What it's for
Neutral density (ND) filters block a fixed amount of light without shifting the image's colors, letting you use much longer shutter speeds even in full daylight — the classic technique for smoothing moving water, streaking clouds or blurring crowds in long-exposure photos. This calculator gives you the new shutter speed once you add an ND filter to your original speed.
Formula used
Each ND filter blocks a fixed number of light stops, corresponding to its factor: stops = log₂(ND_factor). The new shutter speed is new_speed = original_speed × 2^stops. For example, an ND8 filter (factor 8) blocks log₂(8) = 3 stops, multiplying the shutter speed by 2³ = 8.
ND factor table used
ND2 = 1 stop; ND4 = 2 stops; ND8 = 3 stops; ND16 = 4 stops; ND32 = 5 stops; ND64 = 6 stops; ND100 = 6.64 stops; ND400 = 8.64 stops; ND1000 (known as the "big stopper") = 10 stops; ND32000 = 15 stops. These follow the most common optical-density factor numbering on the market (ND followed by the light-blocking factor).
Worked example
A shot metered at 1/125s (0.008s) unfiltered, with an ND1000 filter (10 stops), becomes 0.008 × 1000 = 8 seconds — enough to smooth running water or clouds.
Limitations
The calculation assumes the filter blocks exactly its nominal manufacturer-stated stops — in practice, cheaper ND filters can vary slightly from the nominal value, and may also introduce a slight color cast that needs correcting in post-processing.
Frequently asked questions
How do I find my current shutter speed in decimal seconds?
Divide 1 by the fraction's denominator. For example, 1/125s = 1÷125 = 0.008s; 1/60s = 1÷60 ≈ 0.0167s.
Why does the calculation use log base 2?
Because each photographic "stop" represents a doubling (or halving) of the amount of light — the same principle used for aperture and ISO. Log base 2 converts the filter's blocking factor (e.g. 8x) into the number of stops (3).
Does an ND1000 filter really block exactly 1000x the light?
That's the manufacturer's nominal value (10 stops = 2¹⁰ = 1024x, rounded to "1000"). In practice, the actual density can vary slightly between units and brands.