HOW A NEUTRAL DENSITY FILTER WORKS

Top Reasons to Own a Neutral Density Filter

1. Blurring water motion in waterfalls, rivers, oceans, etc...
2. Reducing depth of field in bright daylight conditions
3. Adding motion blur to moving subjects: people, cars, planes
4. Extended exposure times

What is a neutral density filter?

Neutral Density (ND) filters prevent light from entering the camera in measured amounts. This allows the photographer more control in selecting shutter speed and aperture combinations in a variety of conditions.

Example 1: The first example is a typical wide angle landscape shot with the equivalent of a 24mm wide angle lens. When shooting landscapes, a low ISO setting is desirable to avoid noise. The aperture selection was f/11 in order to accomplish a depth of field where the foreground through the background is sharp and in focus. The final setting is the shutter speed. The shutter speed is pre-determined for this image based on the aperture and ISO selections. The corresponding shutter speed with these settings is 1/60th of a second:

In the resulting image, the clouds are not moving and the water motion seems distracting. The movement of the clouds and water is captured at a shutter speed that is not desirable. However, because both the clouds and water are moving, a slower shutter speed will blur the movement.

The following three images use the exact same settings as the first image, but fixed neutral density filters are used:

A Hoya Solas IRND 0.9 (3-stop) filter allowed the shutter speed to be reduced from 1/60th second to 1/8th second and the result is some sense of motion blur. Because the clouds and water are so far away, a longer exposure will blur the movement more.

A Hoya Solas IRND 1.8 (6-stop) is a denser filter, allowing the shutter speed selection to slow even further to 1 full second. The result is some texture in the water movement, and with a full Hoya Solas IRND 3.0 (10-stop), a shutter speed of 16 seconds blurs the movement enough to achieve the vision of the original frame:

Fixed and Variable ND filters

Fixed neutral density filters have coating(s) evenly distributed across the frame of the filter.

Variable neutral density filters are essentially two polarizers placed together with one plane "blocking" a fixed amount of light and the other rotating to change the amount of light "blocked". 

The downsides to most ND filters are unpredictable “color shift” and/or vignetting. 

Almost all ND filters have some sort of color shift. When an image is produced that has a green color cast, it is often the result of a poorly designed filter. On the other hand, when infrared (IR) light penetrates the camera sensor, the resulting image can have a magenta cast. 

Color shift is amplified by low-quality materials, poor manufacturing, and ignoring the infrared band of the light spectrum.

Quality manufacturing and design resolves these issues.

Vignetting is determined by the amount of light fall off toward the outer edges of your lens. The wider the lens you choose and the longer the exposure time you have, the more vignetting can occur. Poorly designed and manufactured filters will increase the amount of unwanted vignetting. A quality filter will either have no color shift at all, or, at the very least, have predictable results. 

Here are features to look for in deciding which ND filter is best for you: 

• Color Shift: Should be non-existent or at minimum predictable results
• Vignetting: A quality filter does not amplify vignetting from a lens.
• Durability: Filter is resistant to inevitable dings and knocks
• Simplicity: The system is easy and quick to set up and use on location and/or during a shoot

SOURCE: Hoya Filter USA