A neutral density filter is pretty simple. All it does is darken the image by a certain number of stops. For instance, a 10 stop neutral density filter drops the amount of light entering the lens by 10 full stops. The reason it is called a neutral density filter is that it is not supposed to do anything else to the image other than reducing the brightness of the light. In other words it is supposed to be totally neutral with no warming, cooling, or other effects to color. In practice, almost all ND filters affect the colors of things likes skies in varying ways.
Neutral density filters are commonly supplied as threaded filters that screw onto the front of your lens. They are manufactured to specific common 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.
Why use a neutral density filter?
The reason you use an ND filter is to reduce the amount of light that reaches the sensor. Here are a few examples where that can be beneficial.
- A portrait photographer might use something like a 3 stop ND filter to allow them to shoot at a wider aperture in bright sunlight.
- A landscape photographer might use a 10 stop ND filter to allow them to have a long exposure of water in bright day light.
- One might use a 6 stop ND filter so they can use a lower shutter speed with a wider aperture to blur the motion of people or cars moving around.
How do you correct the color aberrations of an ND filter?
It’s really simple, take two images of the same subject, one with the ND filter and one without. Make sure to properly expose them so the final images have the same brightness.
Then adjust your images to match in Photoshop or Lightroom and export the settings you used as a preset. Now when you open any future images taken with that ND filter, just apply the preset you made and voila, you have totally corrected colors. For a more exacting approach, you can also use something like the x-rite color checker.