JP2518908B2 - Metering method for electronic still cameras - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to a photometric method for an electronic still camera, and more particularly to a photometric method for an electronic still camera that also serves as a solid-state image pickup device to quickly perform photometric measurement of subject brightness. It is a thing.

[Conventional technology]

Recently, an electronic still camera for recording captured images on a still video floppy (hereinafter referred to as a video floppy) instead of the silver salt film has been provided. Such an electronic still camera captures an image signal using a solid-state imaging device, for example, an interline transfer CCD (hereinafter abbreviated as CCD).

As a photometric method for electronic still cameras, a method using a light receiving element such as SPD has been generally used, but recently, as described in, for example, JP-A-63-169180, CC
It is known that the exposure time is calculated by measuring the subject brightness based on the brightness signal output from D.

[Problems to be Solved by the Invention]

By the way, the width of the brightness range that can be measured by CCD is, for example, S
The PD is about 21EV, whereas it is about 8EV, which is narrow. Since the width of the brightness range of a general subject is about 5EV, this is the brightness range for CCD photometry, such as 8EV-
When included in 16EV, the brightness value of the subject, for example, the average brightness value can be calculated correctly, and there is no problem.

However, for example, when shooting is performed in the evening, the brightness of the subject is lowered as a whole, so that the brightness range of the subject, which was 9EV to 14EV during the daytime, becomes 4EV to 9EV. For this reason, most of the brightness range of the subject is out of the brightness range of the CCD, and the photometric value calculated based on the brightness signal output from the CCD is far from the proper photometric value.

In order to improve such problems, the electronic still camera is designed to calculate the proper photometric value by repeating the photometry multiple times while automatically changing the central brightness value that is the center of the brightness range measured by the CCD. Has been proposed by the applicant. However, it is not preferable to repeat photometry many times, because the time (so-called time lag) required from the time the shutter button is pressed until the image is actually recorded on the video floppy becomes large, and it is easy to miss the photo opportunity. .

[Object of the Invention]

The present invention has been made to solve the above problems, and provides a photometric method for an electronic still camera capable of promptly measuring accurate subject brightness while performing photometry with a solid-state imaging device having a narrow photometric area. The purpose is to provide.

[Means for solving the problem]

In order to achieve the above object, the photometric method of the electronic still camera of the present invention performs photometry with a predetermined brightness value as the photometric center, and when the difference between the obtained photometric value and the brightness value is smaller than a certain value, When the exposure is performed based on the photometric value and the difference is larger than a certain value, the photometric center is shifted to the photometric value to perform the photometry again, and the number of times of performing the photometry is limited.

When the difference is larger than a certain value, the photometry center may be shifted based on the subject scene to perform the photometry again.

[Action]

According to the above photometry method, first, initial photometry is performed centering on a predetermined luminance value. The difference between the initial photometric value obtained by the initial photometry and the brightness value is compared with a constant value. When the difference is smaller than a certain value, it can be determined that the subject brightness distribution is almost covered by the brightness range of the initial photometry, and thus the initial photometry value is a highly reliable value.
Therefore, the exposure is performed based on the initial photometric value. Further, when the difference is larger than a certain value, the initial photometric value has a low reliability, and therefore the photometric central value is made to coincide with the initial photometric value and re-photometry is performed. Similarly, the reliability of the photometric value is inspected, and the exposure is performed based on the photometric value at the time when the reliability is determined to be high.

Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings.

〔Example〕

In FIG. 1 showing an optical system and a circuit block of an electronic still camera 1 embodying the present invention, the electronic still camera 1
The optical system is a taking lens 1 with a two-group (lens 10a, 10b) configuration.
0, Instantaneous diaphragm type diaphragm 11 arranged between lens 10a and lens 10b, optical low pass filter (OLPF) 12, CCD14
It consists of

A diaphragm drive circuit 16 and a CCD drive circuit 17 controlled by the overall controller 15 are connected to the diaphragm 11 and CCD 14, respectively. The output signal from CCD14 is the preamplifier 2
After being amplified by 0, it is input to the color separation circuit 21 and converted into RGB color signals. The respective color signals are input to the γ correction circuit 22, and as shown in FIG. 2, gamma correction is performed with γ = 0.45. As a result, the brightness range measurable by the CCD14 is E _i
Approximately 8EV between to E _m. This E _i is set so as not to become 0 due to dark current or the like. E ₀ in the figure is the brightness value at the center of photometry,
Below, it is called the central brightness value, and the output level corresponding to this is 60.
It becomes IRE and has proper exposure.

Each RGB color signal gamma-corrected by the γ-correction circuit 22 is
It is converted into a luminance signal and a color difference signal by the matrix circuit 23, and is written in the video floppy by the magnetic head in the recording unit 24 through the encoder 25. Further, an AF distance measuring unit 28 for measuring a subject distance and a shutter button 30 are connected to the overall control unit 15.

Next, the electronic still camera 1 configured as described above
The operation of will be described. First, when the video floppy is loaded into the electronic still camera 1 and the power is turned on, the loaded video floppy starts to rotate and reaches a steady rotational speed of 60 revolutions per second in about 30 ms.

Aim the shooting lens 10 at the subject and press the shutter button 30 halfway. The overall control unit 15 follows a built-in program sequence, as shown in the flowchart of FIG.
The AF distance measuring unit 28 is driven to perform distance measurement and focus adjustment of the subject distance (the drive system of the focusing lens is not shown). When the focus adjustment is completed, the shutter button 30 is fully pressed. The overall controller 15 sends a command signal to the CCD drive circuit 17 to apply a voltage to the electrodes of the CCD 14, so-called start-up, and starts initial photometry.

The subject light that has passed through the shooting lens 10 is CC through the OLPF12.
Upon being incident on D14, the first sweep pulse causes the accumulated charge of CCD 14 to become zero, and at the same time, photoelectric conversion of initial photometry and accumulation of charge are started. The brightness range that can be measured by the CCD 14 is determined by the aperture and the charge storage time, but this initial photometry is performed by setting the aperture of the taking lens 10 to the open state, so the brightness range that can be measured by the initial photometry Is CCD14
Is determined only by the charge accumulation time of. In the charge accumulation time t ₀ at the time of the initial photometry, the average brightness value is the center brightness value E ₀ (for example, 12 EV)
The gamma correction circuit 22
The output level from is predetermined to be 60 IRE.

The charges accumulated in the photoelectric conversion unit of the CCD 14 are read out by the sweep pulse after the charge accumulation time t ₀ has elapsed and are sent to the preamplifier 20. The signal input to the preamplifier 20 is sampled and held at a predetermined sampling frequency and sent to the color separation circuit 21, where it is separated into R, G, and B color signals. Each of these color signals is gamma-corrected by the γ-correction circuit 22 and sent to the overall controller 15.

The overall control unit 15 detects the luminance signal from the R, G, B signals and averages them to calculate the initial photometric value E ₁ , and
The difference | E ₁ −E ₀ | between this and the central luminance value E ₀ that is the photometric central value at the time of initial photometry is calculated, and this value is compared with ε (for example, 1 EV).

As shown in FIG. 4, when | E ₁ −E ₀ | <ε,
Since it is possible to determine that the brightness distribution area of the subject is covered by the CCD14 latitude during initial metering, the initial metering value E ₁
Is a reliable value. Therefore, to calculate the shutter speed T _V and the aperture value A _V corresponding to the charge accumulation time based on the initial photometric value E _1. Following the general control section 15 narrows down the 11 stop by sending a signal to the aperture driving circuit 16 to the aperture value A _V. When the diaphragm 11 is narrowed down, the charge of the CCD 14 is made zero by the sweep pulse synchronized with the next vertical synchronizing pulse, and the exposure is started. Set shutter speed T _V
After the time elapses, the charge accumulated in the photosensitive portion of the CCD 14 is read by the sweep pulse and transferred to the preamplifier 20.

The output signal from the preamplifier 20 is processed by the color separation circuit 21 and the γ correction circuit 22, and then input to the matrix circuit 23 to be converted into a luminance signal and a color difference signal, which are sent to the recording unit 24 via the encoder 25, and then the video signal. Recorded on floppy. After this, the diaphragm 11 is returned to the open state.

As shown in FIG. 5, when | E ₁ −E ₀ | ≧ ε,
Since the initial photometric value E ₁ is a value with low reliability, photometry is performed again. The charge accumulation time t ₁ of the CCD 14 in this re-photometry is
The initial photometric value E ₁ is calculated so as to be the center of photometry. When this calculation operation is completed, the CCD14
The accumulated electric charge of becomes zero and the photometry is started. The charge accumulated in the CCD 14 is read out by the sweep pulse after the time t ₁ has elapsed and is sent to the preamplifier 20.

This signal charge is input to the overall controller 15 via the color separation circuit 21 and the γ correction circuit 22. Thus, the overall control unit 15 calculates the re photometric value E _2, the absolute value of the difference between the re-photometric value E ₂ and the initial photometric values E ₁ | E ₂ -E ₁ | is a constant value epsilon (e.g. 1E
V) is less than or equal to.

When | E ₂ −E ₁ | <ε, it can be judged that the brightness distribution area of the subject is covered by the latitude of the CCD 14 at the time of re-photometry, so the re-photometry value E ₂ is a highly reliable value. .
Thus, the shutter speed T _V on the basis of the re-photometry value E ₂
And the aperture value A _V is calculated, Similarly, exposure, recording is performed.

If | E ₂ −E ₁ | ≧ ε, the re-photometric value E ₂ is a value with low reliability, and therefore photometry is repeated until | E _n −E | <ε (n is an integer). However, in the case of still photography, the increase in the number of photometry increases the time from the time the shutter button 30 is pressed until the exposure is performed, and misses the photo opportunity. And

In addition, for example, when shooting in a room where lighting is not sufficient,
There may be no subject brightness distribution in the initial metering range (EV8 to EV16 in FIG. 4), but in such a case,
The electronic still camera 1 is designed to display a message such as "photometry unavailable", "switch to flash photography", or "gain adjustment" in the viewfinder or the like.

Although the photometric value E ₁ is set to the photometric center at the time of re-photometry in the above embodiment, it is not always necessary to set it to the center. For example, CC
Divided photometric range of D in the central portion and a peripheral portion, - a spot light when "(central photometric value) (periphery photometric value)> e _1", "(central photometric value) - (periphery If the photometric value) <e ₂ ”, it is determined that the subject is backlit. In such a case, the main subject is in the highlight part or the dark part, so the photometric value E _n
The previous photometric value E _n-1 which is the value to be compared with is set to "E _n-1 + e ₁ " for spot light or "E _n-1 + e ₁ " for backlight
_n-1 −e ₂ ”.

Further, although the number of times of re-photometry is two, the present invention is not limited to this, and may be three, for example. In addition, the overall control unit 15
The brightness signal for photometry input to may be taken from the encoder 25.

〔The invention's effect〕

As described above, according to the present invention, photometry is performed with a predetermined luminance value as the center of photometry, and when the difference between the obtained photometric value and the reference luminance value is smaller than a constant value, the photometric value is used as the basis. Exposure, and when the difference is larger than a certain value, re-photometry is performed with the photometric value as the center of photometry, and the number of re-photometric operations is limited.
While measuring light with a solid-state imaging device with a narrow photometry area,
Accurate subject brightness can be measured quickly.

Further, when the difference is larger than a certain value, if the center of photometry is shifted and re-photometry is performed based on the subject scene, it is possible to obtain accurate light even when shooting under non-steady light, such as backlight or spotlight. The subject brightness can be measured quickly.

[Brief description of drawings]

FIG. 1 is a schematic diagram showing the structure of an electronic still camera embodying the present invention. FIG. 2 is a reference diagram showing a state in which the image signal is gamma-corrected. FIG. 3 is a flow chart showing the main part of the sequence program of this embodiment. FIG. 4 is a schematic diagram showing a case where the subject brightness distribution region is covered by the initial photometric range. FIG. 5 is a schematic diagram showing a case where the subject brightness distribution region is not covered by the initial photometry range. 1 ... Electronic still camera 14 ... CCD 15 ... Overall control unit.

Claims (2)

(57) [Claims] 1. A photometric method for an electronic still camera, wherein a solid-state image pickup device arranged on an image plane for converting an optical image into an electric signal and outputting the electric signal is also used for photometric measurement of subject brightness. When the difference between the obtained photometric value and the brightness value is smaller than a constant value, exposure is performed based on the photometric value, and when the difference is larger than the constant value, the center of the photometric value is shifted to the photometric value. A method of metering an electronic still camera, characterized in that the number of times of re-photometry is limited. 2. A photometric method for an electronic still camera, wherein a solid-state imaging device arranged on an image plane for converting an optical image into an electric signal and outputting the electric signal is also used for photometric measurement of subject brightness, wherein a predetermined brightness value is used as a photometric center. When the difference between the obtained photometric value and the brightness value is smaller than a fixed value, exposure is performed based on the photometric value, and when the difference is larger than the fixed value, the center of photometry is shifted based on the subject sheet. A photometric method for an electronic still camera, characterized in that the photometry is performed again and the number of times of performing the photometry is limited.