In high-precision measurement scenarios, the “parfocal accuracy” of ZOOM LENSES serves as a core metric determining system reliability. Its quantitative validation requires integrating optical principles, mechanical compensation mechanisms, and real-time error compensation technologies to establish a multidimensional verification system.

I. Technical Principles of Focusing Precision

The ZOOM LENS maintains a constant image plane position during zoom through a mechanical compensation mechanism, with its core principle being the coordinated movement of the zoom group and compensation group. When the zoom group moves along the optical axis to change the focal length, the compensation group counteracts the focal shift through nonlinear motion, ensuring the image plane remains fixed on the imaging sensor plane.

II. Three Major Sources of Error in Focusing Accuracy

1. Mechanical transmission errors: Displacement deviations in the ZOOM LENS caused by cam curve machining accuracy and clearance between guide pins and slots lead to focal length calculation errors. For example, an industrial lens exhibited a relative focal length error of 0.5% at the telephoto end due to cam curve inaccuracies, directly resulting in focus shift.

2. Optical aberration effects: Spherical aberration, field curvature, and other aberrations cause focal point variations across different wavelengths. When wide-aperture lenses are fully open, peripheral light rays can deviate from the central focal point by several micrometers. Even with adequate mechanical compensation, optical aberrations may still induce focus drift.

3. Environmental Disturbances: Thermal expansion and contraction of the lens barrel due to temperature changes, along with transient mechanical deformation caused by vibration, can disrupt parfocal characteristics. In a lithium battery laminate inspection scenario, environmental temperature fluctuations of ±5°C caused lens focal plane displacement exceeding 10μm—far surpassing the measurement system's tolerance threshold.

III. Quantitative Verification Method for Focusing Precision

1. Static focal plane position verification method: Using a high-precision displacement stage and laser interferometer, fix the lens at the midpoint of the ZOOM LENS range. By fine-tuning the displacement stage, achieve the sharpest image of a standard resolution target (e.g., USAF 1951 target) and record the focal plane position. Repeat testing across the entire focal range (e.g., from wide-angle to telephoto end) and calculate the maximum deviation of the focal plane position.

2. Dynamic Focus Tracking Verification Method: Construct a high-speed visual measurement system. Employ feature extraction-matching algorithms to track target feature point positions in real time. Simulate zoom processes (e.g., motor-driven cam rotation), record coordinate changes of feature points, and calculate displacement values (Δx, Δy) within the affine transformation model to quantify focus drift.

3. MTF (Modulation Transfer Function) Full-Focus-Range Evaluation Method: Using an MTF tester (e.g., OptoEngine MTF-5000), collect MTF values at different spatial frequencies (e.g., 50 lp/mm, 100 lp/mm) across the entire focal range and plot MTF curves. If the curve fluctuates by <10% across the full focal range, it indicates that the focus accuracy meets high-precision measurement requirements.