Flash-Assisted Focal Length Detection in Digital Cameras
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Solution Overview
Problem
Passive auto focus in digital cameras is less accurate in low-light environments, and conventional auto focus methods often fail to correctly focus on human subjects due to reliance on image definition and background detection.
Innovation Solution
A method using a flash to assist in focal length detection by capturing an image with the flash, calculating exposure feature values from face blocks, looking up a focus range comparison table, and performing a focus procedure to determine the target focal length, thereby improving focusing accuracy in low-light conditions and ensuring the human subject is in focus.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Ease of manufacture
If passive auto focus is used to reduce manufacturing cost, then the camera can be manufactured more cheaply, but focusing accuracy deteriorates in low-light environments
Solution Approach 1:
The flash is fired before the main photographing to pre-illuminate the scene and enable the image sensor to capture a pre-flash image. This preliminary action provides sufficient light information for exposure feature calculation and focus detection, resolving the low-light focusing problem without requiring active auto focus hardware
Solution Approach 2:
A focus range comparison table is introduced as an intermediary data structure that stores pre-calculated exposure feature values and corresponding focal lengths. The table acts as a reference guide to quickly determine the initial focus position based on calculated exposure features, improving focusing accuracy in low-light conditions without adding complex hardware
2Device complexity
If conventional auto focus methods rely on image definition, then the system remains simple, but focusing accuracy on human subjects deteriorates when backgrounds are confusing
Solution Approach 1:
The patent applies different processing strategies to different regions of the image. Face detection algorithms specifically identify and prioritize human face regions, applying face-focused exposure feature calculation only to these areas. This local differentiation ensures human subjects are correctly focused even when backgrounds are confusing or have similar contrast characteristics
Data Source
AI summary
A method for assist in focal length detection is adapted to a digital camera having a flash. The method includes the following steps. First, the flash of the digital camera is actuated and an image having a face portion is captured. Afterwards, an exposure feature value of the face portion in the image is calculated. A focus range comparison table is looked up according to the exposure feature value to obtain an initial focus position. Then, a focus procedure is performed according to the initial focus position to obtain a target focal length. After the target focal length is obtained, a flash feature value of the image is further calculated, and a guide number (GN) and a diaphragm value are set according to the flash feature value.