An ND filter is a piece of glass or resin that is darkened to reduce the amount of light entering the lens. A simple way to think of an ND filter is like window tint for a car or truck. The premise is exactly the same, if you want to keep light out you use an ND filter to block a certain percentage of the light that would normally enter the lens.
ND filters come in different strengths, or densities. The amount of light an ND filter blocks out is referred to as its density. Some may block 50% of the light from entering the camera, some may block 99% of the light entering the camera. One key aspect of an ND filter is that they’re never 100% black. Every type of ND filter lets at least a little bit of light into the lens.
ND filter density is measured in 3 different ways; Optical Density, Lens Opening, and Stop Value.
This chart shows how those 3 different types of ND values correlate to one another. When shopping for ND filters some may be rated according to Optical Density, and some may be rated according to Actual Stops. To help you figure all that out, this chart will tell you what correlates to what at a glance.
|OPTICAL DENSITY||LENS OPENING||ACTUAL STOPS/EVs||USECASES|
|.3||ND2||1||Video or wider aperture.|
|.6||ND4||2||Video or wider aperture.|
|.9||ND8||3||Video, wider aperture, long exposure at dusk.|
|1.2||ND16||4||Video, wider aperture, long exposure around sunset.|
|1.5||ND32||5||Video, wider aperture, long exposure around sunset.|
|1.8||ND64||6||10-30 second exposure in evening.|
|2.1||ND128||7||10-30 second exposure in shade.|
|2.4||ND256||8||10-30 second exposure on bright cloudy day.|
|2.7||ND512||9||10-30 second exposure in sunlight.|
|3||ND1024||10||30 second to 1 minute exposure in sunlight.|
|3.6||ND4000||12||1-2 minute exposure in brightest sunlight.|
|4.8||ND65000||16||2-6 minute exposure in sunlight.|
Go ahead and add this page to your bookmarks to keep this chart handy for quick reference.
Neutral density filters are typically designed as threaded filters that screw onto the front of your lens. They are manufactured to standardized sizes that are used by lens manufacturers. You will need to check your lens to determine the size of the front threads on your lens, usually expressed in millimeters.
Some ND filters are made for filter holder systems. There are two main sizes for these types of filters, 100mm systems and 150mm systems. Filter holder systems typically screw onto the front filter threads or attach to the lens via a custom designed harness. The benefit for this type of system is that only 1 filter needs to be purchased to fit on a variety of lenses. This makes these systems somewhat cheaper to use and can reduce the amount of gear required if you want the ability to use a filter on every lens.
True ND filters are not variable filters. A true ND filter absorbs light equally across the scene no matter what angle it is at. Variable ND filters use a polarization effect to block light from entering the lens. Because of this effect variable ND filters often have color distortions and variations in brightness across the image frame.
The “neutral” in neutral density is what makes neutral density filters special. In theory a perfect ND filter would reduce the amount of light evenly across the full spectrum of visible light frequencies. In reality this can be difficult to achieve which means that different ND filters usually have some color cast in the final image. A little bit of color cast is usually not a problem as it is fairly easy to fix in a program such as Lightroom.
How and why do photographers use ND filters? There are only a few technical reasons to use them, and a few creative reasons as well. For photography there honestly aren’t a lot of 100% necessary reasons to use ND filters. The main technical reason is to adjust the light when the scene or subject are extremely bright. Think about photographing the sun, it’s extremely intense light and it can also melt sensor or film. This is particularly true for mirrorless cameras that use the sensor all the time the camera is on. For photographing the sun an ND filter is necessary as it will protect your camera gear from damage. A 12 to 16 stop ND filter is recommended for directly photographing the sun.
Creatively, ND filters can be used to lower the shutter speed to blur anything that is moving in the scene. ND filters can also be used to allow photographers to use a fast prime lens wide open in bright daylight. Even with an f/2.8 lens, you may find once in awhile that the scene is too bright to get the proper exposure. This can be especially true if your camera’s shutter speed is limited to 1/4000th of a second or less.
Another use of ND filters occurs when using strobes in outdoor light. While this is a pretty rare situation for the average photographer to run into, professionals working on commercial shoots will need to know how to use this technique. The problem an ND filter solves when using a flash outdoors is that of flash sync speed. Most cameras have a maximum flash sync in the 1/200 – 1/250th of a second range. In bright sunlight a shutter speed of 1/250th will be far too slow relative to the amount of light. In this case an ND filter can be placed on the lens to lower the exposure. Then the picture can be taken, with properly synced strobes, and correct exposure, at 1/250th of a second. This method is also vastly superior to using HSS because HSS robs the flash of power and reduces the ability of the flash to stop motion. Of course, you will need powerful strobes to work in outdoor light, probably in the range of 600-800 watts of power. Don’t worry about your subject moving either, the short duration of the strobe will stop the motion of your subject better than the shutter will.
Much of photography gear and technology centers around increasing the amount of light in our images, from fast primes and zooms to advanced sensors with high sensitivity to light. As crazy as it seems, sometimes photographers have reason to lower the amount of light entering the lens, and for that there are ND filters.