Reflective Optical System Focus Correction
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Solution Overview
Problem
Conventional image capturing apparatuses with reflective optical systems suffer from lower focus detection accuracy due to changes in Modulation Transfer Function (MTF) with focal position, and existing correction methods do not adequately account for the unique characteristics of reflective optical systems, leading to reduced focus detection precision.
Innovation Solution
An image capturing apparatus that includes an image sensor and a processor to calculate focus evaluation values and detect the focus lens position, with a calculation unit that adjusts focus detection results based on information about the reflective optical system and image forming positions for various spatial frequencies, improving focus detection accuracy.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Volume of moving object
If a reflective optical system is used to achieve a long focal length with a small outer shape, then the imaging lens can capture at high magnification, but the MTF changes differently with focal position compared to normal imaging lenses, resulting in lower focus detection accuracy
Solution Approach 1:
The patent applies parameter changes by calculating a correction value that compensates for the different MTF characteristics of reflective optical systems. The correction value is determined based on the MTF at a first spatial frequency and MTF at a second spatial frequency, which differ from conventional lenses. This parameter adjustment allows the focus detection algorithm to accurately determine focus position despite the unique MTF behavior of reflective systems, thereby resolving the contradiction between compact size and focus detection precision.
2Ease of operation
If conventional focus detection control is used without correction, then the system is simple to operate, but the focus detection accuracy is lower due to the unique MTF characteristics of reflective optical systems
Solution Approach 1:
The patent implements preliminary action by pre-calculating and storing correction values in a correction value storage unit before actual focus detection occurs. The correction values are computed based on the specific MTF characteristics of the reflective optical system at different spatial frequencies. During focus detection, the system automatically retrieves and applies the appropriate correction value without requiring manual intervention, thus maintaining ease of operation while significantly improving focus detection accuracy.
3Device complexity
If a correction value is calculated without considering the MTF of the reflective optical system, then the calculation process is simpler, but the accuracy of the focus detection correction is lower
Solution Approach 1:
The patent resolves this contradiction by changing the parameters used in correction calculation to specifically account for reflective optical system characteristics. The calculation unit computes the correction value using MTF values at two different spatial frequencies that are tailored to the reflective system's unique behavior. This parameter-specific approach increases calculation complexity but ensures high correction accuracy, as the correction values are precisely matched to the optical system's actual performance characteristics.
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
Enhances focus adjustment accuracy in image capturing apparatuses with reflective optical systems by providing a correction value that accounts for the specific MTF characteristics, leading to improved focus detection precision and reliability.
Implementation Method 1
an image signal obtained by photoelectrically converting the subject image in accordance with the image sensor
Data Source
AI summary
An image capturing apparatus comprises an image sensor for capturing a subject image, a focus detection unit configured to, based on an image signal obtained by photoelectrically converting the subject image while performing a scan operation that causes a focus lens to move along an optical axis, calculate a focus evaluation value and detect a position of the focus lens at which the focus evaluation value is a maximum, and a calculation unit configured to, in a case where the imaging optical system includes a reflective optical system, calculate, based on information on the reflective optical system and information on an image forming position for each of a plurality of different spatial frequencies, a correction value for correcting a focus detection result of the focus detection unit.