In-Depth Interpretation of Lens MTF Curves and Avoiding Small Aperture Diffraction Limits

1. The secret method of reading the coordinate axis of the modulation transfer function (MTF curve)

The MTF (Modulation Transfer Function) curve is the only objective physical gold standard for measuring the optical resolution and contrast of a lens:

1. Abscissa (X-axis): from 0 to 21.6mm. 0 represents the perfect imaging point in the center of the sensor, and 21.6mm represents the edge of the full-frame film.
2. Vertical coordinate (Y-axis): from 0 to 1.0 (or 100%). The higher the value, the more perfect the transmittance and reduction.

• 0.8 or above: extremely excellent;
• 0.6 - 0.8: good and usable;
• 0.6 or less: soft and loose visible to the naked eye.

3. Low frequency line (10 lp/mm thick line): reflects the contrast (Contrast) of the lens. The higher the thick line, the stronger the transparency and three-dimensionality of the picture.
4. High-frequency line (30 or 40 lp/mm thin line): reflects the micro-contrast and extremely high resolution (Sharpness) of the lens. The higher the thin line, the clearer the separation between eyelashes, hair strands and tiny architectural brick patterns.
5. Solid line (S, sagittal radial) and dotted line (T, tangential meridian): The closer the two lines are, the more perfectly controlled the astigmatism of the lens is, and the smoother the out-of-focus bokeh will be, without causing dizzying radial pull or bilinearity.

2. Small aperture diffraction limit (Airy Disk) and the sweet spot area of the best image quality

Newbies often have a fatal misunderstanding: "If you want landscape photos to be clear both front and back, the aperture must be opened to the minimum f/16 or f/22."

• Physical backlash of the wave nature of light: When light passes through an extremely narrow aperture physical hole, the edge of the light wave will spontaneously bend and diffraction (Diffraction) will diverge from an original ideal light point on the sensor to form a circle of concentric rings of alternating light and dark, called Airy Disk.
• The diffraction threshold in the high-pixel era is advanced: On a 61-megapixel (such as Sony A7R5) body, the pixel pitch is only about 3.76 microns. At f/8, the diameter of the Airy disk exceeds the size of a single pixel!
• Best image quality aperture (Sweet Spot): Most modern fixed focus and zoom lenses may be affected by spherical aberration and vignetting when wide open from f/1.4 to f/2.8; when narrowed down two stops to f/5.6 or f/8, the peak resolution of the entire frame will be achieved! From f/11, the image quality begins to slowly decline due to the influence of diffraction, and at f/16 and f/22, the image quality collapses off a cliff and becomes blurry**. If you need a large depth of field, you should use depth of field synthesis instead of blindly opening a very small aperture.

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