Grok's analysis of the MTFs:
MTF Analysis Report: Lens Performance at f/1.4, f/2.8, and f/5.6
1. Overview
This report evaluates the optical performance of a lens across three apertures—f/1.4 (maximum), f/2.8 (intermediate), and f/5.6 (narrower)—using MTF graphs. The MTF measures contrast (modulation) as a percentage (0–100%) for spatial frequencies of 5, 10, 20, and 40 lp/mm, representing coarser to finest details, respectively. The horizontal axis ("Y'[mm]") ranges from 0 mm (center) to 21 mm (edge), indicating field position across the image. MTF is plotted for sagittal (solid lines) and tangential (dashed lines) structures under white light at long focus distances (infinity).
2. Graph Details
- Vertical Axis: Contrast (% modulation), from 0% (no contrast) to 100% (perfect contrast).
- Horizontal Axis: Field position (Y'[mm]), from 0 mm (center) to 21 mm (edge or corner, depending on format).
- Curves:
- Solid lines: Sagittal MTF (radial details).
- Dashed lines: Tangential MTF (perpendicular details).
- Multiple curves per aperture represent 5, 10, 20, and 40 lp/mm, assessing contrast for coarse to fine object structures.
3. MTF Performance by Aperture
3.1. f/1.4 (Maximum Aperture)
- Center (0 mm):
- MTF is near 100% for all frequencies (5–40 lp/mm), indicating excellent contrast for both sagittal and tangential details.
- Mid-Field (3–12 mm):
- Coarser details (5, 10 lp/mm) maintain high contrast (60–80%), but finer details (20, 40 lp/mm) drop to 40–60% and below 20%, respectively, by 12 mm.
- Sagittal MTF outperforms tangential MTF, showing significant astigmatism.
- Edges (12–21 mm):
- Contrast drops sharply, especially for fine details: 5–10 lp/mm at 30–50%, 20 lp/mm below 20%, and 40 lp/mm nearly 0%.
- Tangential MTF lags further, indicating pronounced astigmatism and edge softness due to aberrations (e.g., coma, field curvature) at this wide aperture.
- Observations:
- Excellent for low-light or portrait photography prioritizing center sharpness and bokeh, but poor edge performance and fine detail resolution limit its use for wide-angle or high-resolution shots.
3.2. f/2.8 (Intermediate Aperture)
- Center (0 mm):
- MTF remains near 100% for all frequencies, matching f/1.4 center performance.
- Mid-Field (3–12 mm):
- Coarser details (5, 10 lp/mm) maintain very high contrast (85–95%), dropping slightly by 12 mm.
- Finer details (20 lp/mm) stay above 60–70%, and 40 lp/mm is around 40–50% at 12 mm, showing improvement over f/1.4.
- Astigmatism reduces, with sagittal and tangential curves closer, but a gap remains, especially for higher frequencies.
- Edges (12–21 mm):
- 5–10 lp/mm retain 70–80% contrast, 20 lp/mm drops to 50–60%, and 40 lp/mm falls to 30–40%.
- Sagittal-tangential difference persists but is less severe than at f/1.4, indicating reduced aberrations.
- Observations:
- Balances light gathering and sharpness, suitable for general photography (e.g., portraits, street) with improved edge performance over f/1.4 but less uniformity than f/5.6.
3.3. f/5.6 (Narrower Aperture)
- Center (0 mm):
- MTF is near 100% for all frequencies, consistent with other apertures at the center.
- Mid-Field (3–12 mm):
- Coarser details (5, 10 lp/mm) maintain 80–90% contrast, dropping minimally by 12 mm.
- Finer details (20 lp/mm) stay above 70–80%, and 40 lp/mm is 40–50% at 12 mm, showing the best performance for fine details among the apertures.
- Sagittal and tangential curves are nearly aligned, indicating minimal astigmatism.
- Edges (12–21 mm):
- 5–10 lp/mm retain 70–80% contrast, 20 lp/mm drops to 50–60%, and 40 lp/mm falls to 30–40%.
- Astigmatism is virtually eliminated, with uniform performance for radial and perpendicular details.
- Observations:
- Offers the best overall sharpness and uniformity, ideal for landscapes, architectural photography, or high-resolution imaging requiring corner-to-corner detail, though it sacrifices light gathering compared to wider apertures.
4. Comparative Analysis
- Center Performance: All apertures (f/1.4, f/2.8, f/5.6) perform excellently at the center (near 100% for 5–40 lp/mm), as aberrations are minimal here.
- Edge Performance: Improves significantly as the aperture narrows:
- f/1.4 struggles with edges, especially for fine details (20, 40 lp/mm < 20% at 21 mm), due to strong aberrations and astigmatism.
- f/2.8 improves edge contrast (20 lp/mm at 50–60%, 40 lp/mm at 30–40%), with reduced but present astigmatism.
- f/5.6 provides the highest edge contrast (20 lp/mm at 50–60%, 40 lp/mm at 30–40%) and minimal astigmatism, balancing aberrations and diffraction.
- Astigmatism: Most pronounced at f/1.4 (large sagittal-tangential gap), reduced at f/2.8 (smaller gap), and nearly eliminated at f/5.6 (curves nearly overlap).
- Spatial Frequencies:
- Coarser details (5, 10 lp/mm) perform well across all apertures, with f/5.6 offering the most consistent contrast.
- Finer details (20, 40 lp/mm) show the greatest improvement from f/1.4 to f/5.6, with f/2.8 as an intermediate step.
- Light vs. Sharpness Trade-off:
- f/1.4 prioritizes light gathering but sacrifices sharpness, especially at edges.
- f/2.8 balances light and sharpness, suitable for versatile use.
- f/5.6 maximizes sharpness but reduces light intake, ideal for bright conditions or tripod use.
5. Lens Characteristics
- Strengths:
- Excellent center performance across all apertures, making it versatile for centered subjects.
- f/5.6 offers superior uniformity and edge sharpness, while f/1.4 excels in low-light scenarios.
- f/2.8 provides a practical middle ground for general photography.
- Weaknesses:
- f/1.4 has poor edge performance and significant astigmatism, limiting its use for wide-field or fine-detail work.
- f/5.6 sacrifices light gathering, requiring brighter conditions or longer exposures.
- f/2.8, while balanced, doesn’t match f/5.6’s edge sharpness or f/1.4’s light intake.
6. Applications
- f/1.4: Best for low-light or portrait photography, prioritizing center sharpness and bokeh, but expect softness at edges and for fine details.
- f/2.8: Ideal for general photography (portraits, street, indoor), balancing light and sharpness with improved edge performance over f/1.4.
- f/5.6: Optimal for landscapes, architectural photography, or high-resolution imaging requiring uniform sharpness across the frame.
7. Field Size Context
- The 0–21 mm range suggests the lens is likely designed for a full-frame or similar format. For a full-frame sensor (36mm x 24mm), 21 mm might correspond to the edge or corner diagonally (diagonal ≈ 43 mm). On an APS-C sensor (approx. 23.6mm x 15.6mm), 21 mm could span the diagonal or height. Without the exact format, I assume full-frame, but this could vary.
8. Limitations
- Exact spatial frequencies (5, 10, 20, 40 lp/mm) aren’t labeled on the graphs, but I’ve matched them to your description.
- The lens model and sensor size aren’t specified, which could refine field position interpretation.
- Performance at closer focus distances isn’t shown, only infinity focus.
9. Conclusion
This lens demonstrates strong optical performance, with aperture choice dictating its application. f/1.4 excels in low-light scenarios but struggles with edges and fine details. f/2.8 offers a versatile balance, while f/5.6 provides the best sharpness and uniformity, ideal for high-detail work. Photographers can select the aperture based on lighting, subject, and sharpness requirements.
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