Overview
ND filters are essential for outdoor video production. Without them, shooting in daylight at the correct shutter speed for the 180-degree rule is impossible — you'd either overexpose badly or be forced into incorrect shutter speeds that produce stroboscopic motion. This guide explains the logic, the naming conventions, and how to choose what you need.
What You Need
- A camera or phone used for outdoor video
- Your lens filter thread diameter (printed on the lens cap, e.g. Ø77mm)
- Our Exposure Triangle Calculator
Steps
What an ND filter does
A neutral density (ND) filter is like sunglasses for your lens — it reduces the amount of light entering the camera without affecting colour temperature. In photography it allows shallow depth of field in bright light. In video, its primary purpose is to let you maintain the 180-degree shutter rule (1/48s at 24fps, 1/50s at 25fps) outdoors without overexposing. Without an ND, bright daylight forces you to either overexpose or use a faster shutter speed — both wrong.
Understanding ND filter strengths
ND filters are rated in several ways that all mean the same thing. ND number: ND2 = 1 stop, ND4 = 2 stops, ND8 = 3 stops, ND64 = 6 stops, ND1000 = 10 stops. Optical density: ND 0.3 = 1 stop, ND 0.6 = 2 stops, ND 0.9 = 3 stops. Stops of reduction: the most intuitive — each stop halves the light. An ND8 (3 stops) takes a correct exposure at 1/400s and allows 1/50s, which is exactly what the 180-degree rule requires at 25fps in full sun.
Fixed vs variable ND filters
Fixed ND filters have a single density. Reliable, no optical issues, but you need multiple filters for different lighting conditions. A set of ND8, ND64, and ND1000 covers most scenarios. Variable ND filters rotate like a polariser to adjust from 1 to 8+ stops in one filter. Convenient, but cheap variable NDs create an X-shaped darkening artefact at high settings (the "cross-polarisation" problem). Quality variable NDs (Breakthrough, Tiffen) cost £80–£200 but are worth it for run-and-gun shooting.
How to choose the right ND strength
Use this as a guide: Overcast daylight: ND8 (3 stops). Bright partly-cloudy: ND64 (6 stops). Full sun midday: ND64–ND1000 depending on aperture. Golden hour: ND8 or no filter. If you're unsure, use our Exposure Triangle Calculator: enter your frame rate, desired aperture, and current ISO — it will recommend the appropriate ND filter strength.
ND filters for phone cameras
Phone cameras don't accept screw-on filters. Options: clip-on ND filters (e.g. Moment, PolarPro), which clip over the lens; magnetic filter systems designed for specific phone models; or built-in ND modes (some Lumix phones have a physical variable ND). Clip-on filters are affordable and work well for standard lenses but often don't cover ultra-wide lenses. Only use them on the main (1x) camera lens.
Pro Tips
- Internal ND filters (found in many cinema cameras and some mirrorless cameras) are more convenient than external filters — no vignetting, no cross-polarisation issues.
- A polarising filter is NOT a substitute for an ND filter — it only reduces 1.5 stops and changes how reflections appear.
- Always check for colour cast: hold the ND in front of a white surface and look through it. A visible colour cast in a budget ND will affect your white balance.