Abstract:
A lens module for autofocus of a camera in passive mode includes a lens barrel containing a first lens, a second lens, a pressure sensor, an image sensor, and a processor. In the passive autofocus mode, the first lens is located at a first end of the lens barrel and moves within the lens barrel when a distance to a target object changes. The pressure sensor between the first and the second lenses senses both pulling and pushing pressures which the first lens applies. The image sensor is adjacent to an end of the lens barrel and is behind the first and second lenses. The processor can apply formulas to calculate a distance between the target object and the lens module.

Description:
FIELD 
     The subject matter herein generally relates to a lens module and an electronic device having the lens module. 
     BACKGROUND 
     Many image capturing devices (such as cameras and cell phones) employ autofocus systems which can focus on selected points or areas. The autofocus system can be switched between an active autofocus mode and a passive autofocus mode. When the autofocus optical system is under the active autofocus mode, the autofocus system sends some form of energy towards the target object, analyses the reflection to determine a distance between the target object and the active autofocus system, and adjusts the lens according to the distance. When the autofocus optical system is under the passive autofocus mode, the autofocus system analyses the image of the target object without sending any energy towards object. 
     Thus, the image capturing device can determine the distance between the target object and the autofocus system only when the autofocus optical system is under the active autofocus mode. However, the autofocus system is under the active autofocus mode when the image capturing device is located in an environment with a low illumination. That is, the image capturing device cannot determine the distance between the target object and the autofocus system when the image capturing device is located in an environment with a high illumination. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Implementations of the present technology will now be described, by way of example only, with reference to the attached figures. 
         FIG. 1  is a block diagram of an embodiment of an electronic device including a lens module. 
         FIG. 2  is diagrammatic view of the lens module of  FIG. 1 . 
         FIG. 3  is a diagrammatic view showing the lens module of  FIG. 2  under a passive autofocus module. 
         FIG. 4  is a diagrammatic view showing the lens module of  FIG. 2  under an active autofocus module. 
     
    
    
     DETAILED DESCRIPTION 
     It will be appreciated that for simplicity and clarity of illustration, where appropriate, reference numerals have been repeated among the different figures to indicate corresponding or analogous elements. In addition, numerous specific details are set forth in order to provide a thorough understanding of the embodiments described herein. However, it will be understood by those of ordinary skill in the art that the embodiments described herein can be practiced without these specific details. In other instances, methods, procedures, and components have not been described in detail so as not to obscure the related relevant feature being described. Also, the description is not to be considered as limiting the scope of the embodiments described herein. The drawings are not necessarily to scale and the proportions of certain parts may be exaggerated to better illustrate details and features of the present disclosure. 
     The term “comprising,” when utilized, means “including, but not necessarily limited to”; it specifically indicates open-ended inclusion or membership in the so-described combination, group, series, and the like. 
       FIGS. 1 and 2  illustrate an exemplary embodiment of a lens module  100  applied in an electronic device  1 . The electronic device  1  can be a camera, a tablet computer, or a smart phone. 
     The lens module  100  comprises a lens barrel  10 , a first lens  20 , a second lens  30 , a pressure sensor  40 , an image sensor  50 , a distance detector  60 , and a processor  70 . 
     The first lens  20 , the second lens  30 , the pressure sensor  40 , the image sensor  50 , and the processor  70  are mounted inside the lens barrel  10 . The lens barrel  10  has a first end  11 , closest to a subject to be photographed, a second end  12  opposite to the first end  11 , and an optical axis  13  passing through the first end  11  and the second end  12 . The first lens  20  is located at a first end of the lens barrel  10 . The image sensor  50  is located at the second end  12  of the lens barrel  10 . The second lens  30  is positioned between the first lens  20  and the image sensor  50 . The pressure sensor  40  is positioned between the first lens  20  and the second lens  30 . The distance detector  60  is located outside the lens barrel  10  and mounted to the first end  11  of the lens barrel  10 . In at least one embodiment, the image sensor  50  is a complementary metal oxide semiconductor (CMOS) or a charge-coupled device (CCD). 
     The lens module  100  can be manually or automatically switched between an active autofocus function and a passive autofocus function. When the lens module  100  is in an environment with a low ambient illumination, the lens module  100  is switched to an active autofocus mode. When the lens module  100  is under the active autofocus mode, the lens module  100  sends some form of energy towards a target object  2  (shown in  FIGS. 3 and 4 ), analyses the reflection to determine a distance between the target object  2  and the lens module  100 , and adjusts the first lens  20  according to the determined distance (that is, the active autofocus function). When the lens module  100  is in an environment with a high ambient illumination, the lens module  100  is switched to a passive autofocus mode. When the lens module  100  is under the passive autofocus module, the lens module  100  analyses an image resolution of an image of the target object  2  formed in the image sensor  50  without sending any energy towards the target object  2 , and adjusts the first lens  20  according to the image resolution (that is, the passive autofocus function). 
     In at least one exemplary embodiment, the lens module  100  is automatically switched between the active autofocus function and the passive autofocus function. The electronic device  1  further comprises an illumination sensor  200 . The illumination sensor  200  periodically detects an ambient illumination of the environment in which the electronic device  1  is located. The illumination sensor  200  can be a photosensitive diode, a photosensitive triode, or a photosensitive resistor. The detection of the ambient illumination implemented by the photosensitive diode, the photosensitive triode, or the photosensitive resistor is a well known technique, thus a detailed description is omitted. When the detected ambient illumination is greater than a default threshold, the electronic device  1  controls the lens module  100  to switch to the passive autofocus mode. When the detected ambient illumination is less than or equal to the default threshold, the electronic device  1  controls the lens module  100  to switch to the active autofocus mode. In at least one embodiment, the default threshold, usually preset before the electronic device  1  is put on the market, is of about 20 candela. 
     In other exemplary embodiments, the lens module  100  is manually switched between the active autofocus function and the passive autofocus function, and the illumination sensor  200  is omitted. The electronic device  1  may further comprise a button (not shown) for controlling the lens module  100  to switch between the passive autofocus mode and the active autofocus module. The button can be a virtual button or a mechanical button. 
       FIG. 3  illustrates that when the lens module  100  is under the passive autofocus mode, the first lens  20  moves within the lens barrel  10  when distance between a target object  2  and the lens module  100  is changed (that is, when the target object  2  moves towards or away from the lens module  100 ). The lens module  100  is thus enabled to focus on the target object  2  and to perform the passive autofocus function. 
     As shown in  FIG. 2 , the pressure sensor  40  senses a pressure, such as the pressure the first lens  20  applies to the pressure sensor  40  through the expansion or contraction of an elastic member  21  as the first lens  20  moves away or toward the pressure sensor  40 . 
     As previously mentioned, in at least one embodiment, the first lens  20  is connected to the pressure sensor  40  by the elastic member  21  such as a coiled spring. The elastic member  21  is elastically deformed when the first lens  20  moves, thereby applying pressure to the pressure sensor  40 . More specifically, the elastic member  21  is in a natural state when the first lens  20  is in an original position. When the first lens  20  moves towards the target object  2 , the elastic member  21  applies a pulling pressure to the pressure sensor  40 . When the first lens  20  moves away from the target object  2 , the elastic member  21  applies a pushing pressure to the pressure sensor  40 . 
     The processor  70  calculates a moving distance ΔD 2  of the first lens  20  according to a pressure value of the sensed pressure. In at least one embodiment, the pressure value is proportional to the moving distance ΔD 2  of the first lens  20 . 
     The processor  70  further calculates a moving distance ΔD 1  of the target object  2  according to the moving distance ΔD 2  of the first lens  20 , and then calculates a distance D between the target object  2  and the lens module  100  according to the moving distance ΔD 1  of the target object  2 . In at least one embodiment, before the target object  2  moves towards or away from the lens module  100 , a distance between the target object  2  and the first lens  20  is defined as an original target object distance equaling u. A distance between the image sensor  50  and the first lens  20  is defined as an original image distance equaling v, and the focal length of the first lens  20  equals f. Then, the processor  70  can calculate the original target object distance u according to the original image distance v and the focal length f based on the function, 
               1   u     =       1   v     +       1   f     .             
After the target object  2  moves towards or away from the lens module  100 , the moving distance ΔD 1  of the target object  2  can be calculated according to the original target object distance u, the original image distance v, the focal length f, and the moving distance ΔD 2  of the first lens  20  based on the function,
 
               1     u   -     Δ   ⁢           ⁢     D   1           =       1     v   -     Δ   ⁢           ⁢     D   2           +       1   f     .             
Then, the distance D between the target object  2  and the lens module  100  can approximately be calculated according to the original target object distance u and the moving distance ΔD 1  of the target object  2  based on the function, D=u-ΔD 1 .
 
     With the above configuration, when the lens module  100  is in an environment with a high ambient illumination and is switched to the passive autofocus mode, the lens module  100  can also determine the distance D between the target object  2  and the lens module  100 . 
       FIG. 4  illustrates that when the lens module  100  is under the active autofocus mode, the distance detector  60  detects the distance between the target object  2  and the lens module  100 , thereby allowing the lens module  100  to focus on the target object  2  and to perform the active autofocus function. In at least one embodiment, the distance detector  60  is an infrared detector or an ultrasonic detector. 
     It is to be understood, even though information and advantages of the present embodiments have been set forth in the foregoing description, together with details of the structures and functions of the present embodiments, the disclosure is illustrative only; changes may be made in detail, especially in matters of shape, size, and arrangement of parts within the principles of the present embodiments, to the full extent indicated by the plain meaning of the terms in which the appended claims are expressed.