Patent ID: 7848629

Claim:
An auto-focus system for use in an image capture device, comprising: a focus lens; a motor; a flash unit for firing a pre-flash; a metering unit configured for measuring an ambient brightness before the pre-flash and a pre-flash ambient brightness during the pre-flash; a storage unit for storing a preset desired brightness and a table, the table comprising a collection of main flash ratios and a collection of object distances, where each main flash ratio is associated with a corresponding object distance; a digital signal processor configured for calculating a main flash ratio using the formula: R m = Y d - Y p Y p - Y a , where R m is the main flash ratio, Y d is the preset desired brightness, Y p is the pre-flash ambient brightness, and Y a is the ambient brightness the digital signal processor being configured for indexing an object distance corresponding to the calculated main flash ratio using the table and calculating an image distance using the indexed object distance and a number of focus steps of focus lens using the image distance; and a motor controller configured for reading the calculated number of focus steps of the focus lens and controlling the motor to drive the focus lens to move the corresponding number of focus steps.