1. Concept of Spatial Frequency Response
Spatial Frequency Response (SFR) refers to the degree of attenuation of the amplitude (i.e., contrast) of a sinusoidal input as a function of spatial frequency. It translates the subjective perception of “sharpness” by the human eye into measurable physical parameters, and is used to evaluate the ability of an imaging system (such as a camera or lens) to reproduce details at different spatial frequencies.
As shown in the figure above, a scene can be decomposed into numerous frequency components. The imaging system alters the contrast of each frequency component, and the output image is the result of the individual attenuation and superposition of these different frequency components. At the physical level of imaging, generally speaking, the higher the spatial frequency, the greater the contrast loss. The SFR test results provide the contrast at different frequencies, visually demonstrating the contrast transfer capability of the imaging system.

2. Causes of Image Blur
Image blur in digital cameras is the result of multiple interacting factors, which can be mainly categorized into three types:

2.1 Imaging Hardware Level

(1) Geometric aberrations and diffraction of the lens

(2) Pixel aperture and photoelectric crosstalk of the image sensor

2.2 Image Processing Software Level

(1) Algorithmic processing such as noise reduction, background blurring, and beauty filters

2.3 Environmental Factors

(1) Atmospheric turbulence and aerosol interference

(2) Relative movement between the camera and the subject during exposure

(3) Flare phenomena under strong light
3. e-SFR Testing

e-SFR stands for Edge Spatial Frequency Response. Its testing principle involves analyzing the degree of attenuation of the amplitude (contrast) of a sinusoidal input as a function of spatial frequency through the edge spread function, to quantify the sharpness and detail reproduction capability of the imaging system.

3.1 Testing Standards

(1) Standard basis: The international standard for e-SFR testing is ISO 12233, which was first published in 2000, with the latest 5th edition released in 2024.

(2) Calculation principle: The device under test captures an image of a slanted straight edge. The slant of the edge creates different “phases,” forming an oversampled edge spread function to counteract the uncertainty introduced by the shift variance of the discrete pixel array on the sensor. Then, the first derivative of the edge spread function is calculated to obtain the line spread function, which is then subjected to a Fourier transform and modulus operation to yield the spatial frequency response.

3.2 Testing Equipment

The testing equipment used for this e-SFR test includes the LS-CCXL-2S06-IR Vertical Multi-CCT Supplemental Light Source and the CP135 Slanted Edge SFR Test Chart.

The LS-CCXL-2S06-IR Vertical Multi-CCT Supplemental Light Source is a standard supplemental light source specifically designed by Yanding for camera testing, used to simulate environments with different color temperatures and illuminance levels. This light source supports a continuously adjustable color temperature from 2300 K to 10000 K and integrates dual-band near-infrared light sources at 850 nm and 940 nm. The two light sources can be turned on individually or used in combination to meet the testing requirements for both visible light and infrared cameras. The light source supports programmable control interfaces and can be controlled via a computer, significantly improving testing efficiency.

The CP135 Slanted Edge SFR Test Chart complies with the ISO 12233:2017 standard. This chart contains 9 low-contrast rectangular targets slanted at 5°, enabling SFR testing across multiple fields of view and in both horizontal and vertical directions.

3.3 Testing Procedures

(1) Environment Setup: Conduct the test in a dark environment. Adjust the position, orientation, color temperature, and illuminance of the vertical multi-CCT supplemental light source according to testing requirements. Use an illuminance meter to measure and ensure that the illuminance at the center and corners of the chart is identical. Here, 6500K and 800 lux are used as an example.

(2) Sample Capture: Securely fix the DUT (Device Under Test), turn on the power, and bring it into its working state. Adjust the posture of the DUT so that the center of the image aligns with the center of the test chart, and the four surrounding rectangles are located at the 70% field of view. Once the image is stable, capture the sample.

(3) RIQA Analysis: (RIQA is an image quality analysis software independently developed by Yanding, and its Camera module focuses on camera image quality analysis)

a. Open the RIQA software and select the test item corresponding to the required module.

b. Click the “+ Add” button to select the image to be loaded. The “Gamma” in the lower left corner can be changed according to testing requirements.

c. After clicking the “Start” button, the software automatically recognizes the test chart and marks the ROI areas. The ROI areas can be manually adjusted as needed.

d. Click the “Analyze” button. The RIQA software processes and calculates the image, then automatically jumps to the test result window. After processing, the software navigates to the result window, where the central area displays the SFR (MTF) curves for each ROI (which can be switched to Edge curves), and the measured values are displayed below.

e. Click the “Analyze” button. The RIQA software processes and calculates the image, then automatically jumps to the test result window. After processing, the software navigates to the result window, where the central area displays the SFR (MTF) curves for each ROI (which can be switched to Edge curves), and the measured values are displayed below.

f. Click the “Export” button to generate the report.

3.4 Result Interpretation

The report includes measurement results for the image center and the surrounding ROI areas, such as MTF50P, MTF50, MTF30, MTF10, and related charts. These can be retrieved as needed. For more detailed data, you can choose to export a spreadsheet file when generating the report.

MTF50 is the spatial frequency value corresponding to an MTF of 50% on the MTF curve;
MTF50P is the spatial frequency value corresponding to the MTF dropping to 50% of its peak value on the MTF curve;
MTF30 is the spatial frequency value corresponding to an MTF of 30% on the MTF curve;
MTF10 is the spatial frequency value corresponding to an MTF of 10% on the MTF curve.

See Also
camera测试用例、测试用例、riqa简介