ND filter for video

The ND filter for video to buy is a variable one, unless you shoot slow, planned landscapes. Choose the K&F ND2-ND400 for a single filter with a wide range, or the NiSi Swift True Colour kit if colour accuracy matters most, and keep a fixed ND for long exposures.

An ND filter is sunglasses for a lens. Without it, a bright Indian afternoon forces you to close the aperture or raise the shutter speed, and your video loses its look and its motion.

How many stops

Start with the old sunny rule: in full sun at ISO 100, f/16 at 1/100 gives a correct exposure.

• To shoot f/2.8, open up 5 stops from f/16.
• To shoot at 1/50, slow down 1 more stop from 1/100.
• That is 6 stops of ND at ISO 100.
• A log profile that starts at ISO 800 is 3 stops more, so about 9 stops.

So at f/2.8, a filter that reaches about 9 stops covers bright days. ND400 works out at about 8.6 stops, since each stop halves the light. NiSi reaches 1 to 9 stops by pairing a 1 to 5 stop variable ND with a 4-stop ND in its kit, and it says the pair holds accurate colour.

Variable or fixed

A variable ND turns to change strength, so it suits moving between shade and sun. Two polarising layers do that job, which is why a variable ND can show an uneven, dark cross at its strongest settings on wide lenses. Filters with hard stops, such as the Prism FX VND, stop you turning into that range. Whatever you buy, stay a little short of the last click.

Fixed ND filters are simpler and usually cleaner, but changing one takes time. They suit landscapes and long exposures on a tripod.

Buy for your largest lens

Screw-on filters must match a lens thread, shown on the lens front in mm. Buy for your largest thread and use step-up rings for smaller lenses. It costs less than a filter for each lens. Common sizes include 67, 72, 77 and 82mm.

The combination filter

Some filters combine a variable ND with a polariser. A polariser cuts glare from water and windows, which helps at Indian lakes and on glass-heavy streets, but it costs more and you lose the ability to separate the two effects.

Indian conditions

Store filters dry and away from humid air between shoots, because fungus starts with moisture. Wipe off fine dust before you screw a filter on, since grit can scratch the glass and bind in the thread.

What to avoid

Avoid the cheapest unbranded variable NDs, which can tint footage in ways that are hard to correct. Avoid a filter that is thick enough to vignette on your widest lens. If you shoot with an action camera, you need a filter made for that model.

Before you buy

Bring the lens, or at least the thread size written on it. Ask to try the filter on your widest lens at the strongest setting and look at the corners on the screen. Ask for a GST invoice.