ND 2.4

Overview

ND 2.4 refers to a Neutral Density (ND) filter with an optical density of 2.4. The designation follows the logarithmic density scale, where optical density represents the decimal logarithm of the attenuation factor. ND 2.4 reduces the transmitted light by 8 stops, allowing only about 0.4% of the incident light to pass through – making it one of the strongest ND values commonly used in camera work.

The filter is "neutral" because it ideally attenuates all visible wavelengths equally, thus leaving the color temperature and color balance of the image unchanged. Unlike effect or conversion filters, ND is used solely for brightness reduction, not for color or creative influence.

Density, Stops, and Transmission

The conversion follows a fixed logic: for every 0.3 of optical density, the amount of light is halved, which corresponds exactly to one stop. For ND 2.4, this means 2.4 ÷ 0.3 = 8 stops. To put this in perspective with more common ND values:

Transmission is calculated as 10 raised to the power of minus the density: 10^-2.4 ≈ 0.398%. Some manufacturers specify the filter factor instead of the density – ND 2.4 corresponds to a factor of 256× (2⁸).

On-Set Usage

In motion pictures, a strong ND filter like ND 2.4 is primarily used to maintain desired exposure parameters despite bright lighting conditions. Due to the 180-degree shutter rule, the shutter speed is coupled to the frame rate and cannot be arbitrarily shortened; instead of stopping down the aperture significantly (and thus changing the depth of field), excess light is intercepted by the ND filter. This allows for an open aperture and consequently shallow depth of field even in direct sunlight.

• Typical uses: Exterior shots in direct sunlight, looking into bright windows, shooting light sources or reflective surfaces.
• In professional settings, ND 2.4 is often used as a glass filter in a 4×5.65 inch format, inserted into a matte box in front of the lens.
• For digital cinema cameras, this value is often offered as IRND (Infrared-ND). Strong ND filters allow a proportionally higher amount of infrared light to pass than visible light; IRND variants suppress this IR component to prevent color shifts (especially towards red/magenta in black fabrics).

Examples of real products with this value include the Schneider 4×5.65" Platinum IRND 2.4 (8 Stops) and comparable filters from Tiffen and Lindsey Optics.