Photographic Lens Focusing Mechanism for Image Plane Stability

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Solution Overview

Problem

Conventional photographic lenses experience significant variation on the image plane during focusing, particularly when capturing closest distance objects, due to the movement of lens units with negative and positive refractive powers, leading to degraded automatic focusing response and increased weight of the front lens unit.

Innovation Solution

A photographic lens configuration where the third lens unit moves towards the object side and the second lens unit moves towards the image side during focusing, with the image forming magnification of the second lens unit satisfying β2N > 1, maintaining a constant height of the principal ray of off-axis light flux and reducing image plane variation.

Engineering Contradictions & Design Principles

VSEngineering Contradiction Analysis

1Reliability

If the front lens unit having a negative refractive power is moved towards the object side for focusing, then the focusing function is achieved, but the weight of the front lens unit becomes great due to including a plurality of negative lenses of great lens diameter, which degrades the response characteristics of automatic focusing

Engineering Contradiction:

Improvefocusing functionVSAvoidweight of front lens unit

Core Design Contradiction:

ReliabilityVSWeight of moving object

Solution Approach 1:

The lens system is divided into multiple lens units with different refractive powers (first lens unit with negative power, second and third lens units with positive power). This segmentation allows different units to perform different functions during focusing, enabling the front lens unit to be lighter while maintaining focusing capability through coordinated movement of multiple units.

Inventive Principle:

Principle #1Segmentation

Solution Approach 2:

Instead of moving only the front lens unit towards the object side for focusing, the invention inverts the approach by moving the rear lens units (second and third lens units) towards the image side. This reverse movement strategy reduces the burden on the front lens unit, decreasing its weight while achieving the same focusing effect.

Inventive Principle:

Principle #13The other way round (Inversion)

2Reliability

If the second lens unit moves towards the object side for focusing on closest distance object, then the focusing function is achieved, but the height of passage of principal ray of off-axial light flux varies greatly, making it difficult to reduce variation on the image plane

Engineering Contradiction:

Improvefocusing functionVSAvoidimage plane variation

Core Design Contradiction:

ReliabilityVSManufacturing precision

Solution Approach 1:

Instead of moving the second lens unit towards the object side, the invention moves it towards the image side during focusing. This reverse movement direction, combined with the third lens unit moving towards the object side, creates a balanced optical path change that minimizes variation in the height of passage of principal rays and reduces image plane variation.

Inventive Principle:

Principle #13The other way round (Inversion)

Solution Approach 2:

The invention optimizes the image forming magnification of the second lens unit (β2N > 1) and controls the movement distances of the second and third lens units relative to each other. By changing these parameters, the system achieves focusing while maintaining stable principal ray heights and minimizing image plane variation.

Inventive Principle:

Principle #35Parameter changes

3Length of stationary object

If a retrofocus type photographic lens is used to achieve relatively long back focus, then the back focus distance is increased, but the position of rear principal point must be located relatively close towards the image side, requiring complex lens unit arrangements

Engineering Contradiction:

Improveback focus distanceVSAvoidlens unit arrangement

Core Design Contradiction:

Length of stationary objectVSDevice complexity

Solution Approach 1:

The lens system is segmented into multiple lens units (first lens unit with negative power, second and third lens units with positive power) arranged in a specific sequence. This segmentation enables the retrofocus design to achieve long back focus while distributing the optical complexity across multiple manageable units rather than requiring a single complex arrangement.

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

This configuration effectively minimizes image plane variation during focusing, enhancing the sensitivity and optical performance of the lens by controlling the movement and weight distribution of lens units, thereby improving focusing accuracy and reducing the size of the optical system.

Implementation Method 1

during focusing from an infinitely distant object to a closest distance object, the third lens unit moves towards the object side while the second lens unit moves towards the image side

Methodology Applied

Scientific EffectOptical path control through coordinated lens movement: Lens

Data Source

PatentUS8179617B2Photographic lens and image pickup apparatus having the same

Publication Date: 2012.05.15 CANON KK

AI summary

A photographic lens includes, in order from an object side to an image side, a first lens unit having a negative refractive power, a second lens unit having a positive refractive power, a stop, and a third lens unit having a positive refractive power. In the photographic lens, the third lens unit moves towards the object side while the second lens unit moves towards the image side during focusing from an infinitely distant object to a closest distance object. In addition, in the photographic lens, an image forming magnification of the second lens unit during focusing on a closest distance object is appropriately set.