JPH0721579B2 - Focus detection method - Google Patents

Description

The present invention relates to a so-called active type focus detection method used in movie cameras and the like and having a light source such as an infrared light emitting diode.

[Prior Art] A conventional general focus detection method will be described with reference to FIGS. FIG. 3 is a block diagram of the focus detection device. A light spot emitted from a light source 1 such as an infrared light emitting diode is projected onto an object O through a light projecting lens 2. The light spot projected on the object O is imaged by the light receiving lens 3 on the light receiving element 4 composed of SPD or the like.
The light receiving element 4 is composed of elements 4a and 4b. For example, as shown in FIG. 4, the spot S forms an image on the element 4b.

The signal processing circuit 5 represents, for example, a signal (AB) / (A) indicating a focus state by the signals A and B from the two elements 4a and 4b.
+ B) is produced, and the motor 6 is driven so that this signal becomes zero. This signal is divided by the output difference (A−B) of the sum (A + B) of the outputs of the elements 4a and 4b to remove the influence of the light amount. The motor 6 interlocks the light receiving element 4 and the taking lens 7 to move them in the direction of the arrow in FIG.

FIG. 4 shows the above-described operation in the relationship between the spot S on the light receiving element 4 and the light receiving element 4. The light receiving element 4 moves in the arrow direction with respect to the stopped spot S, and the motor 6
If is driven at a constant speed, the light receiving element 4 also moves at a constant speed with respect to the spot S. Then, the signal (A-B) / (A
When the in-focus state where + B) becomes zero is reached, the motor 6 stops. At this time, the image of the subject O is in focus on the photographing means 8 such as a film or CCD through the photographing lens 7.

FIG. 5 shows the change of the signal (A−B) / (A + B) with respect to time. When the time t = 0, the signal (A−B) / (A +)
B) = 0 and the subject is in focus. Further, as shown in FIG. 4, when the spot S is in the element 4b, the signal (AB) /
(A + B) = − 1, the image is out of focus, and conversely the spot S
Is within the element 4a, the signal (AB) / (A + B) = + 1
Similarly, it is out of focus. In FIG. 5, this characteristic can be regarded as a substantially straight line in the vicinity of the focal point within the range of the arrow. Therefore, when (AB) / (A + B) = y is set, the equation y = k · t with k as a coefficient. Can be expressed as

FIG. 6 shows the change of the signal y and the ON / OFF state of the motor 6 when the motor 6 stops after focusing from the state of FIG. As shown in FIG. 6 (a), the actual signal y contains noise. In order to prevent the motor 6 from hunting due to this noise, noise as shown in the figure is shown in the focusing range. Dead zone DB (Dead B
and) are provided. Therefore, in the conventional device, the motor 6 often does not stop at the true focus position but rather stops at the slightly out-of-focus position. In addition, the value of signal y may be a dead zone DB due to noise.
As shown in FIG. 6 (b), the motor 6 is repeatedly turned on and off every time the camera goes in and out, so that the photographic lens 7 moves slightly and the photographic screen becomes unsightly, and when the noise is extremely large, the motor 6 Has a problem that it causes hunting and does not stop.

[Object of the Invention] An object of the present invention is to provide a high-performance focus detection method which eliminates the above-mentioned drawbacks of the conventional example, reduces the dead zone to almost zero, and does not cause inching or hunting even in the focus operation. To do.

[Summary of the Invention] The gist of the present invention for achieving the above-described object is to receive a light beam from a light projecting element onto a subject and receive reflected light from the subject, which has at least two light receiving regions in conjunction with a photographing lens. A method of receiving light by an element and determining focus / non-focus of the photographing lens based on a calculation signal related to a difference between outputs of these two regions, which is a discrete time-series signal based on the calculation signal. Is calculated, and based on an average signal obtained by moving averaging the plurality of time-series signals, the photographing lens is driven so that the average signal approaches zero.

[Examples of the Invention] The method according to the present invention will be described in detail with reference to the examples shown in FIGS. 1 to 2 and 3. The description of the configuration of FIG. 3 and its basic operation is omitted because it has been described above.

FIG. 1 shows the integration processing performed in the signal processing circuit 5,
The outputs A and B of the respective elements 4a and 4b of the light receiving element 4 are integrated, and the integration is terminated at a time τ when (A + B) / 2 reaches a constant level L, for example. At this time, a difference AB of B of the signal A is made, and the blur is converted into a digital amount by an A / D converter incorporated in the signal processing circuit S, and y = (AB) / (A + B), that is, ( A-
B) / (2L) is calculated to obtain the signal y.

FIG. 2 is an explanatory diagram of signal processing in the signal processing circuit 5, where the horizontal axis represents time t and the vertical axis represents y = (AB) / (A + B).
It has the size of. The value of the signal y is detected at each integration time τ, and discrete time series data y (i−n), y (i
-N + 1), ..., y (i-2), y (i-1), y
(I) and y (i + 1) are sequentially obtained. In the active autofocus system, the motor 6 continues to operate when the subject is out of focus and moves toward the in-focus point, that is, y = 0. Therefore, this time-series data should originally decrease monotonically. As shown, the relational expression y in the ideal state with no noise
= Above and below kt, due to noise.
Therefore, in order to reduce the noise component in the time series data and obtain the overall tendency, the signal processing circuit 5 uses Y (i) =
The signal is smoothed by a calculation of {y (i-1) + y (i) + y (i + 1)} / 3 to obtain a moving average. By treating the obtained average value Y (i) as a symbol representing the in-focus state and setting the in-focus state when the average value Y (i) becomes zero, the S / N ratio is improved and the dead zone DB is reduced. Can be made smaller.

Also, in the previous scheme, the signals y (i-1), y (i), y
Three moving averages of (i + 1) were obtained, and further data were taken before and after that in order to further reduce the influence of noise.
Alternatively, a moving average of seven time-series signals may be used.

[Effects of the Invention] As described above, the focus detection method according to the present invention reduces the influence of noise by smoothing the time-series data obtained discretely, thereby reducing the S / N ratio. It is possible to generate a good distance measurement signal of, and it is possible to perform a highly accurate and stable operation.

[Brief description of drawings]

The drawings show an embodiment of a focus detection method according to the present invention. FIG. 1 is an explanatory diagram of integration processing, FIG. 2 is an explanatory diagram of signal processing, FIG. 3 is its configuration diagram, and FIG. Is a relational diagram of the light receiving element and the light spot, FIG. 5 is a relational diagram of time and a signal showing a focused state, FIG. 6 (a) is an explanatory diagram of a focused state when the motor is stopped and the motor is stopped, (B) is an explanatory diagram of an on / off state of the motor. Reference numeral 1 is a light source, 2 is a light projecting lens, 3 is a light receiving lens, 4 is a light receiving element, 5 is a signal processing circuit, 6 is a motor, 7 is a photographing lens, and 8 is an image pickup means.

Claims (1)

[Claims] 1. A light-receiving element which projects a light beam from a light-projecting element onto a subject and reflects light from the subject is received by a light-receiving element which has at least two light-receiving areas in conjunction with a photographing lens, and a difference between outputs of these two areas. Is a method for determining whether the photographing lens is in-focus or out-of-focus on the basis of a calculation signal related to, calculating a discrete time-series signal based on the calculation signal, and performing a moving average of the plurality of time-series signals. Based on the average signal obtained,
A focus detection method characterized by driving the taking lens so that the average signal approaches zero.