Autofocus Control for Rolling Shutter CMOS Sensors
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Solution Overview
Problem
Conventional digital cameras with CMOS sensors using the rolling shutter system face challenges in achieving accurate autofocus due to overlapping exposure and focus lens driving periods, leading to low accuracy and prolonged focusing times.
Innovation Solution
An automatic focusing device that calculates and synchronizes autofocus evaluation values with focus lens positions during overlapping exposure periods, allowing for precise focus adjustments and increased focus lens movement, utilizing a focus lens position calculating unit to ensure accurate correspondence between evaluation values and lens positions, and employing coarse and fine adjustment units for rapid and accurate focusing.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If the exposure period and focus lens driving period are made to not overlap by reducing the driving amount of the focus lens for one frame, then the accuracy of autofocus operation is improved, but the focusing time becomes longer
Solution Approach 1:
The patent applies preliminary action by calculating the focus lens position at the time of exposure for the focusing area in advance, before the actual autofocus evaluation is performed. This allows the system to pre-determine the correct correspondence between evaluation values and lens positions, enabling accurate autofocus even when the exposure and driving periods overlap. The focus lens position calculating unit computes the position based on exposure timing information, so that when the autofocus evaluation is completed, the result can be immediately and accurately mapped to the correct lens position without requiring reduced driving amounts or non-overlapping periods.
2Productivity
If the focus lens driving amount for one frame is increased to shorten focusing time, then the productivity is improved, but the correspondence between autofocus evaluation value and focus lens position becomes inaccurate
Solution Approach 1:
The patent implements feedback by using the calculated focus lens position at the time of exposure as a reference point to guide the focus lens driving. The autofocus control unit uses the relationship between the exposure timing and lens position to determine the correct mapping between autofocus evaluation values and lens positions. This feedback mechanism allows the system to maintain high correspondence accuracy even when driving the focus lens at full speed during the frame period, because the system continuously references the calculated exposure timing positions to correct and guide the lens movement, ensuring that the final focused position accurately corresponds to the evaluation maximum.
Data Source
AI summary
An automatic focusing device in accordance with the present invention comprises an image sensor being subjected to exposure performed in a rolling shutter system, a focusing system including a focus lens for focusing on the image sensor, an AF evaluation value calculating unit calculating an AF evaluation value based on an imaging signal obtained from the image sensor, and an AF control unit controlling an operation of the focusing system based on an AF evaluation value of a focusing area set in a part of the imaging picture by the image sensor, and the AF control unit makes the AF evaluation value of a focusing area of each imaging picture and a focus lens position at the time of exposure for the focusing area correspond to each other, and drives the focusing system during the period which overlaps with an exposure period while searching the focus lens position where the AF evaluation value becomes maximum.