Causes of Striping Artifacts in Sensor Phase Detection Autofocus (PDAF Striping) Under Strong Backlight Contrast

1. The hardware cost of extremely fast focusing in mirrorless cameras: on-chip shielded pixels (Shielded Pixels)

SLR cameras have independent dedicated focus modules, while mirrorless cameras must integrate focus pixels directly on the surface of the imaging sensor (CMOS):

• On-Chip Phase Detection Pixels:
• Sensor manufacturers (such as Sony Semiconductor) select tens of thousands of specific imaging pixels (usually distributed in pairs on specific horizontal lines) during wafer manufacturing;
• Use an extremely small layer of metal aluminum light-shielding mask (Metal Mask) to physically block the left or right half of these pixels;
• In this way, the left pixel only receives light from the right exit pupil of the lens, and the right pixel only receives light from the left exit pupil. By comparing the phase difference of the photoelectric signals on both sides at the microsecond level, lightning-fast autofocus is achieved.
• Missing imaging data and in-camera interpolation:
• Since half of it is blocked by metal, these dedicated focus pixels actually lose half of their normal light-sensing and light-transmitting capabilities;
• When the camera image processor (ISP) generates RAW and JPEG, it relies on the normal pixels in the adjacent upper and lower rows to fill the holes left by these focused pixels through the interpolation algorithm. In 99% of everyday lighting conditions, this color guessing interpolation is completely invisible to the naked eye.

2. Physical trigger boundary of PDAF Striping/Banding

Why does this kind of interpolation masking completely fail in certain large backlight scenes?

• Spatial parallax interference of extreme oblique light:
• When a strong point light source (such as a low-angle sunset direct lens, glaring car headlights, or a powerful stage laser) strikes the sensor from the edge of the screen at a very large angle;
• The light undergoes severe light wave diffraction and oblique internal scattering at the edges of the microlens and metal light shielding mask;
• Normal photosensitive pixels and PDAF pixels with metal light shields produce huge nonlinear faults in the physical scattering absorption rate of this large-angle oblique stray light**;
• Algorithm compensation overflow failure:
• The conventional linear interpolation algorithm in the machine is completely paralyzed in the face of this extreme optical oblique deviation, over-compensating or under-compensating;
• Visual Symptoms: Specific horizontal lines containing PDAF pixels appear on the screen as fine horizontal light and dark stripes and purple picket fences (PDAF Lines) that are neatly arranged and run through the entire high-contrast edge of the backlight!
• The Nikon Z6/Z7 generation and Sony's early A7 series models all encountered heated discussions about this optical physical phenomenon under extremely high contrast and large backlighting.

3. Post-algorithm stripe removal magic weapon: targeted pixel row filter

If such horizontal stripes already appear in the large backlit blockbusters, avoid using ordinary smearing to reduce noise:

• Strict geometric regularity of PDAF stripes:
• Stripes are not random noise, their physical row number coordinates on the sensor wafer are absolutely fixed (for example, a pair appears every 12 rows);
• Specialized open source repair tools (like RawTherapee/PDAF Lines Filter):
• The free open source professional RAW development software RawTherapee has built-in specialized "PDAF Lines Artifact Removal";
• The software directly compares the wafer mask map of a known sensor model to accurately lock the specific pixel rows that generate stripes before demosaic interpolation;
• Apply high-order bicubic spline filter interpolation to instantly and 100% smooth all horizontal stripes** while leaving intact the sharpness of surrounding hair and skin's microscopic pores!

4. On-site avoidance and the ultimate evolution of modern dual pixel array

• Onsite Avoidance Guidelines:
• Use a deep lens hood to prevent strong light sources from directly irradiating the front lens at a large oblique angle to induce oblique scattering;
• Avoid setting the lens aperture to a very small limit (such as f/16f/22, strong light diffraction will aggravate mask scattering), and keep it at the golden aperture of f/5.6f/8.
• Ultimate hardware evolution - Dual Pixel CMOS AF and Quad Pixel:
• Canon's full pixel dual-core completely abandons physical metal occlusion;
• Each pixel is built with two independent complete photodiodes (left A / right B) connected in parallel, which can independently output phase signals, and can be instantly combined to output a 100% full photosensitive signal during exposure synthesis!
  -Sony's latest generation of stacked omnidirectional sensors also moves towards micro-lens spectroscopic technology, completely bidding farewell to the historical pain points of PDAF light-blocking stripes at the root of the hardware.

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