JPH0488760A - Automatic focusing camera - Google Patents

Abstract

PURPOSE:To prevent malfunction of automatic focusing control by driving an image pickup lens over the entire area from the infinity pickup position to the closest pickup position once, storing a position relating to a maximum AF value and returning the image pickup lens to proper position. CONSTITUTION:An image formed on an image pickup element 18 by an image pickup lens 12 is converted into an electric signal. An image pickup element driving circuit 20 gives a scanning signal to the element 18. The electric signal from the element 18 is given to a video signal processing circuit 22, in which the signal is converted into a luminance signal. Simultaneously the luminance signal from the circuit 22 is given to an AF evaluation value detector 24. The detector 24 generates an AF evaluation value based on the luminance signal by the absolute value integration processing. Upon the receipt of an AF command externally, a microcomputer 16 allows a step motor 14 to drive the lens 12 while monitoring the AF evaluation value outputted from the detector 24 in response to a selection command representing whether a maximal value retrieval or a maximum value retrieval.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to an autofocus camera, and more particularly to a one-shot autofocus device for a camera-integrated VTR and an autofocus device for a still camera.

[Prior art]

Conventionally, automatic focusing of camera-integrated VTRs and still cameras (
AF) methods include external ranging methods such as infrared irradiation methods, and internal ranging methods such as the so-called "r-mountain climbing method" that searches for the focal point position where the high frequency component power (energy) of the video signal has a maximum value. An example of an autofocus video camera using the "mountain climbing method" is disclosed in Japanese Patent Laid-Open No. 1-284181.

When applying this type of "mountain climbing method" to a camera-integrated VTR or still camera, the video signal itself output from the image sensor in the camera can be used as focusing information, which improves both cost and accuracy. This is a very expensive method.

[Problem to be solved by the invention]

However, in the ``mountain climbing method'', if the subject has a narrow spatial frequency distribution in the horizontal axis direction, such as vertical stripes, the focus may be incorrect. This is because in the process of moving the focus of the imaging lens when photographing a vertically striped subject, the high frequency component of the video signal (energy) exhibits several maximum values, and the maximum value may be found at a position other than the in-focus point. In some cases, the focus may be fixed at that position and may not be in focus after all.

To explain in more detail, the out-of-focus state of the imaging lens has the effect of a low-pass filter on the video signal, and the frequency characteristics of this low-pass filter are as shown in FIG. The horizontal axis in FIG. 3 is the horizontal spatial frequency μ of the image to be projected onto the image sensor, which is proportional to the video signal frequency. The vertical axis shows the gain g.

Among FIGS. 3(a) to 3(f), FIG. 3(a) shows the most out-of-focus state, and the pass band of the low-pass filter is the narrowest. Figures 3(b) to 3(
As shown in e), the pass band widens as the focus is adjusted, and FIG. 3(f) shows an ideal state in which the focus is perfect and the diameter of the circle of confusion is zero.

In FIG. 3, μ0 is assumed to be the horizontal spatial frequency of a vertically striped object, and for such an object with a narrow frequency distribution, the amount of signal passed through the low-pass filter does not increase monotonically as the focus progresses. Therefore, the automatic focus control evaluation value (hereinafter simply referred to as rAF evaluation value) does not increase monotonically, and an AF evaluation value curve as shown in FIG. 4 is drawn. In FIG. 4, the horizontal axis shows the distance ld between the image sensor and the real image, and the vertical axis shows the AF evaluation value.

In the "mountain climbing method," the method searches for the maximum value of the AF evaluation value obtained while moving the focus, so in some cases, the focusing operation may end when the d≠00 sidelobe peak is searched. Sometimes it happens.

Therefore, the main objective of this invention is to provide an autofocus camera that does not cause malfunctions.

[Means to solve the problem]

Simply put, this invention consists of an imaging lens that forms a subject image on an image sensor, an image sensor that converts the formed subject image into a video signal, and an evaluation that provides a guideline for the degree of focus based on the video signal. A value detection means for outputting a value (IT value detection means), a first drive means for driving the imaging lens over the entire area from the infinity photographing position to the closest photographing position in response to a focusing command; a storage means for storing a position obtained by the maximum evaluation value when the first evaluation value is being driven;
A second step of driving the imaging lens to the position where the maximum evaluation value is obtained after driving the imaging lens to the entire area by the driving means.
An autofocus camera comprising drive means.

[Effect]

When a focusing command signal is given, the first driving means G once drives the imaging lens over the entire range from the infinity photographing position to the closest photographing position. During this sweep, AP(I
The position where the maximum value of f is obtained is stored by the self-storage means. Thereafter, by returning the imaging lens to the position using the second driving means, the focus is focused on the position where the AP value is the maximum value.

[Effects of the Invention] According to the present invention, even if there are multiple AF evaluation value peaks when photographing an object with a narrow spatial frequency distribution such as vertical stripes, the focus of the imaging lens is always set to the maximum AF evaluation value. Since it is adjusted to the position where the value can be obtained, no malfunction occurs in the automatic focusing cylinder.

The above objects, other objects, features and advantages of the present invention will become more apparent from the following detailed description of embodiments with reference to the drawings.

〔Example〕

FIG. 2 is a block diagram showing an autofocus camera 10 according to an embodiment of the present invention. The configuration of this autofocus camera 10 is basically the same as the "mountain climbing method." That is, an image formed by the image sensor 18 through the image sensor 12 is converted into an electrical signal by the image sensor 18 . The image sensor 18 is driven by an image sensor drive circuit 20, for example, by being given a scanning signal. Image sensor 1
The electrical signal from 8 is given to the video signal processing circuit 22,
It is converted into video signals such as luminance signals and chroma signals, and sent to the subsequent recording system. At the same time, the luminance signal from the video signal processing circuit 22 is given to the AF evaluation value detector 24.

The AF evaluation value detector 24 generates an AF evaluation value based on the luminance signal through processing such as absolute value integration.

When the lens focus controller, that is, the microcomputer 16 receives an AF command from the outside, it monitors the AF evaluation value output from the AF evaluation value detector 24 in response to a selection command for local maximum value search or maximum value search. At the same time, the imaging lens 12 is driven by the step motor 14. Note that detectors 26 are provided at both ends of the focus control range of the imaging lens 12, and the detectors 26 provide end point detection signals to the microcomputer 16, respectively.

FIG. 1 is a flow diagram showing the operation of the lens focus controller, that is, the microcomputer 16 when the maximum value search method is selected. When an AF command is input in this first step S1, the microcomputer 16
In step S2, the focal point of the imaging lens 12 is once set to an infinite position. However, this setting may be set in advance before the AF command signal is input.

In the following step S3, the microcomputer 16 clears the maximum value variable Emax stored in a predetermined area of its RAM (not shown), and sets the variable 5TEP indicating the focus position to "o".

When the initial setting is completed in this way, the microcomputer 16 receives the AF evaluation value E corresponding to the current imaging lens focal position output from the AF evaluation value detector 24 in the next step s4. value variable Em
It is determined whether or not it is larger than ax. If E>Emax
If so, in step S5, the maximum AF evaluation value Emax up to that point is updated with the AF evaluation value E at that time, and 5TEPO is updated with 5TEP.
The focal position where the maximum AF evaluation value was obtained is stored in O.

Thereafter, in the same way as in the case where "No" is determined in the previous step S4, the process proceeds to step S6, and the microcomputer 16 determines whether the imaging lens 12 has reached the closest photographing position based on the signal from the end point detector 26. do. If it is the closest position, the process proceeds to step S7, and if not, in step S8, the microcomputer 16 sends a command signal to the step motor 14 to move the focal point of the imaging lens 12 one step toward the closest position. Then, in step S9, the focus position variable 5TEP is incremented, and the process returns to step S4. In this way, if the series of operations from steps S4 to S9 is repeated until the condition in step S6 is satisfied, 5
The position where the maximum AF evaluation value is obtained is stored in TEPO. Therefore, in step S7, the microcomputer 16 provides a signal to the stepper motor 14 to return the imaging lens 12 to its recorded position.

Note that after that, after passing through automatic exposure control, etc., the process shifts to operations such as main exposure.

In this embodiment, when the maximum value search method is set, it operates in the same way as the "hill climbing method" described above.

Although the above-mentioned embodiments have been explained in the case of a camera-integrated VTR, it goes without saying that the present invention can also be applied to still cameras and the like.

Further, in the embodiment, the focus is first set at infinity and the position indicating Emax is stored when driving to the nearest end point, but this may be done in the opposite direction.

[Brief explanation of the drawing]

FIG. 1 is a flow diagram showing the operation of an embodiment of the present invention. FIG. 2 is a block diagram showing an embodiment of the present invention. FIG. 3 is a graph showing the characteristics of the low-pass filter effect exerted on the video signal by the out-of-focus state of the imaging lens. FIG. 4 is a graph showing the AF evaluation value for a subject with a narrow spatial frequency distribution using the distance between the image sensor and the real image as a parameter. In the figure, 10 is an autofocus camera, 12 is an imaging lens, 4 is a step motor, 6 is a microphone computer, 8 is an image sensor, and 26 is an end point detector.

Claims (1)

[Scope of Claims] An imaging lens that forms a subject image on an imaging device, an imaging device that converts the formed subject image into a video signal, and an evaluation value that serves as a guideline for the degree of focus based on the video signal. an automatic focus control evaluation value detection means for outputting an automatic focus control evaluation value, a first driving means for driving the imaging lens in the entire range from an infinity photographing position to a closest photographing position in response to a focusing command; storage means for storing a position where the maximum value of the evaluation value is obtained when the imaging lens is being driven; An autofocus camera comprising a second drive means for driving the imaging lens to the obtained position.