ND Filter Chart: Stops, Density and ND Numbers

Three ways to name one filter

A three-stop neutral density filter can be sold as ND 0.9, ND8 or “3-stop ND”. Cinema filters and camera menus usually give the optical density (0.3, 0.6, 0.9…); stills filters usually give the filter factor (ND2, ND4, ND8…), the number of times the exposure has to be increased. Both describe the same thing, and both convert to stops, which is the number you actually work with.

Density, transmission and stops

ND filter chart

The figures in brackets are the exact stops. They drift below the round numbers by about 0.003 stop per 0.3 of density, which is still only 0.03 stop at ND 3.0. Stills filters at the dense end are often named by rounded factor (ND1000, sometimes ND1024), and both are ten stops for practical purposes.

Half and third stops

A third of a stop is a density of 0.1 (exactly 0.10 ÷ 0.301 = 0.33 stop), and half a stop is 0.15. Cameras and meters work in thirds, so a reading that is “f/8 and a third” against a target of f/8 needs ND 0.1, or a third of a stop on the iris. Most filter sets step in 0.3, so the last third usually comes off the iris, the shutter or the EI.

How much ND you need

Meter at the camera's EI and exposure time, then compare the stop the meter gives with the stop you want to shoot at. Two f-numbers are 2 × log₂(N₁ ÷ N₂) stops apart, and each stop is 0.3 of density.

Stacking filters

Densities add and filter factors multiply. ND 0.9 and ND 0.6 together are ND 1.5: three stops plus two, five stops in all, or ND8 × ND4 = ND32. That is why density is easier on set: add the numbers and divide by 0.3.

Every extra piece of glass is two more surfaces for flare, reflections and dust. Stack as few filters as you can, and when the camera has internal ND, use it for the bulk and keep a single external filter for the remainder.

Internal ND and the matte box

Built-in ND saves filter changes, but the steps are coarse on many cameras. The ALEXA 35's motorized filter has four positions: clear, 0.6, 1.2 and 1.8, so two, four and six stops. A three-stop requirement becomes internal 0.6 plus ND 0.3 in the matte box; seven stops becomes 1.8 plus 0.3.

Matte-box filters come in rectangular sizes such as 4 × 5.65 in and 6.6 × 6.6 in. ARRI's FSND set, for example, runs from 0.3 to 2.4 in 0.3 steps in both sizes, so any whole number of stops up to eight is one filter.

IR, variable ND and color

• Variable ND is two polarizers turned against each other. It is quick for run-and-gun work, but at the strong end of its range it can darken unevenly across the frame, especially on wide lenses, and the markings are approximate. Check it against a fixed filter if the exposure has to match.
• Color shift varies by filter and by batch. Use one maker's set across a scene, and white balance with the filter in.
• Graduated ND darkens only part of the frame, usually the sky. It is rated by the density of its dark part, so a 0.6 grad takes two stops off the top of the frame.

ND in CineMath

The Exposure calculator turns any change in stops into an ND filter, using density = stops × 0.3, and gives the marking in both systems: two stops more light comes out as “Add ND 0.6 (ND4)”. It covers ND 0.3 (ND2) to ND 3.0 (ND1024). A change that is not a whole number of stops shows the exact density and the nearest filter, such as “ND 0.45 (nearest ND 0.6 · ND4)” for a stop and a half. The same screen shows the T-stop, shutter angle and ISO alternatives. The free exposure calculator gives the same ND labels in your browser.

Next steps

• Find the ND for any change in stops, thirds included, with the free exposure calculator.
• See how ND fits with the iris, shutter and EI in the exposure triangle guide.
• Start from a stills meter's EV reading with the EV chart.
• Check what the stop you are holding does to focus with the Depth of Field calculator.

Next time the light changes mid-setup, CineMath's free Exposure calculator gives the ND to add or pull before the AC has finished asking.