JPH0815598A - Focusing method for rear focus type zoom lens - Google Patents

Abstract

PURPOSE:To obtain focus information so as to rapidly perform focusing in the case a rear focus type zoom lens is focused by a stepping motor. CONSTITUTION:As for the zoom lens consisting of four lens groups; in the case of moving the 2nd lens group L12 being a variable power lens and the 4th lens group L14 being a compensator lens on an optical axis by the stepping motors 11 and 12 respectively, the 4th lens group L14 is oscillated on the optical axis by the motor 12 with fine amplitude in a state where out-of-focus amount is small, and the 2nd lens group L12 is oscillated on the optical axis by the motor 11 with the fine amplitude in a state where the out-of-focus amount is large on a zoom telephoto side, so that the focus information is obtained based on an image signal obtained from an imaging device 15.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a focus adjusting method for a rear focus type zoom lens used in a video camera or the like.

[0002]

2. Description of the Related Art In recent video cameras and the like, a conventional cam ring is used for the purpose of downsizing and weight reduction in place of a zoom lens in which a variable magnification lens group and a compensator lens group for correcting image plane variation due to variable magnification are interlocked. By eliminating the cam ring, the variable magnification lens group and the compensator lens group that corrects the image plane variation due to the magnification variation and the image plane variation due to the object distance variation are driven by independent motors so as to maintain a predetermined relationship. Cam type rear focus type zoom lenses have come to be used.

In a video camera equipped with such a zoom lens, as shown in FIG. 4, the position of the variable power lens L2 is detected by a potentiometer 1 using a resistor to detect the variable power lens L2 and the initial position. The variable power lens L2 is driven by the DC motor 2 while the position of the compensator lens L4 having the reset sensor switch 5 is maintained in a predetermined relationship, and the compensator lens L4 is driven by the stepping motor 3
Generally, the information for focusing is obtained from the image signal obtained from the image pickup device 4 by vibrating the compensator lens L4 on the optical axis with a small amplitude by the stepping motor 3.

[0004]

By the way, in the video camera using the rear focus type zoom lens of the above-mentioned conventional example, when the amount of defocusing is small, the compensator lens L4 is moved by the stepping motor 3 with a very small amplitude. By vibrating on the optical axis, the image sensor 4
The information for focusing is obtained by detecting the change in the sharpness of the image on the image sensor 4 from the image signal obtained from the above. However, when the amount of out-of-focus is large, the sharpness of the image on the image sensor is low, and there is little change in the sharpness of the image on the image sensor from the image signal from the image sensor 4 due to a slight vibration of the compensator lens L4. Therefore, in many cases, information for focusing cannot be obtained. In this case, there is a problem that the compensator lens L4 has to be moved between the infinity end and the close end to search for the focus position. In particular, since the depth of field is shallow on the zoom telephoto side, there is a problem that a large out-of-focus blur is likely to occur.

In view of the above-mentioned problems of the conventional example, the present invention provides a focus adjusting method for a rear focus type zoom lens capable of adjusting focus with proper focus information regardless of the amount of defocus. With the goal.

[0006]

In order to achieve the above object, the focus adjusting method for a zoom lens according to the present invention comprises a first lens group having a positive refractive power and a second lens group having a negative refractive power in order from the object side.
A lens unit, a third lens unit having a positive refractive power, and a fourth lens unit having a positive refractive power, wherein the first and third lens units are fixed, and the second lens unit that is a variable power lens unit is The fourth lens group, which is a compensator lens group, is moved in the optical axis direction by driving the stepping motor to change the magnification, and the stepping motor is driven to move the fourth lens group on the optical axis so as to correct the image plane variation. , A zoom lens that moves the fourth lens group to focus a focus shift due to a change in object distance, and images the fourth and second lens groups by vibrating on the optical axis with a small amplitude by a stepping motor. The focus information is obtained from the image signal obtained from the element. Further, it is preferable that the second lens group vibrates on the optical axis when the blur amount is large.

[0007]

According to the above focus adjustment method of the zoom lens, when the amount of blurring of the focus is small, the fourth lens group is vibrated on the optical axis with a small amount of amplitude by the stepping motor. Information for focusing is obtained, and when the amount of defocus on the zoom telephoto side is large, the second lens group is vibrated on the optical axis with a small amount of amplitude by a stepping motor, and You can get information for focus.

[0008]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS One embodiment of the present invention will be described below with reference to FIGS. FIG. 1 shows a schematic configuration diagram of this embodiment. In the figure, the zoom optical system includes, in order from the object side, a first lens unit L11 having a positive refractive power, a second lens unit (magnifying lens unit) L12 having a negative refractive power, and a third lens unit L having a positive refractive power.
13 and a fourth lens unit (compensator lens unit) L14 having a positive refractive power. A numerical example of this zoom optical system is shown in the chart of FIG.

When zooming from the wide end to the tele end, the second lens unit L12 is moved to the stepping motor 11
Is driven to move to the image plane side, and the image plane variation due to zooming is corrected by driving the stepping motor 12 of the fourth lens unit L14. Further, focusing is also performed by driving the fourth lens unit L14 by the stepping motor 12. Further, the stepping motors 11 and 12 are connected to the microprocessor 16 via drivers 17 and 18, respectively.

The second lens unit L12 and the fourth lens unit L1
The position 4 is detected by counting the number of pulses of the stepping motors 11 and 12, respectively. When the power is turned on, the positions are detected by the reset sensor switches 13 and 14, respectively.
4 are each connected to the microprocessor 16. An image sensor 15 is provided on the image plane of the zoom optical system.

If the ratio of the image plane position change amount to the minute position change amount of the second lens unit L12 and the fourth lens unit L14 in the optical axis direction is taken as the position sensitivity, a numerical example of this embodiment is obtained. In this case, the position sensitivity is the value shown in the characteristic diagram of FIG. In the figure, the vertical axis represents the position sensitivity, and the horizontal axis represents the movement amount of the second lens group from the wide end to the tele end. That is, the fourth
The position sensitivity of the lens unit L14 is a value close to +1 from the wide end to the tele end. The position sensitivity of the second lens unit L12 is +0.27 at the wide end, 0 at the middle, and -2.74 at the tele end.

In the present embodiment having the above-mentioned structure, when the amount of defocusing is small, the image obtained by the image pickup device 15 is obtained by vibrating the fourth lens group L14 on the optical axis with a small amplitude by the stepping motor 12. The change in the sharpness of the image on the image sensor 15 is detected from the signal to obtain the information for focusing. In the state where the depth of field is shallow and the amount of defocusing is large on the zoom telephoto side where the focus is large and the image is easily blurred, a slight amount of vibration of the fourth lens unit L14 causes an image signal from the image pickup device 15 to move on the image pickup device 15. In some cases, changes in the sharpness of the image cannot be detected. In this case, the stepping motor 11 vibrates the second lens unit L12 having a high position sensitivity on the optical axis with a small amount of amplitude to obtain information for focusing from the image signal obtained from the image sensor 15. .

[0014]

As described above, according to the present invention, the first lens group having a positive refractive power, the second lens group having a negative refractive power, the third lens group having a positive refractive power, and the positive lens group are arranged in order from the object side. In a zoom lens having a fourth lens group having a refractive power, the second lens group which is a variable power lens group and the fourth lens group which is a compensator lens group are moved on an optical axis by driving a stepping motor, By vibrating the second lens group on the optical axis with a small amount of amplitude by a stepping motor, focus information can be properly obtained from the image signal obtained from the image sensor to adjust the focus, and especially the amount of blurring of the focus can be reduced. When it is large, the second lens group is vibrated on the optical axis with a small amount of amplitude, so that even if the amount of defocus on the zoom telephoto side is large, the focus is higher than the image signal obtained from the image sensor. There is an effect that it is focusing to obtain information properly.

[Brief description of drawings]

FIG. 1 is a schematic configuration diagram of a rear focus type zoom lens for an embodiment according to the present invention.

FIG. 2 is a table showing a numerical example of lens groups constituting the zoom lens.

FIG. 3 is a characteristic diagram showing the position sensitivities of the second lens group and the fourth lens group shown in the numerical example of FIG.

FIG. 4 is a schematic configuration diagram of a conventional electronic cam type rear focus type zoom lens.

[Explanation of symbols]

L11 ... first lens group, L12 ... second lens group, L
13 ... 3rd lens group, L14 ... 4th lens group, 11
..Stepping motor for driving the second lens group, 12 ...
Stepping motor for driving fourth lens group, 13 ...
Reset sensor switch for detecting the initial position of the lens group, 1
4 ... Reset sensor switch for detecting the initial position of the fourth lens group, 15 ... Image sensor.

─────────────────────────────────────────────────── ─── Continuation of the front page (51) Int.Cl. ⁶ Identification code Internal reference number FI Technical display location G03B 13/34 H04N 5/232 A // G02B 15/16

Claims (2)

[Claims] 1. A first lens unit having a positive refracting power, a second lens unit having a negative refracting power, a third lens unit having a positive refracting power, and a fourth lens unit having a positive refracting power, which are arranged in this order from the object side. Then, the first and third lens groups are fixed, the second lens group, which is a variable power lens group, is moved in the optical axis direction by driving a stepping motor to perform variable power, and the fourth lens, which is a compensator lens group, is moved. In a zoom lens that moves the lens group on the optical axis by driving a stepping motor so as to correct the image plane variation due to zooming, and moves the fourth lens group to focus the focus shift due to the object distance variation, Rear focus type zoom lens characterized in that focus information is obtained from an image signal obtained from an image pickup device by vibrating the fourth and second lens groups on the optical axis with a small amount of amplitude by a stepping motor. How to adjust the focus of the lens. 2. The focus adjusting method for a rear focus type zoom lens according to claim 1, wherein the second lens group vibrates on the optical axis when the amount of blur is large.