Phase Difference Focus Detection in Starlit Sky Imaging
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Solution Overview
Problem
Conventional image capturing apparatuses face challenges in achieving high-speed and accurate focus adjustment for capturing starlit skies due to shifting infinity focal positions, environmental changes, and low light conditions, which affect image quality and focus detection speed.
Innovation Solution
An image capturing apparatus with a pair of focus detection pixels that receive light through different exit pupil areas, a detection unit for phase difference calculation, and a switching mechanism to adjust shift amounts based on capture modes, enabling high-speed and accurate focus adjustment in starlit sky image capture mode.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Ease of manufacture
If the focus lens is moved to the infinity focal position set by design, then the image capture setup is simple, but the infinity focal position shifts due to change over time, temperature, and camera orientation, resulting in poor focus accuracy
Solution Approach 1:
The system performs preliminary focus detection using phase difference AF before image capture to identify the accurate focus position, then moves the focus lens to this pre-determined position. This preliminary action compensates for shifts in the infinity focal position caused by environmental changes and ensures accurate focus despite the simplicity of the initial setup.
2Measurement precision
If contrast AF is used to perform focus adjustment, then focus detection can be performed, but the exposure time needs to be extended to obtain necessary contrast in dark starlit sky conditions, resulting in slow focus adjustment speed
Solution Approach 1:
The system replaces contrast AF with phase difference AF, substituting a method that relies on intensity variations (contrast) with one that uses optical path difference measurement. Phase difference AF can detect focus status even in low-contrast conditions by measuring the phase shift between light rays passing through different pupil areas, eliminating the need for extended exposure times and enabling fast focus adjustment in dark starlit sky conditions.
3Reliability
If pixel signals are added in the readout direction to improve the S/N ratio, then the signal quality improves, but the resolution decreases making it difficult to detect small stars
Solution Approach 1:
The system segments the photoelectric conversion portion of each pixel into multiple sections, with each section receiving light flux from different areas of the pupil. This segmentation enables phase difference detection while preserving the ability to detect small stars, as the segmented structure allows for both signal accumulation (improving S/N ratio) and maintaining spatial resolution for star detection.
4Measurement precision
If the diaphragm is opened to improve the S/N ratio without adding pixel signals, then the resolution is maintained, but the shift amount between two images increases, resulting in longer focus calculation time
Solution Approach 1:
The system dynamically adjusts the shift amount used in phase difference calculation based on the aperture value. When the diaphragm is opened (smaller f-number), the system automatically increases the shift amount to compensate for the larger pupil area, maintaining accurate focus detection. This dynamic adjustment allows the system to use open aperture settings for better S/N ratio and faster focus calculation without compromising precision.
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 allows for precise and rapid focus adjustment in starlit sky conditions, improving image quality by optimizing phase difference detection and reducing the time required for focus calculation.
Implementation Method 1
an image sensor having a pair of focus detection pixels that each receive light flux that has passed through different exit pupil areas of an imaging optical system
Data Source
AI summary
An image capturing apparatus includes an image sensor having a pair of focus detection pixels; a detection circuit for performing, on a pair of image signals that were obtained from each of the focus detection pixels, shift processing for relatively shifting the pair of image signals, and detecting a phase difference of the pair of image signals; a switching circuit for switching between a starlit sky image capture mode and a normal image capture mode; and a control circuit for performing control such that a shift amount of one step by which the pair of image signals are relatively shifted in the starlit sky image capture mode is smaller than a shift amount of one step by which the pair of image signals are relatively shifted in the normal image capture mode.