ND Filter Calculator

What is an ND Filter?

A neutral density (ND) filter is a darkened glass or resin filter placed in front of the camera lens. It reduces the amount of light entering the lens without affecting color, allowing you to use slower shutter speeds, wider apertures, or both in bright conditions.

ND Filter Numbering

ND filters are labeled by their light reduction factor. An ND8 filter reduces light by a factor of 8 (3 stops), meaning only 1/8th of the light passes through. Common naming conventions:

• ND number (ND2, ND8, ND64...): The factor by which light is reduced. ND8 = 8x reduction.
• Stops: Each stop halves the light. Stops = log₂(ND factor). ND8 = 3 stops.
• Optical density: Density = log₁₀(ND factor). ND8 = 0.9. Sometimes written as ND 0.9.

The Calculation

The new shutter speed is simply the base shutter speed multiplied by the ND factor:

• New Speed = Base Speed × ND Factor
• Example: 1/250 s with ND8 = (1/250) × 8 = 1/31 s ≈ 1/30 s
• Example: 1/60 s with ND1000 = (1/60) × 1000 = 16.7 s

Stacking ND Filters

You can combine multiple ND filters by multiplying their factors together. For example, stacking ND8 and ND64 gives ND512 (8 × 64 = 512), equivalent to 9 stops of light reduction.

Common Uses

• Waterfalls & rivers: ND8 to ND64 for silky smooth water (0.5 – 4 s exposures).
• Daytime long exposure: ND256 to ND1000 for cloud streaks and empty streets (30 s – several minutes).
• Wide aperture in bright light: ND2 to ND8 to shoot at f/1.4 or f/2 outdoors for shallow depth of field.
• Video: ND4 to ND16 to maintain the 180° shutter angle rule (shutter speed = 2× frame rate) in bright conditions.

Practical Tips

• Use a tripod: Any exposure longer than about 1/30 s requires a tripod to avoid camera shake.
• Focus first: Compose and focus before attaching strong ND filters (ND64+), as the viewfinder will be too dark to see through.
• Timer or remote: Use a 2-second timer or remote shutter release to avoid vibration when pressing the shutter button.
• Bulb mode: For exposures longer than 30 s, switch to Bulb mode and time with a stopwatch or intervalometer.

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