==== Resolution ====
**1. Definition**\\
Resolution is the smallest spatial detail that an imaging system can distinguish under specified conditions. It is generally classified as either **subjective visual resolution**, determined by human visual assessment, or **objective resolution**, expressed using metrics such as MTF10.\\

**2. Subjective Visual Resolution**\\
Subjective visual resolution is the highest spatial frequency on a test chart that an observer can clearly distinguish. It is commonly expressed in line widths per picture height (LW/PH).

ISO 12233:2023 specifies the use of a black-and-white hyperbolic wedge target for this assessment. Since digital imaging systems may produce aliasing artifacts at high spatial frequencies, visual judgments can vary between observers. The standard therefore defines visual assessment criteria, including the continuous-resolution principle and rules for determining whether features are unresolved.

Captured images may also be analyzed using software such as HYRes to obtain objective measurements. These measurements generally correlate with visual assessments. Where software results differ substantially from visual observation, the visual assessment is used as the basis for judgment.

**2.1 Test Chart**\\
The CIPA resolution test chart is the preferred choice. This chart features black-and-white hyperbolic wedge targets, with spatial frequency varying linearly along the length of the wedges; a single chart covers the range from low to high frequencies. This chart is suitable for measuring cameras with resolutions below 2,500 LW/PH; for cameras with higher resolutions, the shooting distance or focal length can be adjusted so that the test chart fills only a portion of the frame, and the correct LW/PH value can be calculated using a scaling factor (magnification).
{{ :yanding:成像质量评价:图像质量指标:cpia.png?400 |}}

**Chart Material and Contrast Requirements:**\\
  * **Reflection Charts:** The substrate should be approximately Lambertian (diffuse reflection), with an effective height of no less than 20 cm.
  * **Transmission Charts:** These must be backlit by a diffuse light source, with an effective height of no less than 10 cm.
  * **Contrast:** The contrast of the test chart—determined by the reflectance ratio $R_{max}/R_{min}$ or the transmittance ratio $T_{max}/T_{min}$—should be no less than 40:1. The modulation ratio for finer hyperbolic wedge features should be 18 or greater.
  * **Measurement Directions:** Typically include four fundamental orientations: horizontal, vertical, +45°, and -45°.

**2.2 Evaluation Criteria**\\
  * **Continuous Resolution Principle:** Assessment begins from the low-frequency side and moves toward the high-frequency side. A specific frequency is considered "resolved" only if all lower spatial frequencies have already been successfully resolved. The resolution limit is defined as the frequency immediately preceding the first appearance of an unresolved line feature.
  * **Unresolved Feature Determination:** A spatial frequency is judged as "unresolved" if any of the following occur:
**(1) Polarity Reversal:** The luminance of the black and white lines is inverted (black becomes white and white becomes black).\\
**(2) Line Fusion:** The lines become blurred or merged, such that the number of distinguishable lines is fewer than the actual number of lines present on the test chart.\\
{{ :yanding:成像质量评价:图像质量指标:判断准则.png?280 |}}
As shown in the figure, the boundary between the resolvable region (point 1) and the non-resolvable region (point 2) is indicated by the dashed arrow, which corresponds to the resolution value under test.\\

**3. Objective Resolution**\\

Objective resolution is a metric that utilizes the Modulation Transfer Function (MTF) to quantitatively characterize the spatial frequency response of an imaging system. It overcomes the limitations of human visual assessment by precisely measuring the system's ability to transfer contrast across various spatial frequencies, providing a repeatable and non-subjective numerical evaluation.

Common resolution evaluation parameters, such as MTF50 and MTF10, are derived from the MTF curve. Among these, MTF10 (the spatial frequency at which the MTF value drops to 10%) is typically regarded as the objective measure of limiting resolution and exhibits a strong correlation with subjective visual resolution.

For a detailed definition of MTF, testing methodologies, and curve analysis, please refer to the entry for [[en:yanding:image_quality_evaluation:image_quality_factors:mtf_sfr_analysis_and_testing|MTF (Modulation Transfer Function)]].