Auto-focus systems are used in many different kinds of camera systems including digital still cameras, digital video cameras, and any number of non-digital cameras. An auto-focus system provides the camera with a capability of automatically adjusting a focus lens so as to achieve an optimal level of sharpness in an image during an image capture procedure. For example, some cameras detect the activation of the shutter button and, before capturing an image, performs an iterative focusing procedure whereby a focus lens is moved back and forth until an optimal focus is determined by the circuitry of the auto-focus system. Then, after the focus lens is set to an optimal setting, the camera captures the image.
Typically, one of two types of auto-focus systems are used in conventional camera systems; active and passive, both of which are described in more detail below. Some cameras may have a combination of both types, depending on the complexity and price of the camera. In general, less expensive point-and-shoot cameras use an active system, while more expensive SLR (single-lens reflex) cameras with interchangeable lenses use a passive system.
An active auto-focus system typically utilizes an emitted signal from the camera in order to induce a signal echo. Based on the echo that bounces off the target object, the active system is able to determine a distance to the target object and, thereby, set the auto-focus system to a focus level corresponding to the determined distance. Such active systems typically use an infrared signal or a sound wave to determine the distance to the target object. For example, a camera having an active auto-focus system may emit an infrared signal when the shutter button is depressed and after a signal is received back from bouncing off the target, the auto-focus system of the camera sets a focus lens to a setting that is based on the returned signals. The camera may then capture the image in a conventional manner.
A passive auto-focus system, however, does not emit any signal and typically uses an analysis of image being captured to set the auto-focus system. The camera will analyze, in real-time, an image being registered at a capture point in the camera system. The capture point may be a pixel strip or pixel array that is able to convert incident light into electrical signals in order to determine the sharpness of the captured image by comparing pixels that are adjacent to each other. For example, when a camera having a passive auto-focus system is aimed at a target, an image of the target, or portion thereof, is captured at a pixel strip. The data from the pixel strip is then analyzed by a processor to determine the sharpness of adjacent portions of the captured image. The auto-focus system then maneuvers a focus lens while at the same time performing subsequent analyses of recaptured data until the captured image attains the sharpness desired.
Cameras having an auto-focus system, whether active or passive, require a lens assembly to be maneuvered in order to attain the sharpness desired. That is, the lens assembly, which focuses light through at least one focus lens onto a capture medium, such as film or a pixel array, is maneuvered toward or away from the medium such that the focal point of the focus lens changes with respect to the distance from the medium. Thus, in order to maneuver the lens assembly, a motor or group of motors is typically required. As is described below in a conventional auto-focus system, motors are bulky and power hungry.
FIG. 1 is a diagram of a conventional auto-focus system that uses a motor 122 to maneuver a lens assembly 105. The auto-focus system 100 includes a lens assembly 105 that is able to be moved by a drive mechanism 115 that is powered by a motor 122. The motor 122 is powered by a power supply 123 and controlled by a processor 132 that receives data from an auto-focus circuit 131. The nature and operation of this conventional auto-focus system 100 is described in more detail in the following paragraphs.
The lens assembly 105 typically includes a number of different lenses such as a focus lens 110, a zoom lens 111 and/or a filter lens 112. Each of these lenses are typically used to modify light, i.e., an image, that is directed through the lens assembly 105 toward a sensor array 130. The lenses of the lens assembly 105 along with a redirecting mirror 113 form the optical train in which light is directed toward the sensor array 130. As such, the lens assembly 105 may be moved with respect to the sensor array 130 and/or the redirecting mirror 113 in order to alter the manner in which the light is redirected. That is, the focal point (not shown) of the focusing lens 110 changes as the lens assembly 105 is moved such that any image that is incident upon the sensor array 130 can be maneuvered until the image is in focus.
In order to maneuver the lens assembly 105 to correctly focus an incident image on the sensor array 130, one or more relatively large motors 122 (only one shown) are required to provide the energy to move the lens assembly 105. In FIG. 1, the motor 122 is coupled to a drive mechanism 121 that translates rotational torque into lateral motion along a shaft 125. The shaft 125 is coupled to another translation point 120 that is able to maneuver the focusing lens separate from the rest of the lens assembly 105 through a screw drive mechanism 126. In other conventional systems not shown, the motor 122 and drive mechanism 121 may be coupled to the lens assembly such that the entire lens assembly is moved with respect to the sensor array 130.
The motor 122 is controlled by a processor 132 in conjunction with an auto-focus circuit 131. Typically, the sensor array 130 includes a certain number of sensors (not shown individually) that are dedicated to the auto-focus system, i.e., a focus strip 135. That is, instead of using the sensors of the focus strip 135 to collect data about an image for capture, the sensors of the focus strip 135 are used to determine whether or not the image about to be captured is in focus or not. A typical sensor array 130 is a charge-coupled device (CCD) that provides input to the auto-focus circuit 131 that is able to compute the contrast between actual picture elements. The focus strip 135 is typically a single strip of 100 or 200 pixels.
As such, the data collected from the focus strip 135 is analyzed with respect to one another to determine whether the image is focused correctly on the focus strip 135 and, subsequently, the sensors of the sensor array 130 that will be used for image capture. Various auto-focus algorithms and mathematical analyses for determining whether an image is focused are well known in the art and are not discussed in greater detail herein. Suffice it to say, data collected from the focus strip 135 through the auto-focus circuit 131, when analyzed by the processor 132 allow the processor 132 to control the motor 122 such that the lens assembly 105 is moved in one direction or the other in a repeating and iterative process until data collected from the focus-strip 135 indicates that the image is optimally focused.
Several problems exist in conventional auto-focus systems, such as the auto-focus system 100 of FIG. 1. For one, larger motors, such as motor 122, require a larger amount of space inside a camera assembly. With traditional camera systems, space is typically available for larger motors. However, as cameras are being realized in much smaller packages and assemblies, such as cell phones and the like, larger motors become more difficult to fit into the tighter space available for the camera assembly. As a result, motors to manipulate a focusing lens may not be feasible in such small places.
Additionally, these larger motors, even if used in a camera with enough space, still require a large amount of power that is undesirable in devices that are typically always powered on. That is, if power is required for a motor-driven auto-focus system in a camera that is part of a cell phone, the battery supplying the entire cell phone is drained all the faster when the motor is required to manipulate the lens assembly for focusing.
Other solutions, such the use of piezoelectric motors which may be smaller than the above-described motor 122 are also impractical because the power required to utilize the piezoelectric motors when manipulating the lens assembly is still at an undesirable high level.
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