Neutral Density filter with optical density 2.1, reducing light by seven stops (ND128), enabling extreme wide apertures for minimal depth of field and extreme slow motion.
Optical Density and Aperture Value
The ND 2.1 filter (also known as ND128) has an optical density of 2.1 and reduces incident light by seven stops. This means the amount of light is reduced to one 128th.
The ND128 designation system indicates: ND128 means light is divided by 128. This corresponds to a reduction of 7 stops (7 x halving = 1/128).
Typical Applications
Extreme Lighting Situations with Large Format Cameras
The ND 2.1 is ideal for professional cinema cameras with large sensors (Alexa, RED, Venice) when you want to shoot at extreme brightness with apertures like T1.4 or T2.0. This is the maximum that practical daylight cinematography allows.
Slow Motion with Minimal Depth of Field
The ND 2.1 makes 120fps or 240fps possible in daylight without having to stop down to f/22. You can fully utilize the minimal depth of field of your lens.
Artistic Separation from the Background
With the ND 2.1, you can film subjects with an extremely wide aperture – the background becomes completely blurred, which has a very isolating psychological effect.
Difference from Variable ND Filters
Variable ND filters can theoretically cover the ND 2.1 range, but they are:
- Practically Unusable: At ND128 settings, strong color casts occur
- Extreme Vignetting: The X-shaped patterns are clearly visible at wide angles
- Moiré Effects: With fine textures, patterns that are difficult to correct emerge
- Optical Quality: The loss of sharpness is significant
The fixed ND 2.1 is the only practical solution for such extreme reductions.
Relation to the 180-Degree Shutter Rule
The ND 2.1 becomes necessary when you:
- Shoot in extreme daylight (100,000+ Lux)
- Use slow motion (120fps or higher)
- Use a wide aperture (T1.4-T2.0)
- Want to adhere to the 180-degree shutter rule
This is technically the limit for practical daylight cinematography. Beyond these parameters, special solutions (IR filters, special cameras) are necessary.
Practical Use Cases
Scenario 1: Slow Motion with Minimal Depth of Field
- Recording Format: 120fps (2x longer exposure)
- Light: Bright midday sun (100,000+ Lux)
- Desired: T1.4, 180-degree shutter
- ND 2.1 is the solution
Scenario 2: Emotional Close-Up
- Time of Day: Midday sun
- Desired: Maximum separation from the background (T1.4)
- ND 2.1 makes this possible at 25p
Scenario 3: Artistic Imagery
- Concept: Psychological isolation of a character
- Environment: Bright, highly textured background
- ND 2.1 makes this complete focus possible
Specifications
- Optical Density: 2.1
- Light Loss: 7 stops
- Multiplier: ND128
- Typical Transmission Rate: 0.78%
- Application Range: Extreme daylight to very extreme lighting situations
- Standard Format: Matte Box 4x4", 4x5.65", 6.6x6.6"
Combinations with Other Filters
- ND 2.1 + ND 0.3 = ND 2.4 (8 stops)
- ND 2.1 + ND 0.6 = ND 2.7 (9 stops)
- ND 2.1 + ND 0.9 = ND 3.0 (10 stops - practical limit)
Practical Application Examples
- Slow-Motion Epics: 120fps+ in daylight with a wide aperture
- Portraits in Midday Sun: Extreme background blur
- Water Surface Shots: Reflection + Slow Motion = Dramatic
- High-Speed Sequences: 240fps+ without stopping down
- Vertical Format Cameras: Alexa 65, RED 8K, Panavision DXL
Important Notes
The ND 2.1 is the practical limit for external ND filters. Stronger filters (ND 2.4 or higher) require:
- High-quality optical glass (Schott B270 glass)
- Precise anti-reflection coatings
- Specialist manufacturers (Schneider, Formatt-Hitech)
- High costs (800-1500 Euros per filter)
For even more extreme reductions, special techniques are necessary (stacked filters, internal camera ND systems, IR filters).