Are variable NDs better?
Q
What are the advantages of a variable ND filter as opposed to a fixedstrength ND?
Dan Redford
A
In a nutshell, the advantage is in its flexibility and how quickly you can alter the strength of the filter in reaction to changing light. An ND – or Neutral Density filter – has one job, and that’s to reduce the amount of light passing through the lens and hitting the sensor. It has no other effect.
A fixed-strength ND holds back light at one density, so if light intensity strengthens and you want to maintain the exposure, you’d need to swap out one ND for a stronger one. With a variable ND, depending on its density range, this is done with a twist of the filter, because a variable ND consists of two circular layers of polarised glass that are in opposition. When the filter is rotated, the layers hold back more or less light, depending on what’s required.
If you don’t think speed is an issue, a good fixed-strength ND will have less impact on image quality, but you may need several filters and a holder to mount them over the lens, unless you intend to hand-hold them. That said, a good variable ND should have little or no impact on image quality.
While a variable ND is useful for photography, as it allows you to control light intensity for creative reasons, it’s essential if you want to use your camera for shooting video. When doing this, you want to maintain your exposure at a constant shutter speed, so a variable ND allows you do this easily without having to change aperture as light levels rise and fall, as they always tend to do.