Want to shoot wide open in bright conditions? Then you need a good ND filter. Here's how it works.
What is an ND Filter?
The ND Filter is used to block a certain amount of light from your camera, allowing you to shoot longer exposures or at a wider aperture than you would otherwise be able to. ND Filters are also called grey filters, because that's how they work — they put a certain amount of grey in front of your lens, darkening your image.
ND Filters are widely used in filmmaking, primarily because you can stay at 24 frames per second at a wide aperture like 2 or 2.8, even though it's in the middle of the day.
There are ND filters available in different strengths — check out the ND Filter Chart below — and different sizes, each corresponding to their respective workflow. Popular ND filters are circular filters that you screw on to your lens and square or rectangular filters that you put in your matte box filter holder.
There are also Variable ND Filters, which allows you to step through different strengths of ND filtration.
ND Filter Chart
This table shows you each ND Factor, ND Density and the stops of light each specification blocks.
| ND Factor | Absorbance | Stops |
|---|---|---|
| ND2 | 0.3 | 1 |
| ND4 | 0.6 | 2 |
| ND8 | 0.9 | 3 |
| ND16 | 1.2 | 4 |
| ND32 | 1.5 | 5 |
| ND64 | 1.8 | 6 |
| ND128 | 2.1 | 7 |
| ND256 | 2.4 | 8 |
| ND512 | 2.7 | 9 |
| ND1024 | 3.0 | 10 |
This means that a ND filter of the ND Factor of ND128 equals a filter of the optical absorbance 2.1, which gives you a 7 stop decrease in light.
How much ND do you need?
Most people I talk to tend to understand the basics of ND filters, but they always have to ask how strong ND filtration they need in order to get the right exposure.
Below is a complete table with all the corresponding F-stops that quickly shows you how many stops you need to block in order to get the right exposure.
| F-Stop | 1 | 2 | 3 | 4 | 5 | 6 | 7 |
|---|---|---|---|---|---|---|---|
| 22 | 16 | 11 | 8 | 5.6 | 4 | 2.8 | 2 |
| 16 | 11 | 8 | 5.6 | 4 | 2.8 | 2 | 1.4 |
| 11 | 8 | 5.6 | 4 | 2.8 | 2 | 1.4 | 1 |
| 8 | 5.6 | 4 | 2.8 | 2 | 1.4 | 1 | - |
| 5.6 | 4 | 2.8 | 2 | 1.4 | 1 | - | - |
| 4 | 2.8 | 2 | 1.4 | 1 | - | - | - |
| 2.8 | 2 | 1.4 | 1 | - | - | - | - |
| 2 | 1.4 | 1 | - | - | - | - | - |
| 1.4 | 1 | - | - | - | - | - | - |
This means that if you are in a situation where you need to stop down your lens to f/16 in order to get the correct exposure — like on a bright, sunny day — and you want to shoot at f/2, you need a ND filter capable of blocking 6 stops of light.
And, as we can see in the ND Filter Chart, a 6 stop ND filter rates as ND64 (ND Factor) or 1.8 (Optical Absorbance). Even if stores have a tendency to label these filters in different ways, this means that you can purchase a ND64 or a 1.8 ND filter and you'll be good to go; they feature the same ND strength.
Best ND Filters
So which ND filter should you buy?
Best Circular ND Filters
The best circular ND filters to screw on your lens is this filter.
Best Square ND Filters
The best square ND filter to fit in your matte box is this filter.
Best Rectangular ND Filters
The best rectangular ND filter — also known as a 4x5.65" filter — is this filter.
Best Variable ND Filter
The best variable ND filter available is this filter.