Digital Camera Sensor Microlens Technology and Image Quality Degradation at Corner Edges Due to Oblique Light Incidence

1. CMOS pixel aperture ratio crisis and surface microlens array

As sensor pixel density soars (individual pixel size shrinks to less than 3 microns), most photons simply cannot fall inside the photodiode without optical intervention:

1. Light concentration of On-Chip Microlens:

• On the outermost surface of the sensor chip, a convex lens microlens invisible to the naked eye is custom-cast for each independent pixel;
• Function: Like countless micro funnels, it captures and refracts all the light that shines on the dead corners of the metal wiring around the pixel, and focuses it on the center of the tiny silicon photodiode at the bottom, increasing the quantum efficiency of the sensor several times.

2. Contradiction between vertical incidence at the center and oblique incidence at the edge:

• In the center of the frame: light enters at a vertical chief ray angle (CRA - Chief Ray Angle) of nearly 90 degrees, and the microlens is perfectly focused;
• But at the very edge of the frame and the four dead corners: the light must hit the microlens surface at an extremely sharp angle!

2. The "transfer disaster" of old range-axis symmetrical wide-angle lenses

Many photographers eagerly screw expensive old classic rangefinder lenses (such as Leica Super-Angulon 21mm, Biogon 28mm symmetrical structure wide-angle) to ordinary Sony/Nikon mirrorless cameras through adapter rings, and the resulting photos have unsightly edges:

1. The physical structure of the rear lens close to the sensor:

• The rangefinder wide-angle lens eliminates the SLR reflector, and the last piece of glass at the rear of the lens almost touches the film surface (the physical distance is only a few millimeters);
• When oblique rays emit from the rear group, the chief ray incident angle (CRA) reaching the edge pixel is extremely steep, with an inclination angle as high as 35° to 45°!

2. Refraction blocking of modern mirrorless high-thickness sensing front glass (Cover Glass):

• The surface of the film is only a thin layer of gelatin emulsion, and the film has no hindrance to the full absorption of obliquely incident light;
• But modern digital camera sensors are covered in front with a stack of sensor protection glass (IR-Cut filter, low-pass filter, dust-proof glass) up to 1.5mm to 2.5mm thick;
• When the extremely oblique light passes through this thick glass, it produces huge astigmatism and field curvature deformation, causing the originally sharp edge of the picture to be completely stretched and disintegrated into a blurry smearing (Smearing).

3. "Red Shift Door" (Italian Flag Effect - Italian flag color cast):

• The light incident obliquely cannot accurately fall on the corresponding photosensitive point after penetrating the microlens. Instead, it penetrates obliquely and "leaks" into the color filter of the neighboring pixel;
• As a result, the color in the center of the picture is normal, but a large area of purple appears on the left edge, and a large area of dark green appears on the right edge. The entire photo is seriously damaged by color cast.

3. The evolutionary game of micro-lens technology among major camera brands

In order to completely solve the problem of image quality collapse under large-angle oblique light, the semiconductor process has made two revolutionary improvements:

1. Microlens edge progressive shift technology (Shifted Microlenses):

• Representative models: Leica M9 / M10 / M11 special range axis sensor.
• When the chip is manufactured, the microlens in the center of the sensor is placed strictly in the center;
• As it moves toward the edge, the physical position of the microlens gradually shifts toward the center of the frame relative to the underlying pixels!
• At the very edge, the microlens has moved a certain distance inward, intercepting the 40-degree oblique light incident obliquely, and perfectly refracting it back to the deep well photosensitive area at the bottom, achieving a sharp transition to the wide-angle edge and completely eliminating red shift.

2. Ultra-thin sensor cover glass:

• The thickness of the protective glass of Leica M series body sensors is extremely extreme (as thin as 0.8 mm, only one-third that of conventional Japanese cameras), completely eliminating edge dispersion and field curvature collapse caused by thick glass.

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