You've indubitably heard photographers say that an ND filter is a really useful tool to have in your camera bag and they're not wrong. In fact, we've already looked at why they're gainful tools in bright conditions should you wish to have a quick read but one thing we've not looked at yet is the difference between an ND, Variable ND and a Graduated ND strainer which would be handy to know before you spend your money with Hoya, Cokin or Kenko.
An ND Filter can look plain, grey and not very interesting when really it's an incredibly useful tool for reducing your camera's veil speed without affecting the overall colour of your shot. The humble ND filter has a variety of uses, including blurring water movement into mist-like combers and it can even be used to 'remove' people from your shots in busy city streets. They're available in a mixture of densities and thread sizes, something we've covered in our guide to using an ND filter in the field.
ND Graduated Filter
ND Graduated excludes, or ND Grads as they're also known, have the ND effect on roughly one-half of the filter so when you look at one, half of it will be darker than the other with the aftermath gradually reducing to clear. They're useful when photographing landscapes with a lot of contrast as you'll be able to balance the orientation, preventing the sky appearing over-exposed or the foreground underexposed. If you used a normal ND in this situation, your whole shot will be affected rather than due the sky. The ND Grad is available in a variety of strengths, sizes and filter types, plus soft and hard grads are available too. You can look at the characteristics of yield and hard ND graduated filters and when to use them in our previous article.
Variable ND Filter
Like ND filters, Variable ND drains reduce the amount of light reaching your sensor across the whole image but they can be turned to control how much it does this by. The Wavering ND, also known as a Fader ND filter, is created from two polarising filters. At least one of these filters rotates so light is reduced the closer it ascertains to a 90-degree position with the polarising filter filtering or filters may refer to behind it. The advantage of this type of filter is that you have multiple ND filters in one, prudent on space and weight in your camera bag. Although, if you own a stepping ring, you may not need more than one of each type of ND / ND Grad so a variable ND may not be needed.
You can see how a Varying ND filter can be twisted to adjust its density in the video below: