Fixed Focal Length Objective With DOE Focus Group for Fast Autofocus

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Solution Overview

Problem

Existing fixed focal length lenses suffer from unsatisfactory chromatic longitudinal error progression over the distance setting range and are heavy, making autofocus applications slow due to the use of low-dispersion glass and cemented members.

Innovation Solution

A fixed focal length lens design incorporating a focusing group with a diffractive optical element (DOE) and up to two lens elements, arranged near the stop, allowing for fast autofocus by minimizing chromatic aberrations through optimized optical power ratios and aspherical surfaces.

Engineering Contradictions & Design Principles

VSEngineering Contradiction Analysis

1Length of stationary object

If a single lens focus element is used, then the overall length of the lens is reduced, but chromatic aberration correction becomes insufficient

Engineering Contradiction:

Improveoverall length of the lensVSAvoidchromatic aberration correction

Core Design Contradiction:

Length of stationary objectVSReliability

Solution Approach 1:

The patent combines refractive lens elements with a diffractive optical element to create a composite focus group. The refractive elements (lens members with positive and negative optical power) correct chromatic aberrations through material dispersion properties, while the diffractive element provides additional optical power and focuses light. This composite structure achieves satisfactory chromatic correction with a single movable focus group, resolving the contradiction between compact size and aberration correction capability.

Inventive Principle:

Principle #40Composite materials

2Reliability

If low-dispersion glass is used for focus elements, then chromatic aberration is corrected, but the glass becomes sensitive during machining and vis-à-vis environmental influences

Engineering Contradiction:

Improvechromatic aberration correctionVSAvoidmachining sensitivity

Core Design Contradiction:

ReliabilityVSEase of manufacture

Solution Approach 1:

The patent uses a combination of refractive lens members with different dispersion properties (positive optical power member and negative optical power member) together with a diffractive optical element. This composite approach achieves chromatic aberration correction through the complementary optical properties of the components rather than relying solely on sensitive low-dispersion glass, improving manufacturability while maintaining optical performance.

Inventive Principle:

Principle #40Composite materials

3Reliability

If a cemented member is provided for focusing, then chromatic and monochromatic aberrations are corrected, but the focus group becomes heavy and moves slowly

Engineering Contradiction:

Improveaberration correctionVSAvoidweight of the focus group

Core Design Contradiction:

ReliabilityVSWeight of moving object

Solution Approach 1:

The patent replaces the traditional heavy cemented glass member with a lighter composite focus group consisting of refractive lens members and a diffractive optical element. The diffractive element, which can be manufactured as a thin coating or molded feature, provides the necessary optical power without the weight of thick cemented glass, enabling faster autofocus while maintaining aberration correction through the combined refractive-diffractive optical design.

Inventive Principle:

Principle #28Mechanics substitution (Replace mechanical system)

4Reliability

If multiple lens elements are used in the focus group, then chromatic aberration is corrected, but the complexity and cost of mechanical control increases

Engineering Contradiction:

Improvechromatic aberration correctionVSAvoidmechanical control complexity

Core Design Contradiction:

ReliabilityVSDevice complexity

Solution Approach 1:

The patent divides the focus group into functionally distinct segments: refractive lens members (with positive and negative optical power) and a diffractive optical element. Each segment performs a specific optical function, and together they achieve chromatic aberration correction. This segmented design allows for simpler mechanical control compared to moving multiple independent lens elements, as the entire focus group can be moved as a single unit while the internal segmentation provides the necessary optical correction.

Inventive Principle:

Principle #1Segmentation

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 lens design achieves reduced weight and fast focusing speeds by minimizing chromatic aberrations and Petzval sum, enabling compact and efficient autofocus performance.

Implementation Method 1

a diffractive-refractive lens doublet as an achromatic imaging lens is known from document US20122002/003 660 A1

Methodology Applied

Scientific EffectDiffraction: Diffraction

Implementation Method 2

a diffractive-refractive lens doublet as an achromatic imaging lens is known from document US20122002/003 660 A1

Methodology Applied

Scientific EffectRefraction: Refraction

Data Source

PatentUS12560783B2Fixed focal length objective

Publication Date: 2026.02.24 LEICA CAMERA AG

AI summary

The invention relates to a fixed focal length objective having a stationary lens group forward in the direction of light (VG), a stationary diaphragm (BL) with an adjustable aperture, a stationary rear lens group (HG) and a focussing group (MG) displaceable relative to the diaphragm along the optical axis of the objective for imaging objects at different distances onto a stationary image plane (IM), in which lens the focussing group (MG) consists of at least one lens element and a diffractive optical element (DOE) integrated into the focussing group (MG).