The impact of the thickness of scratch-resistant glass (Cover Glass) on digital camera sensors on the edge image quality of adapted rangefinder wide-angle lenses and modification for thinning

1. The ultimate pain point of the adapter party: Leica’s top wide-angle head “collapses at the edge” on the Sony body

In order to enjoy the excellent image quality of Leica M-mount ultra-thin rangefinder wide-angle lenses (such as Leica 28mm f/2 Summicron, 21mm Super-Elmar, Voigtlander 15mm/21mm), many photographers buy adapter rings and install them on Sony full-frame mirrorless cameras (A7 series). However, the landscape photos taken are breathtaking:

1. Catastrophic collapse of edge quality:

• The center of the picture is as sharp as a scalpel, but from the 2/3 area to the four corners of the picture, the resolution drops off a cliff, and the leaves and architectural details completely turn into a paste-like radial blur;
• There are serious Red Shift Color Cast (Italian Flag Color Cast) and heavy vignetting around the screen;

2. It’s not that the quality of the lens is bad, but that the thickness of the sensor protective glass (Cover Glass) is fatally mismatched!

2. Optical underlying physical mechanism: severe field curvature and astigmatism caused by parallel flat glass

1. Extra large incident angle (Telecentricity Failure) of paraxially symmetric wide-angle lenses:

• Due to the existence of the reflector mechanical box of traditional SLR lenses, the rear lens group is extremely far from the sensor (flange distance is more than 44mm), and the light exit angle is very close to vertical incidence;
• The rangefinder lens has an extremely short flange distance (Leica M is 27.8mm), and many classic wide-angle lenses adopt a symmetrical double Gaussian (Biogon) structure, with the rear lens almost attached to the sensor surface;
• This causes edge rays to hit the edge of the sensor at extremely tilted and huge angles (even up to 35° to 45°).

2. Refraction disaster of sensor cover glass (Cover Glass/Filter Stack):

• The surface of the digital sensor is covered with a composite glass cladding (Filter Stack) composed of an IR-Cut Filter, a low-pass filter and anti-scratch protective glass;
• The thickness of protective glass varies greatly among major camera manufacturers:
• Leica M series digital cameras (M9/M240/M10/M11): specially designed for rangefinder lenses, the thickness of the glass cladding is extremely stringently compressed between 0.8mm and 1.0mm;
• Sony mirrorless camera (Sony Alpha series): cladding thickness up to 2.0mm to 2.5mm;
• Canon and Nikon mirrorless cameras (R/Z series): The thickness is also generally between 1.5mm and 2.0mm.

3. Optical damage of thick glass under high-angle light:

• According to the law of refraction, when a large-angle oblique beam passes through a flat glass with a thickness of up to 2.5mm, the glass is equivalent to introducing a severely negative power optical element;
• Instantly induces severe asymmetric astigmatism and severe reverse field curvature;
• The optimal focal plane of the lens is flat in the center. When it reaches the edge, it is forcibly pulled forward and bent by the thick glass, causing the edge pixels of the sensor to be completely in a severely defocused out-of-focus area!

3. Non-destructive saving techniques and aperture contraction range

1. Stopping Down:

• Shrink the aperture to f/8 or even f/11, relying on a larger depth of field to cover up edge field curvature out of focus to a certain extent;
• However, on ultra-wide-angle lenses, shrinking the aperture cannot eliminate astigmatism, and f/11 will induce diffraction softening, which is a helpless compromise that treats the symptoms but not the root cause.

2. Choose a digitally optimized lens with telecentric rear group design:

• Choose rangefinder lenses that have been optimized for thick glass sensors in recent years (such as the new digitally optimized version of Voigtlander VM Vintage Line, or some lenses from the Zeiss ZM series), and add a telecentric negative lens to the rear group to correct the angle of the outgoing light.

4. The ultimate surgical solution: Thin-Glass Sensor Mod

For hardcore gamers who pursue the ultimate in image quality, the only physical ultimate solution is to send the camera body to a professional optical modification agency (such as Kolari Vision):

1. Dust-free disassembly and peeling of sensor:

• On a class 100 ultra-clean and dust-free workbench, disassemble the camera mainboard and take out the CMOS sensor core assembly;
• Use precision hot air deglue or ultrasonic knife to completely peel off Sony's original infrared protection filter, which is more than 2.0mm thick, from the glass layer;

2. Implantation of 0.7mm ultra-thin high-transmittance optical glass:

• Precision-fitting and customized high-transmittance ultra-thin optical filter (thickness is only 0.7mm, built-in high-performance infrared cut-off coating and multi-layer anti-reflective AR coating);
• Recalibrate CMOS flange plane micron levelness and sensor displacement stabilization (IBIS) zero reference.

3. The actual measurement effect of the modified machine has leapt:

• The modified Sony body is connected to Leica 21mm / 28mm / 35mm top rangefinder lenses. Hairline-level details at the edges are completely revived, and the sharpness of the four corners of the image is almost as sharp as the center of the image!
• At the same time, the image quality of this body will not be negatively affected at all when using native Sony E-mount original lenses, completely opening up the physical gap between rangefinder ultra-thin optical aesthetics and full-frame high-performance digital bodies.

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