JP4674472B2 - Digital camera - Google Patents

Description

  The present invention relates to a digital camera capable of accurately performing focus adjustment (focusing) during low-luminance photography such as astronomical photography and night scene photography.

  In an autofocus (AF) camera, due to the sensitivity characteristics of the focus detection sensor, it may take time to focus or may not be able to focus when the brightness is low such as at night. Therefore, many cameras have a function of illuminating a subject by automatically irradiating AF auxiliary light when it is determined that the luminance is low (see, for example, Patent Document 1).

  On the other hand, there is an AF control method in which a focusing optical system is driven to a plurality of focus positions to perform imaging, a focus evaluation value is calculated from each image, and a focus position is detected from those focus evaluation values. In Patent Document 2, since the point light source is likely to be saturated when photographing night scenes and the like, it is difficult to detect the in-focus position from the focus evaluation value. It is described to do.

JP-A-63-44611 JP 2000-50147 A

  By the way, in low-intensity long-time exposure in which the exposure time is several minutes or more, it is a heavy burden to repeat photographing many times, so it is desirable to avoid focus mistakes as much as possible. However, especially in astronomical photography, it is extremely difficult for the photographer to focus correctly with manual focus while checking the viewfinder. On the other hand, the focus position detected by AF may not be accurate depending on conditions. , I'm worried about leaving everything to AF. Needless to say, the above-described AF auxiliary light is not used for astronomical photography or night scene photography.

A digital camera according to any one of claims 1 to 10 includes a driving unit that drives a focusing optical system of a photographic lens to perform focus adjustment, and an imaging unit that photoelectrically converts a subject light beam that has passed through the photographic lens to generate an image. And first preliminary photographing for controlling the driving means and the imaging means to drive the focusing optical system to the first position and acquiring the first image when the low-luminance focusing mode is set, and the focusing optical system at the first position. The second preliminary shooting is performed in which the second image is acquired by driving to a second position different from the first position, the first image is stored in association with the first position, and the second image is associated with the second position. Control means for controlling to store and display means capable of displaying the first image and the second image, and the control means selects either the first image or the second image displayed on the display means One of the selected first image and second image The driving means is controlled so as to drive the focusing optical system to the first position or the second position stored in association with .

According to the first to tenth aspects of the present invention, focusing at low luminance can be performed accurately and easily .

An embodiment of the present invention will be described with reference to FIGS.
FIG. 1 is a block diagram of a digital camera according to this embodiment. The image sensor 1 is composed of, for example, a CCD, and receives a subject light beam that has passed through a photographing lens (here, an AF lens) LE, photoelectrically converts it, and generates an electrical image signal. The generated image signal is subjected to various processes in the image processing unit 2 and then recorded on a recording medium such as a memory card MC by the recording unit 3.

  The camera control unit 4 includes a CPU, a memory, and the like, and controls the image processing unit 2, the lens driving unit 5, and the external display unit 6. The lens driving unit 5 drives the focusing light optical system FL of the AF lens LE in the optical axis direction in order to adjust the focus in accordance with an instruction from the camera control unit 4. The external display unit 6 displays an image, a menu screen, or the like on the liquid crystal monitor 6A, for example, in accordance with an instruction from the camera control unit 4. The information input unit 7 includes switches that are turned on and off in conjunction with the operation of a release button and other operation members.

  The camera control unit 4 usually operates the lens driving unit 5 in response to a half-press operation of the release button, and focuses the AF lens LE on the subject. The AF method does not matter, but in any case, it is often impossible to focus at low luminance. Therefore, the camera has a low brightness focusing mode in addition to the normal AF mode. As will be described later, preliminary shooting is performed at a plurality of focus positions, and a plurality of images obtained thereby are used for focus adjustment, which is suitable for shooting that is difficult to focus on, such as astronomical shooting. . Switching between the normal AF mode and the low-intensity focusing mode may be performed using a dedicated operation member or a menu screen.

Details of the low brightness focusing mode will be described.
In this embodiment, a single-lens reflex digital camera is assumed. When the low-luminance focusing mode is set, the photographer sets the focusing optical system FL at a desired focus position by a manual focusing operation. For example, in the case of astronomical photography, the focusing optical system FL is set to the end on the infinity side (this position is not infinity) or slightly closer to it. This is because the possibility of focusing on the subject in this vicinity is high.

  Next, the photographer determines the composition so that the subject is positioned within the focus area of the shooting screen, and presses a release button, for example. In response to this, the camera control unit 4 performs the first preliminary shooting with the current focus position as the initial position, and further moves the focusing optical system FL at a predetermined pitch via the lens driving unit 5, that is, Preliminary photographing is performed a plurality of times while changing the focus position stepwise. Here, photographing represents an operation of driving a diaphragm or shutter (not shown) so that a predetermined amount of light is received by the image sensor 1 to obtain the image signal, and this does not change between preliminary photographing and main photographing. However, since the image signal obtained by the preliminary shooting is only temporarily used, it is not left on the memory card MC.

  The number of preliminary photographing, the moving pitch and moving direction of the focusing optical system FL are set in advance by the photographer on the menu screen or the like. As the moving direction, for example, it is possible to select whether to move to both the infinity side and the close side with respect to the initial position, or to move only in one direction. The shutter time for preliminary shooting may be specified by the photographer or automatically determined by the camera, but may be shorter than the actual shooting because it is only necessary to determine the in-focus state. . In the case of astronomical photography, the exposure time for main photography is several minutes to several tens of minutes or more, but in preliminary photography, it may be about 5 seconds to 10 seconds.

Images obtained by each preliminary shooting (hereinafter referred to as preliminary shooting images) are temporarily stored in the memory in association with the focus position at that time. The preliminary photographed image does not need to store the entire image, and only the focus portion may be cut out and stored. The focus part here refers to a very narrow area including the focus area, and by storing only that part, a plurality of images can be stored without increasing the memory capacity. In the case of a camera having a plurality of focus areas, the area selected by the photographer or camera is targeted. As is well known, the focus position can be taken into the camera from a distance encoder provided in the lens.
If a sufficient memory capacity can be secured, the entire captured image may be stored.

  When all the preliminary shootings are completed, the camera control unit 4 extracts the best focused state from the stored preliminary shooting images. For example, a known contrast AF method may be used to determine the in-focus state. In contrast AF, focusing on the fact that there is a correlation between the degree of blur of an image and the contrast, focusing is performed using the fact that the contrast of the image is maximized when the image is focused. The magnitude of the contrast can be evaluated by the magnitude of the high frequency component of the image signal. Therefore, the camera control unit 4 extracts the high frequency components of each preliminary image signal, and integrates the absolute value of each high frequency component as the focus evaluation value. Then, the image with the maximum focus evaluation value is determined as the image with the best focus state.

  Thereafter, the camera control unit 4 displays a thumbnail of the stored plurality of pre-photographed images on the liquid crystal monitor 6A. For an image that is determined to be in the best focus state, for example, the frame portion of the image is replaced with another image By using a different color, the photographer is clearly informed that the image is currently selected. At this point, however, the selection has not been finalized.

  The photographer compares a plurality of thumbnail images and searches for the best one in focus. In the thumbnail display, a plurality of images are displayed on one screen in the order of shooting, and individual images can be selectively enlarged as necessary. Since each image is obtained by enlarging the focus portion, it is possible to easily confirm the in-focus state by visually enlarging the image. In particular, an image determined to be the best by the camera or an image close to the focus position is carefully checked, and if it is determined that the determination by the camera is correct, the “decision” operation may be performed as it is. On the other hand, if it is determined that the camera judgment is not correct, the image determined to be the best is selected and the “decision” operation is performed. The selection state is changed by shifting the frame of the different color by a predetermined operation.

  When the “decision” operation is performed, the selection of the pre-photographed image that was in the selected state at that time is confirmed. The camera control unit 4 reads the focus position stored in association with the selected image, moves the focusing optical system FL to the focus position, and fixes at the position. If there is no problem, the photographer performs the main photographing by long exposure at the focus position.

  On the other hand, if none of the displayed pre-photographed images is in satisfactory focus, the best one is selected for the time being, and the focusing optical system FL is moved to the corresponding focus position. This focus position is not an accurate subject focus position, but there is no doubt that it is close to it. Therefore, the focus adjustment in the low brightness focusing mode is performed again with this position as the initial position. In this case, setting the movement pitch of the focusing optical system FL smaller than the first time increases the possibility of capturing a focus peak that could not be captured at the first time. In this way, the focusing optical system FL can be finally brought to the in-focus position of the subject by gradually reducing the moving pitch and performing focusing twice or three times. By performing actual shooting in this state, it is possible to reliably prevent defocusing.

FIG. 2 shows a flowchart for realizing the above-described low-luminance focusing mode processing. It is assumed that the number N of preliminary photographings and the movement pitch P of the focusing optical system FL are already set and stored.
When the release button is operated while the low brightness focusing mode is selected, this program is started by the camera control unit 4. First, in step S1, the preliminary shooting number counter n is set to an initial value of “0”. In step S2, preliminary shooting is performed, and in step S3, the focus portion is cut out from the image obtained by the preliminary shooting, and is temporarily stored in association with the focus position at that time.

  In step S4, n is counted up, and in step S5, it is determined whether or not the value of n has reached a preset number of times N. If N has not yet been reached, the focusing optical system FL is moved by the moving pitch P in step S6, and the process returns to step S2. If n has reached N, it is determined that N times of preliminary shooting have been completed, and the process proceeds to step S7, where the best in-focus state is selected from the N stored preliminary images. The selection method is as described above. In step S8, the stored N pre-photographed images are displayed as thumbnails on the liquid crystal monitor, and the image determined to be in the best focus state is selected.

  Thereafter, the process goes through the loop of steps S9 and S10, and waits for the “decision” operation or the image selection state to be changed. When the selection state changing operation is performed, the selection display is moved according to the operation in step S11, and the process returns to step S9. When the determination operation is performed, in step S12, the focus position corresponding to the image in the selected state at that time is read, and in step S13, the focusing optical system FL is moved and fixed to the focus position.

The case where the AF lens is mounted has been described above, but the case where the MF (manual focus) lens LE1 is mounted will be described below. FIG. 3 is a block diagram when the MF lens is mounted.
Since the lens cannot be driven on the camera side when the MF lens is mounted, the photographer moves the focusing optical system FL while checking the distance scale on the lens side in the preliminary shooting in the low brightness focusing mode. . The camera control unit 4 cuts out and stores the focus portion of the obtained image every time preliminary shooting is performed in the same manner as described above. However, since the distance information (focus position) cannot be obtained from the lens side, the focus position is I don't remember. For this reason, the photographer needs to read the distance scale on the lens side during the preliminary photographing, and write down the indicated value as the focus position. At that time, a numerical value indicating the order of preliminary shooting is associated and recorded. In order to perform more precise focusing, it is desirable to attach a gauge with a fine scale to the position of the distance pointer of the lens.

  When preliminary shooting is completed a desired number of times, the photographer inputs that fact to the camera by operation. In response to this, the camera control unit 4 extracts the best in-focus state from the preliminary photographed image in the same manner as described above. Preliminary shot images are displayed as thumbnails, and the best focus state is clearly indicated. The difference from the above is that a numerical value indicating the shooting order is attached to each thumbnail image. What is necessary is just to display this superimposed on each image. The photographer compares a plurality of thumbnail images to find the best in-focus state, and if found, confirms the number assigned to the image. Then, the focus position (scale indication value) corresponding to the number is recognized from the user's memo, and the manual focus operation is performed so that the indication state is obtained. Thereafter, photographing is performed at the focus position. It should be noted that more precise focusing can be performed by changing the lens moving pitch as described above and repeating the above processing.

FIG. 4 shows a flowchart for realizing the low-intensity focusing mode processing when the MF lens is mounted.
When the release button is operated while the low brightness focusing mode is selected, this program is started by the camera control unit 4. First, in step S21, the counter n of the number of times of preliminary photographing is set to “0” which is an initial value. In step S22, preliminary photographing is performed, and in step S23, n is counted up. In step S24, the focus portion is cut out from the image obtained by the preliminary shooting, and temporarily stored in association with the value of the counter n at that time. In this case, n represents the order of preliminary shooting.

  Next, the process loops through steps S25 and S26, and waits for an operation to end the preliminary shooting or a next preliminary shooting start operation (for example, a release operation). The photographer can change the focus position and start the next preliminary shooting. When the next preliminary shooting start operation is performed, the process returns to step S22, and when the preliminary shooting end operation is performed, the process proceeds to step S27. In step S27, the best in-focus state is selected from the n stored pre-photographed images. In step S28, n stored pre-photographed images are displayed as thumbnails on the liquid crystal monitor, and a numerical value indicating the order of the photographing (value of n at the time of storing each image) is superimposed on each image. An image that is determined to be in the best focus state is set to the selected state. The photographer looks at the display image and performs manual focusing as described above.

FIG. 2 is a control block diagram of a digital camera according to an embodiment of the present invention, showing a configuration when an AF lens is mounted. The flowchart which shows the procedure of the low-intensity focusing mode process at the time of AF lens mounting | wearing. The block diagram of the camera shows the configuration when the MF lens is mounted. The flowchart which shows the procedure of the low-intensity focusing mode process at the time of MF lens mounting | wearing.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Imaging device 2 Image processing part 3 Recording part 4 Camera control part 5 Lens drive part 6 External display part 6A Liquid crystal monitor 7 Information input part FL Focusing optical system LE AF lens LE1 MF lens MC Memory card

Claims (10)

Driving means for adjusting the focus by driving the focusing optical system of the taking lens;
Imaging means for photoelectrically converting a subject luminous flux transmitted through the photographing lens to generate an image;
When the low-luminance focusing mode is set, a first preliminary photographing that acquires the first image by controlling the driving unit and the imaging unit to drive the focusing optical system to the first position, and the focusing optical system Drive to a second position different from the first position to perform a second preliminary photographing to acquire a second image, store the first image in association with the first position, and correspond to the second position And control means for controlling to store the second image;
Display means capable of displaying the first image and the second image ,
Wherein, when said one of the display means on the displayed first image and the second image is selected, correspondence to any one of the selected first image and the second image is in the digital camera, wherein the controller controls the driving means to drive said focusing optical system in the first position or the second position is stored. The control means performs control so as to perform main shooting while fixing the focusing optical system at the first position or the second position when a main shooting operation is performed by a photographer. 1. The digital camera according to 1. 2. The control unit according to claim 1, wherein when the low-luminance focusing mode is reset by a photographer, the control unit performs the preliminary photographing again using the first position or the second position as an initial position. Digital camera. Determining means for determining an in-focus state of the first image and the second image;
4. The digital camera according to claim 1 , wherein the display unit clearly displays an image that is determined to be in a good focus state by the determination unit . 5. 5. The digital camera according to claim 1, further comprising a number setting unit capable of setting the number of times of preliminary photographing . 6. The digital camera according to claim 1 , further comprising pitch setting means capable of setting a movement pitch of the focusing optical system in the preliminary photographing . The digital camera according to claim 1, wherein the first image and the second image are images obtained by cutting out a part of the image output from the imaging unit. The control unit allows the plurality of preliminary shootings when the low-luminance focusing mode is set, cuts out a focus portion from an image obtained by each preliminary shooting, and displays each cut image on the display unit. The digital camera according to any one of claims 1 to 7, wherein the display image clearly indicates which display image was taken. The digital camera according to claim 1, further comprising a low-luminance focusing mode setting unit capable of setting the low-luminance focusing mode. The digital camera according to claim 1, further comprising a storage unit capable of storing the first image and the second image.

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