Defocus Amount Selection for Low Contrast Focus Control
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Solution Overview
Problem
Conventional image capturing apparatuses using phase difference detection methods for focus control face inaccuracies when capturing low contrast or low luminance objects, such as human faces under backlight conditions, due to smaller image signals and variable evaluation values influenced by settings like F-number and object luminance.
Innovation Solution
An image capturing apparatus that acquires and processes two signals from different pupil areas in the optical system, calculates defocus amounts and reliability across various frequency bands, and selects a defocus amount for focusing based on a reference reliability, ensuring accurate focus control even in low luminance or low contrast conditions.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If phase difference detection method is used for focus control, then focus detection can be performed using image sensor output signal, but focus control accuracy deteriorates when capturing low contrast or low luminance objects
Solution Approach 1:
The invention segments the frequency spectrum into multiple bands (first frequency band and second frequency band higher than the first) and processes signals in each band separately to calculate different defocus amounts. This allows the system to select appropriate frequency bands based on object characteristics, improving focus control accuracy for low luminance and low contrast objects by using the second defocus amount calculated from the higher frequency band when reliability is sufficient.
Solution Approach 2:
The invention changes the frequency band parameter used for defocus calculation based on reliability assessment. By calculating defocus amounts in multiple frequency bands and selecting based on reliability (comparing second reliability with first reference reliability), the system adapts to different object conditions, maintaining accuracy across varying luminance and contrast levels.
2Productivity
If evaluation value is used for focus control, then focus assessment can be performed, but evaluation accuracy decreases when capturing low luminance or low contrast objects due to variable evaluation values
Solution Approach 1:
The invention segments the evaluation process into multiple frequency bands, calculating defocus amounts separately for the first and second frequency bands. This segmentation allows the system to assess focus accuracy through multiple independent evaluations, improving reliability for low luminance and low contrast objects by comparing results across different frequency bands rather than relying on a single evaluation value.
Solution Approach 2:
The invention implements feedback through reliability assessment, where the second reliability (comparing second defocus amount with reference reliability) feeds back into the selection process. This feedback mechanism allows the system to adjust its focus control strategy based on the quality of evaluation values obtained from different frequency bands, ensuring accurate focus control even when object luminance or contrast varies.
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
The solution provides improved accuracy in focus control for low luminance or low contrast objects by selecting the appropriate defocus amount based on reliability, enhancing in-focus rates and reducing focus errors.
Implementation Method 1
an image sensor configured to capture an object image
Data Source
AI summary
An image capturing apparatus includes an acquirer configured to acquire a first signal and a second signal from different pupil areas in an image capturing optical system, a calculator configured to calculate a first defocus amount based on the first signal and the second signal in a first frequency band after each filter process by a plurality of filter processes, and to calculate a second defocus amount and a second reliability based on the first signal and the second signal in a second frequency band higher than the first frequency band, and a controller configured to select a defocus amount used for focusing among a plurality of defocus amounts based on the second reliability. The controller selects the second defocus amount as a defocus amount used for the focusing when the second reliability is equal to or higher than a first reference reliability.