Optimization of rear lens elements for edge oblique light caused by the physically extremely short flange focal distance of mirrorless camera mounts

1. The physical definition of flange distance and the “reflector shackles” in the SLR era

Flange Focal Distance (FFD) is the absolute physical straight-line distance from the metal contact surface of the camera mount to the focal plane of the photosensitive element (CMOS/film):

• Physical tightrope in the SLR era (44mm - 46.5mm):
• In order to insert a 45° violently moving mirror box between the rear lens group and the focal plane of the SLR camera, the flange distance is forced to be extremely long (Canon EF is 44.0mm, Nikon F is as high as 46.5mm);
• Wide-angle lens nightmare: Retrofocus / Angénieux Design:
  To design a 20mm ultra-wide-angle lens, the physical back focal length of the lens originally only needs 20mm. But in order to avoid the 46.5mm reflector, engineers were forced to add a huge negative lens concave lens group at the front of the lens, which forced the focus to the sensor beyond 46.5mm;
  As a result, the SLR wide-angle lens is extremely bulky and bulky, the front lens is as huge as a light bulb, and edge coma, astigmatism and barrel distortion are extremely difficult to correct.

2. Physical leap in the mirrorless era: short flange distance (16-20mm) and super large bayonet

After canceling the reflector, the modern mainstream bayonet parameters have undergone a subversive reconstruction:

• Sony E mount: Flange distance 18.0mm, mount inner diameter 46.1mm;
• Canon RF mount: Flange distance 20.0mm, mount inner diameter 54.0mm;
• Nikon Z mount: Flange distance 16.0mm (shortest in full frame!), mount inner diameter up to 55.0mm (largest in full frame!);
• Optical Design Bonus Burst:

1. The rear lens is completely close to the sensor: The last lens in the rear lens can be made into an extremely large ultra-large-diameter lens, only a few millimeters away from the CMOS;
2. Symmetric and quasi-symmetrical optics renaissance: There is no need to use complex anti-telephoto structures. Wide-angle lenses can adopt elegant symmetrical optical configurations, reducing the size by more than 40%, and the edge resolution at full aperture skyrockets;
3. Dream Super Large Aperture Breaking the Ice: Thanks to the 16mm short flange and the 55mm large aperture, Nikon has successfully created the unprecedented Noct 58mm F0.95 top optical lens. However, in the SLR era, the physical limit of the bayonet mount can only barely accommodate F1.2.

3. By-products of short flange distance: Engineering salvation of large-angle incident light at the edge

Everything has two sides, and having the rear lenses too close also brings severe physical challenges:

• Edge ray chief ray angle (CRA: Chief Ray Angle) increased:
  The rear lens is very large and close to the edge. The angle of light hitting the four corners of the sensor increases sharply, which can easily lead to edge microlens shadows and vignetting;
• "Telecentric Design" balance of modern lenses:
  Top mirrorless wide-angle lenses (such as Sony FE 24mm F1.4 GM / Canon RF 15-35mm F2.8L) is not blindly close to the sensor; the last two lenses in the rear group are usually designed as large-diameter plano-concave correction lenses. At the last millimeter moment when the light exits the lens barrel and enters the camera, it forcibly refracts the oblique light back to a gentle angle that is almost vertical. It perfectly fits the pixel micro-lens deep well structure of the digital sensor, achieving impeccable and ultimate wide-open image quality in the center and edges.

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