dark exposure, in order, when a long-exposure shooting is carried out; an image signal processor that processes image-pixel signals that are generated by the main exposure and are read from the image sensor; and a noise reduction processor that reduces dark current components in the image-pixel signals on the basis of dark current components in the dark exposure. The exposure controller operates the image sensor for heating between the main exposure and the dark exposure.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to a camera that is capable of long-exposure photography, and particularly to a noise reduction process that reduces noise caused by long-exposures.

2. Description of the Related Art

A digital camera configured for long-exposure photography can be used for shooting night scenes, fireworks, celestial bodies, and so on. This so called “bulb shooting” photography allows a trace of a star's movement or the headlights of the vehicle to be imaged on a picture.

In bulb shooting, dark current occurs as noise in an image sensor such as a CCD. The amount of dark current in bulb shooting is greater than that from normal shooting. Also, the value of dark current is different in each pixel. To reduce this fixed pattern noise, an image sensor is exposed in a state in which light is completely blocked after a long exposure is carried out (hereinafter, the first exposure will be referred to as a “main exposure” and the second exposure as a “dark exposure”). A noise component included in image signals is sampled from the difference between output signals of the main exposure and output signals of the dark exposure, so that the noise component can be removed. This process is described, for example, in JP2000-209506A.

Moreover, the temperature of an image sensor increases as the length of exposure increases. Accordingly, dark current increases with increasing period of exposure. One method of reducing a noise component that increases with temperature is to repeatedly take a dark exposure. Then, a noise component can be calculated from the average of a series of detected dark currents. This process is described in U.S. Pat. No. 7,636,113.

When the ambient temperature is extremely low at a photography location, the increase in the temperature of the image sensor is restricted. For example, when photographing a celestial body using bulb shooting at a high-altitude location with a low ambient temperature, the temperature of the image sensor decreases as the exposure time passes. The dark current output level depends upon the temperature of the image sensor; therefore, the lower the temperature, the greater the decrease in the output level of the dark current. Consequently, a dark exposure should last long enough to obtain dark current components accurately, and a totally long photograph time losses convenience.

SUMMARY OF THE INVENTION

An object of the present invention is to provide a camera that is capable of accurately detecting a dark current in a short exposure period, regardless of low ambient temperature conditions.

A camera according to the present invention has an image sensor; an exposure controller that carries out a main exposure and a dark exposure, in order, during long-exposure photography; an image-signal processor that processes image-pixel signals that are generated in the main exposure and read from the image sensor; and a noise-reduction processor that reduces dark current components included in the image-pixel signals on the basis of dark current components obtained from the dark exposure. The exposure controller operates the image sensor for heating between the main exposure and the dark exposure.

A camera according to another aspect of the present invention has an image sensor; an exposure controller that carries out a main exposure and a dark exposure, in order, during long-exposure photography; an image-signal processor that processes image-pixel signals that are generated in the main exposure and read from the image sensor; and a noise-reduction processor that reduces dark current components included in the image-pixel signals on the basis of dark current components obtained from the dark exposure. During the dark exposure the exposure controller operates at least one device that is activated by an electric power supply.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Hereinafter, the preferred embodiments of the present invention are described with reference to the attached drawings.

FIG. 1is a block diagram of a digital camera according to a first embodiment.

The digital camera10is an SLR-type camera that is equipped with an interchangeable lens unit11mounted on the front surface of the camera10, a viewfinder22, and an LCD47arranged on the back surface of the camera10. A system control circuit31including a CPU, a ROM unit, a RAM unit, etc., controls the motion of the camera10. A program for the control of the camera10is stored in the ROM unit.

When electric power is activated by the operation of a power lever (not shown), an electric power supplier49supplies electric power to the system control circuit31, a CCD driver49in an initial circuit36, and so on. Then, the digital camera10is activated in accordance to the operation control program. While electric power is in the ON state, a given photography mode is set.

A mode dial (not shown) is provided on the upper surface of the camera10and is operated by a user when selecting from a series of photography modes. A user can select a point-and-shoot auto mode that automatically focuses on a subject image and sets exposure values, or select a manual-exposure mode in which exposure values may be set manually.

The interchangeable lens unit11is equipped with photographing optical lenses14and15, which are driven by a lens controller17to adjust a focal length. Light passing through the lens unit11is directed to the viewfinder22by a quick return mirror21. A user views a target subject through the objective lens of the viewfinder22.

When a release button (not shown) is depressed halfway, the system control circuit31detects an operation signal fed from a release switch54and carries out an AF adjustment process and an exposure calculation process. In the AF adjustment process, an AF sensor52outputs luminance signals of the target subject and the system control circuit31outputs a control signal to the lens controller17on the basis of the luminance signals so that the photographing optical lenses14and15are repositioned to bring the subject image into focus.

Also, the system control circuit31calculates an F value and a shutter speed (a period of exposure) from an exposure program diagram, in response to a subject brightness signal fed from a metering sensor53. An aperture16provided in the lens unit11is driven by an aperture driver32on the basis of the calculated aperture value. An exposure display57displays exposure information such as the aperture value and shutter speed along the bottom line of the visual field that is formed by the viewfinder22.

When the release button is depressed completely, a photographing or shooting process is carried out in response to an operation signal fed from a release switch54. Concretely, the quick return mirror21moves upward and a focal plane shutter25opens for the exposure period corresponding to the calculated shutter speed. Thus, the photographing optical lenses14and15form an object image on the photo-receiving surface of the CCD33. The quick return mirror21and the focal plane shutter25are driven by a mirror driver34and a shutter driver35, respectively.

After the exposure period ends, one frame's worth of image-pixel signals are read from the CCD33by the CCD driver38and are fed to an A/D converter39. On the light-receiving surface of the CCD33a color filter array, in which R, G, and B elements are Bayer-arrayed, is disposed. Accordingly, R, G, and B image-pixel signals are output from the CCD33. The R, G, and B image-pixel signals are converted from analog signals to digital signals and output to an image-signal processing circuit40.

The image-signal processing circuit40applies an image-signal process such as white-balance processing to the input R, G, and B image-pixel signals to generate color image data. The generated color image data are temporarily stored in the RAM41and output to the system control circuit31to compress the image data. The compressed still-image data are then recorded in a memory card48such as a SD card via a card I/F43.

Also, a recorded still image is temporarily displayed on the LCD47soon after the photography process. An LCD driver45drives the LCD47on the basis of image data fed from the image-signal processing circuit40and turns a backlight on in accordance to the displayed still image.

On the other hand, when the manual exposure mode is selected, a user can set an aperture value and/or shutter speed (i.e., a period of exposure) to a preferred value by operating a cross button or an electronic dial (neither of which are shown in the drawings). The system control circuit31sets the input values on the basis of an operation signal fed from an electronic dial switch58.

In the case of shooting night scenes or celestial bodies, a shutter speed is set for a relatively long exposure compared to that in a normal photography mode. Generally, such shooting based on long-exposures is also known as “Bulb shooting.” In the present embodiment, when a shutter speed (i.e., an exposure period) is set to a period longer than a predetermined period (e.g., more than 0.5 or 1 second), it is regarded as long-exposure shooting. Accordingly, an exclusive noise-reduction process is applied to one frame's worth of image-pixel signals. Note that a user may optionally perform the noise reduction process after bulb shooting by selecting a user setting.

In the bulb shooting, an exposure based on the calculated shutter speed (hereinafter, called a “main exposure”) is carried out first, and a further exposure is then carried out in a state when the shutter25is closed (hereinafter, called a “dark exposure”) after the main exposure.

Image-pixel signals from the main exposure are temporarily stored in the RAM unit provided in the system control circuit31. On the other hand, electric charges that accumulate during the dark exposure are output from the CCD as dark current components. The image signal processing circuit40carries out the noise reduction process that decreases the image-pixel signals with dark current components. This process is carried out for each pixel. Thus, fixed pattern noise is removed from the R, G, and B image-pixel signals.

A temperature sensor27is positioned near the CCD33. The system control circuit31monitors a temperature signal that is output from the temperature sensor27during the Bulb shooting that includes the main exposure and the dark exposure. Then, as described below, the CCD33is operated to increase the temperature of the CCD33.

Next, the relationship between the CCD temperature and a generated dark current is explained with reference toFIGS. 2-3.

FIG. 2is a graph indicating the change in temperature during the main exposure and the dark exposure in a state where the temperature of the CCD increases.FIG. 3is a graph indicating the change in temperature during the main exposure and the dark exposure in a state where the temperature of the CCD decreases. The two exposures shown inFIG. 2andFIG. 3are carried out continuously. Note that a reading period “TM” for reading one frame's worth of image-pixel signals occurs between the main exposure and the dark exposure.

As is well known, a dark current is noise caused by heating of an image sensor such as the CCD33. The value of the dark current varies with the changing CCD temperature. InFIG. 2, a curved line “L1” represents the changing temperature of the CCD33.

The total amount of dark current that occurs in the main exposure or the dark exposure is calculated for each pixel by using the following formula. Note that “Sn” represents a total amount of dark current at a time “tn” after the start of the main exposure, “Tn” represents the temperature of the CCD33at the time “tn”, “Ts” represents the temperature of the CCD33at the start of the main exposure, and “a” represents a temperature coefficient of the CCD33.
Sn=2(Tn-Ts)/α(1)

In the present embodiment, dark current components to be removed from the image-pixel signals are generated by the dark exposure. A period of the dark exposure is adjusted such that the amount of dark current in the dark exposure becomes equal to that in the main exposure. In the dark exposure, all of the accumulated charges become dark current components. Note that the absolute amount of dark current is different in each pixel. However, by subtracting a value of dark current from a corresponding pixel value of image-pixel signals, dark current components can be removed or reduced from the total of one frame's worth of image-pixel signals.

As can be seen from the formula (1), even if the CCD temperature changes during the main/dark exposure due to a change in the outside-air temperature, the amount of dark current in the dark exposure can be adjusted to equal that of the main exposure by adjusting the period of the dark exposure. When shooting in a normal photography environment, the temperature of the CCD33increases by heat produced while the CCD33is driven. The longer a period of exposure, the higher the temperature increases. Therefore, the temperature of the CCD33generally increases substantially after the main exposure. When the period of the main exposure is “E1” (=T1−T0) and the amount of dark current obtained by the formula (1) is“S1”, the period of the dark exposure “E2” (=T3−T2) is shorter than the period “E1” because the amount of dark current “S2” soon reaches the total amount of dark current “S1” because of the increase in the temperature.

On the other hand, when the outside-air temperature is very low, the temperature of the CCD33decreases. As can be seen from a curved line “L3” inFIG. 3, the temperature of the CCD33does not increase regardless of the motion of the CCD33; inversely, the temperature decreases with the lapse of the main exposure. The decrease in the temperature continues during the period when the image-pixel signals are read. In this case, the output of dark current decreases (see the formula (1)). Consequently, the period of the dark exposure “E2” (T3−T2) that is necessary for obtaining an amount of dark current “S2” that is equal to “S1” becomes longer than the period of the dark exposure “E1” (=T1−T0).

In this embodiment, the CCD33is operated for heating between the main exposure and the dark exposure to increase the CCD temperature temporarily. Hereinafter, the heating motion of the CCD33is explained withFIG. 4.

FIG. 4is a graph showing the change in the CCD temperature when the CCD33is operated for heating.

After the period of the main exposure “E1” is finished and the image-pixel signals charged in the CCD33are read from the CCD33during the period “TM”, the CCD33goes into a “heating motion” before the dark exposure starts. Concretely, a heating motion that reads electric charges accumulated in a shutter-closed state is carried out. This forced motion of the CCD33generates heat and increases the temperature of the CCD33. This motion is separate from the reading of image-pixel signals that forms a photographic image. The read image-pixel signals are abandoned without utilizing a photographing process or an image recording process. Note that various motions may be applied as a heating motion. The CCD33may be subjected to an arbitrary heating motion that is substantially useless with respect to a shooting or image recording process.

The temperature of the CCD33temporarily increases by operating the CCD33for a predetermined period “TD”. Since the temperature at the start of the dark exposure is relatively high, a high output level of dark current can be maintained in spite of a decrease of the temperature during the dark exposure. Consequently, the period of the dark exposure “E2” is shortened. InFIG. 4, a curved line “L3” represents the change in the CCD temperature with the heating motion of the CCD33, whereas a curved line “L3′” represents the change in the CCD temperature without the heating motion.

The timing of the completion of the dark exposure is adjusted such that the amount of dark current obtained in the dark exposure is the same as that obtained in the main exposure. Concretely, regarding a given predetermined pixel (s), the amount of dark current in the main exposure is first calculated based on the formula (1). Then, the amount of dark current in the dark exposure is successively calculated based on the formula (1) and it is determined successively whether the amount of dark current in the dark exposure is equal to that in the main exposure. The dark exposure is finished at a time when the amount of dark current in the dark exposure is equal to or greater than that in the main exposure.

The detection of the CCD temperature is carried out at short time intervals (e.g. 100 msec) from the main exposure to the dark exposure. During the dark exposure, a calculation of dark current, a comparison of it with the total dark current from the main exposure, and a determination of the completion timing of the dark exposure is carried out at the short time intervals.

In this case, the system control circuit31calculates a total amount of dark current occurring in the main exposure by using the following formula. Note that “S1” represents the total amount of dark current in the main exposure, “k” represents the time of temperature detection after the start of the main exposure, “T0” represents a temperature at the start of the main exposure, “Tk” represents a detected temperature at the time “k”, “m” represents the time of temperature detection at the end of the main exposure, and “Tm” represents the temperature at that time.

Then, when the dark exposure starts, a total amount of dark current is calculated by the following formula at the predetermined time intervals. Note that “S2” represents a total amount of dark current in the main exposure, represents a time of temperature detection after the start of the dark exposure, “T0” represents the temperature at the start of the dark exposure, “Tj” represents the detected temperature at time “j”, “l” represents the time of temperature detection at the end of the main exposure, and “Tl” represents the temperature at that time. When “S2” is equal to or greater than “S1”, the dark exposure is completed.

Such control of the completion timing of the dark exposure allows a positive adjustment to be made to the heating motion period “TD” and the period of the dark exposure. For example, a relationship between a change in the CCD temperature, an increase in the temperature due to the heating motion of the CCD, and the timing of the completion of the dark exposure can be established empirically and its data can be stored in a memory. Then, the period of a heating motion and the period of the dark exposure can be set in accordance to a change in temperature during the main exposure based on the stored information.

Herein, the period of the heating motion “TD” is set such that the period of the dark exposure is shorter than that of the main exposure. Concretely, the period “TD” is set to a period in which a decreasing temperature returns to a temperature that is close to the temperature at the start of the main exposure (seeFIG. 4).

Next, a long-exposure photography process is explained with reference toFIGS. 5 and 6.

FIG. 5is a flowchart of the long-exposure photography process.FIG. 6is a timing chart of the photography process.

When a shutter speed, i.e., exposure period, is set to a value equal to or longer than a given period in the manual exposure mode and a release button is depressed, the long-exposure photography process is carried out (S101and S102). Herein, it is determined that the long-exposure photography process should be conducted when the exposure period is greater than or equal to 0.5 second.

During the long-exposure photography process, temperature data are output from the temperature sensor27at constant time intervals (e.g., 100 msec). In the main exposure, it is determined whether or not the temperature of the CCD33decreases during the main exposure (S103). Herein, the temperature at the end of the main exposure is compared with the temperature at the start of the main exposure.

When it is determined at Step S103that the temperature of the CCD33decreases, the CCD33is operated for a given interval “TD” after the image-pixel signals are read from the CCD33, as shown inFIG. 6(S104). This heating motion increases the temperature of the CCD33. Then, the dark exposure is carried out after the heating motion (S105). Consequently, dark current components are generated. Furthermore, a noise reduction process is carried out (S106). On the other hand, when the temperature does not decrease in the main exposure, the CCD33is not operated and the dark exposure is directly started.

In this way, in the present embodiment the CCD33is operated after the main exposure and before the dark exposure. Thus, a decrease in dark current is prevented even if the ambient temperature at a photography location is low, so that Bulb shooting can be effectively carried out. And because the temperature is successively detected, the CCD33is operated only when necessary. Also, the period of the dark exposure can be optionally set in accordance to a change in the temperature by utilizing the above described formula.

Next, a digital camera according to the second embodiment is explained with reference toFIGS. 7-9. The second embodiment is different from the first embodiment in that some devices incorporated in the camera, particularly devices other than the CCD, are operated for heating. Other constructions are substantially the same as those in the first embodiment.

FIG. 7is a graph showing the change in the CCD temperature according to the second embodiment.

In the second embodiment, during a dark exposure an image signal processing circuit40, back light46, and a metering sensor53(all shown inFIG. 1) are operated for heating. Concretely, image pixel signals that have “zero” levels are input to the image signal processing circuit40in which a series of image signal processes are applied to the input image-pixel signals, but the generated image data are not stored in the RAM41. Such image signal processing during the dark exposure does not affect the dark exposure or the image signal process.

Also, the back light46is turned ON in a state when a recording image is not replayed. Furthermore, the metering sensor57is turned ON so that the metering sensor57is driven during the dark exposure.

The heating motions of the above electric circuit or electronic device only cause heat and are not directly associated with a photography process. The generated heat is transmitted to the CCD33. Consequently, a decrease in the CCD temperature is restricted during the dark exposure, which allows for the dark exposure to be shortened. InFIG. 7, “L4” represents the change in CCD temperature when the above devices are operated and “L4′” represents the change in CCD temperature without the above devices being operated.

Next, a long-exposure photography process is explained with reference toFIGS. 8 and 9.

FIG. 8is a flowchart of a photography process according to the second embodiment.FIG. 9is a timing chart of a photography process according to the second embodiment.

Similarly to the first embodiment, when a release button is depressed in a state when an exposure is equal to or longer than a predetermined period, the main exposure is carried out and it is determined whether the CCD temperature decreases (S201-S203). If the CCD temperature decreases, the image signal processing circuit40, the back light46, and the metering sensor53are driven in accordance to the start of the dark exposure (S204and S205). Herein, the start of the motion of these devices is almost simultaneous with the start of the dark exposure.

When the dark exposure ends, the heating motion of the devices finish accordingly, and a noise reduction process is carried out (S206and S207). On the other hand, when the CCD temperature increases, the devices are not operated for heating.

In this way, in the present embodiment, some devices that receive electric power are operated to increase the temperature of the CCD33. Note that one or more devices may be operated for heating. The devices may be operated for either part or all of the total period of the dark exposure.

A combination of the first and second embodiment may be applied. Also, a camera other than an SLR-type camera may be applied.

The present disclosure relates to subject matter contained in Japanese Patent Application No. 2011-170958 (filed on Aug. 4, 2011), which is expressly incorporated herein by reference, in its entirety.