Variable neutral density (ND / VND) polarizer cross dead black cross (X-Pattern) avoidance

1. The truth about the physical structure of Variable ND

Many people think that adjustable ND is some kind of "liquid crystal glass that can adjust the concentration". This is completely wrong optical common sense!

• The essence is the face-to-face overlap of two linear polarizers!
• The first layer of front lens is a polarizer (Polarizer A);
• The second rear lens is another polarizer (Polarizer B).
• Malus's Law:
  When the gratings of the two polarizers are parallel, the light transmittance is the highest (the basic light reduction is about 1 stop to 1.5 stops); when you rotate the outer ring and the angle between the gratings of the two polarizing filters gradually approaches 90 degrees perpendicularly, the light is theoretically 100% blocked and intercepted by the two vertical gratings (the light reduction can be up to 5 stops to 9 stops), achieving continuous stepless light reduction.

2. Devastating disaster: How was the "X-Cross Pattern" born?

When you use VND on a wide-angle lens (such as 16mm, 24mm) and try to turn the light reduction setting to the maximum setting, a terrible picture appears: a huge, pitch black, X-shaped or cross-shaped dark spot will burst out in the center of the photo, with extremely bright surrounding corners, and the entire film will be scrapped!

• Geometry and Physics Origin: The wide-angle lens has a very large field of view. The actual physical optical distance (Distance) of the light incident at the edge at a large angle and penetrating the two layers of polarizers is much greater than the light incident from the vertical center of the lens! When the angle between the central rays reaches 70 degrees, the edge tilt rays have already reached the 90-degree complete extinction threshold in geometric projection, causing the light to be forced to undergo severe geometric polarization destructive asymmetry on the two-dimensional sensor.

3. The golden rule to completely avoid cross black spots

1. Focal segment safety red line:

• Wide-angle end (< 24mm): Adjustable ND must not exceed 3 to 4 stops of light reduction;
• Focal length above 35mm: can be safely used to 5th to 6th stop;
• Telephoto end above 70mm: almost no cross-black spots.

2. Look for professional VND with “hard stops”:

• It is strictly prohibited to buy low-quality, unlicensed adjustable NDs that can rotate 360 degrees infinitely!
• Professional brands (such as PolarPro Peter McKinnon series / Nisi True Color) have precision-designed physical mechanical limit latches on the metal frame: strictly limiting the rotation range to the safe range of 1-5 gears (ND2-ND32) or 6-9 gears (ND64-ND512), and completely locking it physically and mechanically, so that no matter how hard the user twists, it is impossible to screw into the cross spot danger zone.

3. Top-level landscape long exposures are still dominated by fixed single-stop ND:

• When you need a 10-stop (ND1000) ultra-slow door during the day to shoot silky water and waves, resolutely use a fixed single-stop neutral gray filter and stay away from any adjustable polarization ND.

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