Of all the filters in my kit, the one that I pull out the most is the neutral-density filter. A neutral-density filter, or “ND filter”, reduces the amount of light entering the camera. It’s as simple as that. There aren’t any color corrections or emphasis; all wavelengths are reduced with equal intensity (hence the “neutral” descriptor). Similarly, there are no polarizing effects. It’s about the simplest no-frills filter you can own, but it’s incredibly useful, especially if you want fine control over your exposure in difficult lighting situations.
Using Neutral-Density Filters
ND filters are generally classified by the amount, or stops, of light they reduce. (If you’re not familiar with the concept of stops, just think of it as a halving or doubling of light from your current exposure). An ND2 filter will reduce your exposure value (EV) by two whole stops, an ND4 by four, and so on.
Say the light meter in your camera is giving you a proper exposure at an ISO 100, a shutter speed of 1/500, and an aperture of f/4. Applying an ND2 filter to your lens will halve the exposure value, and then halve it once more. So to gain those stops back, we need to adjust some or all of our settings in to compensate: We could set our shutter speed to 1/125. We could set our ISO to 400. We could open our aperture up to f/2.8 or 1.8. We could do some combination of all three.
Here’s a more practical example, one that I ran into just the other day: Say you’re out taking pictures of peonies on a bright, sunny day. You want to completely obscure the background and isolate your subject (as one does when photographing pretty flowers), so you need a shallow depth-of-field, and therefore a very large aperture. Your current lens goes to f/1.4, so that’s perfect. The only problem is… your camera is quite old, and the highest shutter speed you can get is 1/1000. Opening up to f1/1.4 on a sunny day would need a shutter speed that’s well beyond that, perhaps even up to 1/4000. Well, the simplest solution would be to slap an ND2 filter on the front, and we’re back down to 1/1000.
To explain further what happened there: We wanted to decrease our shutter speed by two whole stops. Halving the shutter speed from 1/4000 to 1/2000 halves the amount of light, so that’s one full stop. Halving it again gets us back to 1/1000, which is two full stops from the original speed. We got there with an ND2 filter. The “2” indicates that it reduces your exposure value by two full stops.
This stuff can be a little daunting, especially if you’re new to photography. In that case, I recommend checking out this great article on the “Exposure Triangle” by PetaPixel, which is a brilliant jumping-off point for understanding stops and exposure values in depth.
Types of Neutral-Density Filters
There are two primary types of ND filters you can get: fixed and variable.
Fixed ND filters are exactly what they sound like: they only provide one exposure value. They often come in “kits”, like this one, which has four filters ND2 to ND16*. The distinct advantage to this type is that they’re relatively inexpensive, and in addition, there are no moving parts (i.e. fewer things to go malfunction in the field). The downside, of course, is that there are more pieces to keep track of. Furthermore, if you are in rapidly changing lighting conditions, it can be a pain to be constantly swapping them.
Variable ND filters are, conveniently, also exactly what they sound like. They are a single screw-on filter that can be set to a given range of ND values by turning a ring on the front. This one, for example, claims to be able to achieve a range of ND2-400, which is quite a wide range indeed. Most of them, however, are ND2-8 or 2-16, and that’s all I really need. They are certainly more expensive, but I like them because it’s only one piece of kit that I have to keep track of (and I am predisposed to losing things, so that’s for the best).
*(Be sure you’re getting the proper size filter for your lens. The filter size of your lens will usually be printed on its front element and look something like ‘ø 67mm’. This tells us that the lens takes a 67mm screw-on filter.)
A Real-World Example
Here I’m set up to shoot some wave-action at sunset when the weather got something close to cooperative. Now, this is a fairly low-light situation as it is, but here’s the thing: I want to completely blur and flatten the waves and the surface of the water to get a dreamy, painterly effect as they crash over that driftwood. I decided that about a 4-second shutter speed would be sufficient.
Even at ISO 100 and stopped all the way down to f/22, there’s no way I’m getting that kind of shutter speed and maintaining proper exposure, especially as my camera is more or less pointed right into the setting sun. The fact that it was behind cloud cover still didn’t help.
Enter the handy, dandy ND8 filter. Throwing that on the front of the lens allowed me to, after some trial and error, get the shot as you see below. It’s more or less what I was going for, and I think it looks nice. But I’ll let you be the judge.