Imaging Lens Autofocus Speed via Inner Focus Group Segmentation
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Solution Overview
Problem
Existing imaging lenses face challenges in achieving a balance between reducing size and weight for improved portability and autofocus speed while maintaining high optical performance and minimizing aberration fluctuations during focusing.
Innovation Solution
The design incorporates a three-group lens configuration with a first lens group having positive refractive power, a second lens group with positive refractive power that moves during focusing, and a stationary third lens group with negative refractive power, including an aperture stop between the first and third lens groups, with specific focal length and Abbe number conditions to optimize refractive power distribution and aberration correction.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Speed
If a front focus type configuration is used where the first lens group and second lens group move during focusing, then the lens system can achieve focusing functionality, but the weight of the focus group increases and autofocus speed decreases
Solution Approach 1:
The lens system is divided into three separate lens groups (first, second, and third) with different functions. Only the second lens group moves during focusing, while the first and third groups remain stationary. This segmentation allows the moving mass to be minimized to just one lens group, significantly improving autofocus speed compared to configurations where multiple groups must move.
Solution Approach 2:
Instead of using a conventional front focus type where the front lens groups move, this patent inverts the approach by using an inner focus type where the middle lens group (second lens group) moves. This inversion allows the aperture stop to be positioned between the first and third lens groups, enabling the stationary first and third groups to provide structural support while the lighter second group performs the focusing motion.
2Volume of moving object
If the first lens group is composed of multiple lenses (three or more), then the refractive power distribution can be optimized, but the overall lens system size increases
Solution Approach 1:
Each lens group is designed with specific local characteristics optimized for its function. The first lens group has positive refractive power for light convergence, the second lens group has negative refractive power for aberration correction and focusing, and the third lens group has positive refractive power for final image formation. This localized optimization allows each group to achieve its function with minimal lens elements, reducing overall system size while maintaining optical performance.
Solution Approach 2:
The patent optimizes specific parameters including the focal lengths of each lens group (f1, f2, f3), the positions of the aperture stop, and the refractive indices of the lens materials. By carefully controlling these parameters, the system achieves proper refractive power distribution and aberration correction with a compact three-group configuration, avoiding the need for additional lens elements.
3Device complexity
If the third lens group consists of only a negative lens, then the lens system can be simplified, but the correction of astigmatism and suppression of incident angle of off-axis rays deteriorates
Solution Approach 1:
The third lens group uses an asymmetric configuration with a negative lens followed by a positive lens, rather than a simple symmetric single lens. This asymmetric two-lens design allows the negative lens to correct astigmatism while the positive lens controls the incident angles of off-axis rays. The asymmetric arrangement of different power lenses enables simultaneous correction of multiple aberration types that cannot be achieved with a single symmetric lens.
Solution Approach 2:
The positive lens in the third lens group acts as an intermediary element between the negative lens and the image plane. It mediates the optical paths of off-axis rays, reducing their incident angles on the image sensor. This intermediary positive lens works in conjunction with the preceding negative lens to achieve comprehensive aberration correction, with each lens performing a specific corrective function.
4Speed
If the second lens group is made lighter to increase autofocus speed, then the focusing performance improves, but the optical performance and aberration correction may deteriorate
Solution Approach 1:
The second lens group, which moves during focusing, is designed as a composite optical system combining multiple lens elements with different refractive indices and Abbe numbers. This composite structure allows the group to maintain adequate optical performance and aberration correction capabilities even with reduced mass. The careful selection of glass materials and lens configurations ensures that the moving second group can perform both focusing and optical correction functions effectively.
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
This configuration results in reduced size and weight, increased autofocus speed, and minimized aberration fluctuations, particularly in chromatic aberration, while maintaining high optical performance.
Implementation Method 1
a first lens group having a positive refractive power, a second lens group having a positive refractive power, and a third lens group having a negative refractive power
Data Source
AI summary
The imaging lens includes, as lens groups, only a positive first lens group, a positive second lens group, and a negative third lens group in order from the object side. An aperture stop is disposed between a lens surface closest to the image side in the first lens group and a lens surface closest to the object side in the third lens group. During focusing, the first lens group and the third lens group do not move, and the second lens group moves. The first lens group consists of a negative lens and a positive lens in order from the object side. The third lens group has a negative lens and a positive lens in order from the object side.