How do ND filter numbers work?

ND filters are labelled by their filter factor (ND2, ND4, etc.) or by stops of light reduction. ND2 = 1 stop, ND4 = 2 stops, ND8 = 3 stops, and so on. Each stop doubles the required exposure time. ND1000 (10 stops) is a very popular choice for long-exposure daytime photography.

What ND filter do I need for smooth water?

For smooth, silky water effects, you typically need a shutter speed of 1-30 seconds. An ND6 (6 stops) or ND10 (10 stops) filter achieves this in daylight. Start with your base exposure without the filter and use this calculator to find the new shutter speed.

Can I stack ND filters?

Yes, the stops are additive. An ND3 (3 stops) stacked with an ND6 (6 stops) gives 9 stops total. However, stacking can cause vignetting, colour casts, and reduced image quality. A single stronger filter is usually preferable.

What is optical density?

Optical density (OD) is the logarithmic measure of filter strength. OD = stops × 0.3. So a 10-stop filter has OD 3.0. This notation is commonly used on Lee and Formatt-Hitech filters.

PhotoCalcs

ND Filter Calculator — Adjusted Shutter Speed

Calculate the new shutter speed when using neutral density filters. Supports ND2 through ND1000000.

Base Shutter Speed (without filter)

Your exposure before adding the ND filter

ND Filter Strength

Number of stops of light reduction

How We Calculate This

An ND filter reduces the light reaching the sensor by a specified number of stops. The new shutter speed is calculated as:

New shutter speed = Base shutter speed × 2^stops

For example, if your base exposure is 1/125s and you add a 10-stop ND filter:

1/125 × 2^10 = 1/125 × 1024 = 8.2 seconds

The ND number equals 2^stops (ND1000 ≈ 2^10 = 1024). Optical density = stops × 0.3.

Frequently Asked Questions

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Last updated: February 2026

All calculations are estimates based on standard optical and photographic formulas. Results may vary with specific equipment.