Passive auto-focus device and method

A passive auto-focus method operable to be carried out by a passive auto-focus device, comprising the following steps:

• comparing a range of a variation area of a current scene with a predetermined range to determine a focus-searching range, in which the variation area of the current scene is defined according to the comparison between the current scene and a preceding scene, and the focus-searching range indicates one of several different ranges;
• generating a current step setting value according to the focus-searching range or by comparing the definition pertaining to the focus-searching range with a first predetermined definition in which the current step setting value indicate one of several different steps;
• and
• generating a plurality of image records in connection with the focus-searching range and the current step setting value by order.

A passive auto-focus method, comprising the following steps:

• comparing the range of a variation area of a current scene with a predetermined range to determine a focus-searching range in which the variation area of the current scene is defined according to the comparison between the current scene and a preceding scene while the focus-searching range indicates a narrow focus-searching range if the range of the variation area is within the predetermined range or else the focus-searching range indicates a wide focus-searching range;
• and
• generating a plurality of image records in connection with the focus-searching range,
• wherein the narrow focus-searching range falls within the range of the variation area while at least some of the wide focus-searching range falls outside the range of the variation area.