What's a Neutral Density Filter?

Simply put, a neutral density (ND) filter reduces the amount of light entering your digital SLR lens.

You can choose how much the light is reduced by getting a 2x, 4x or even 6x ND filter.

As the number gets higher, the filter becomes darker and less light enters the lens.

What's the Point?

Hold on a sec.

Isn't photography all about getting more light into the camera?

There are a large number of lenses with wide maximum apertures (that let in more light) and many digital SLRs can be set to high ISOs (increasing the camera's sensitivity to light).

Both of these features would indicate that more light is better.

It is, unless you really enjoy using slow shutter speeds.

When you use a slow shutter speed, it blurs any objects in the photo that are in motion.

Slow shutter speeds are commonly used to turn cascading water into a smooth sheet, and to show motion in crowded spaces.

Let's say that you're taking a photo of Niagara falls in the middle of the day, and want to use a slow shutter speed to blur the water.

If it's a bright sunny day, you won't be able to.

There is a lower limit on shutter speed in the middle of the day - you can only slow it down so much before the photo becomes over-exposed.

If you put a neutral density filter on your lens (and reduce the amount of light) then you can slow the shutter speed down enough to get the right effect.

The more available light, the more powerful ND filter you must use.

The Catch

If you don't already own a tripod, you'll have to get one to use with your ND filter.

Since the point of this filter is to reduce light and slow down shutter speeds, your camera needs a stable base of support.

Without a tripod you won't just get blurred water: your entire photo will turn out blurry.

Graduated Neutral Density

A graduated ND helps capture shadow and light

A graduated neutral density filter is specialized, but is useful when it comes to digital SLR cameras.

Here's why: your eyes have incredible dynamic range.

If you standing on a rocky shore watching the sun go down, your eyes are able to see detail in both the shadows and the highlights.

You digital SLR camera can't - it has a limited range from shadow to light.

If you want to take a photo of the scene in front of you, you have to make a choice. If you correctly expose for the bright sunlight, anything in shade is going to be under-exposed .