The MTF (Modulation Transfer Function) curve graph serves as a critical analytical tool for evaluating the optical performance of lenses. By quantifying the lens’s ability to preserve contrast across varying spatial frequencies, it visually illustrates key imaging characteristics such as resolution, contrast fidelity, and edge-to-edge consistency. Below is a detailed explanation:
I. Interpretation of Coordinate Axes and Curves
Horizontal Axis (Distance from Center)
This axis represents the distance from the center of the image (starting at 0 mm on the left) to the edge (ending point on the right), measured in millimeters (mm). For full-frame lenses, particular attention should be paid to the range from 0 to 21 mm, which corresponds to half the diagonal of the sensor (43 mm). For APS-C format lenses, the relevant range is typically limited to 0 to 13 mm, representing the central portion of the image circle.
Vertical Axis (MTF Value)
The vertical axis indicates the degree to which the lens preserves contrast, ranging from 0 (no contrast preserved) to 1 (perfect contrast preservation). A value of 1 represents an ideal theoretical scenario that cannot be achieved in practice, while values closer to 1 indicate superior performance.
Key Curve Types
Spatial Frequency (Unit: line pairs per millimeter, lp/mm):
- The 10 lp/mm curve (represented by a thick line) reflects the lens’s overall contrast reproduction capability. An MTF value above 0.8 is generally considered excellent. – The 30 lp/mm curve (represented by a thin line) indicates the lens’s resolving power and sharpness. An MTF value exceeding 0.6 is regarded as good.
Line Direction:
- Solid Line (S / Sagittal or Radial): Represents test lines extending radially outward from the center (e.g., resembling spokes on a wheel). – Dotted Line (M / Meridional or Tangential): Represents test lines arranged in concentric circles (e.g., ring-like patterns).
II. Performance Evaluation Criteria
Curve Height
Central Region (Left Side of Horizontal Axis): Higher MTF values for both 10 lp/mm and 30 lp/mm curves indicate sharper central imaging. High-end lenses often achieve central MTF values above 0.9.
Edge Region (Right Side of Horizontal Axis): Lower attenuation of MTF values toward the edges signifies better edge performance. For example, an edge MTF value of 30 lp/mm greater than 0.4 is acceptable, while exceeding 0.6 is considered excellent.
Curve Smoothness
A smoother transition between the center and edge suggests more consistent imaging performance across the frame. A steep decline indicates a significant drop-off in image quality towards the edges.
Closeness of S and M Curves
The proximity of the sagittal (solid line) and meridional (dashed line) curves reflects the lens’s astigmatism control. Closer alignment results in more natural bokeh and reduced aberrations. Significant separation may lead to issues such as focus breathing or double-line artifacts.
III. Additional Influencing Factors
Aperture Size
Maximum Aperture (e.g., f/1.4): May yield higher central MTF but can result in edge degradation due to optical aberrations.
Optimal Aperture (e.g., f/8): Typically offers a more balanced MTF performance across the frame and is often highlighted in blue on MTF graphs.
Zoom Lens Variability
For zoom lenses, MTF curves should be evaluated separately at the wide-angle and telephoto ends, as performance may vary with focal length.
IV. Important Considerations
Limitations of MTF Analysis
While MTF provides valuable insights into resolution and contrast, it does not account for other optical imperfections such as distortion, chromatic aberration, or flare. These aspects require additional evaluation using complementary metrics.
Cross-Brand Comparisons
Due to variations in testing methodologies and standards among manufacturers, direct comparisons of MTF curves across different brands should be avoided.
Curve Stability and Symmetry
Irregular fluctuations or asymmetry in the MTF curves may indicate manufacturing inconsistencies or quality control issues.
Quick Summary:
Characteristics of High-Performance Lenses: – The entire 10 lp/mm curve remains above 0.8 – Central 30 lp/mm exceeds 0.6 – Edge 30 lp/mm exceeds 0.4 – Sagittal and meridional curves are closely aligned – Smooth and gradual MTF decay from center to edge
Primary Evaluation Focus: – Central 30 lp/mm value – Degree of edge MTF attenuation – Proximity of S and M curves
Maintaining excellence in all three areas strongly indicates superior optical design and build quality.
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Company Name: Shangrao Jinyuan Optoelectronics Technology Co., Ltd.
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Country: China
Website: https://www.jylens.com/
