Long Exposure Photography: Neutral Density Filters
One problem encountered when experimenting with long exposure photography is having too much light. You can’t get the aesthetic effect of those long shutter speeds without over-exposing the image. If it’s relatively dark – like dawn or dusk – that’s not much of a problem. But what if you want to express the illusion of time when it’s bright and sunny? Neutral density filters are the answer.
Neutral density (ND) filters reduce the amount of light entering the camera, allowing for longer exposure times than would be possible without them. The key is that they reduce light uniformly, so contrast and dynamic range are not affected – unlike a graduated neutral density (GND) filter. The primary purpose of ND filters is blocking light from reaching the image sensor.
ND filters are nothing more than darkened glass placed in front of the lens to absorb a percentage of the incoming light. They are available in different “strengths” usually designated by either the number of stops it slows down the exposure or in terms of optical density strength (see the ND filter strength conversion table below). A 3-stop or 0.9 density ND is ideal for waterfalls in bright sunlight, slowing the exposure to a second or so, depending on the f-stop and ISO used. A strong 10-stop or 3.0 ND filter can blur clouds over several minutes, even on a bright sunny day.
Neutral Density Filters Strength
The image below, Iguazu Falls on the border of Brazil and Argentina, would not have been possible without the help of a neutral density filter. Here is the exposure data from this image: 4.0 seconds, f/22, ISO 100, 6-stop ND filter. The correct exposure data without the ND filter would have been the same except for the shutter speed, which would have been 1/15 of a second (6 stops faster). As you can see, I am already at the smallest f-stop (f/22) and the lowest ISO (100) possible for my camera and lens.
There’s nothing else I can do in camera to slow things down to achieve the desired effect. Without the ND filter, the correct exposure would have been 1/8 second so a 4-second exposure would have badly overexposed the image. There is simply no way to create the smooth, silky water I desired under those bright, sunny skies without the ND filter absorbing some of the light that was reaching my camera’s sensor. Unless, of course, I wished to wait for less intense light once the sun went down or a cloud passed overhead. But then there’s that pretty little rainbow I would have missed.
So you see, neutral density filters may not exactly be essential, but they will certainly help achieve longer exposures and help you get shots that you wouldn’t have taken otherwise.
Circular or Square Neutral Density Filters?
When it comes to neutral density filters, you have two choices: circular screw-in filters or square/rectangle slide filters. Each have their advantages and disadvantages.
Circular ND filters screw into the front element of your lens. Since you probably have several different lenses, each with a different size front element, you should also own a set of step-up rings for each lens rather than buying a separate filter for each size. Get one ND filter for the lens with the largest front element diameter (77mm for example) and step-up rings for the smaller sizes (52mm or 67mm, just to name a few).
The circular screw-in filters are convenient to carry around and store in your camera bag. They are also more durable and difficult to break. But stacking filters for more ND strength or adding a polarizer can darken or vignette the image corners. Singh-Ray makes a circular screw-in Vari-ND that allows you to adjust the strength of the filter’s density (1 to 8 stops as mentioned earlier) as well as a model with a built-in polarizing filter. But I have found these filters difficult to use and the filters are so thick that they vignette when used with wide-angle lenses.
Square or Rectangle filters (above) are glass or resin slides that fit onto the front of your lens with an adapter ring and filter holder. These filter systems come in different sizes. They usually do not vignette with wide-angle lenses, even when with filters stacked together. They are much more cumbersome to carry around and store in your camera bag, however, when compared to the circular variety. You should always weigh the options of both and decide which is best for you.
What Do I Use?
Over the course of my professional photography career, I have tried just about every type of neutral density filter on the market and I’ve settled on the filters made by Kase Filters (All links below to Amazon).
Kase Wolverine Shockproof 100mm ND8 3-Stop Neutral Density Filter
Kase Wolverine Shockproof 100mm ND64 6-Stop Neutral Density Filter
Kase Wolverine Shockproof 100mm ND1000 10-Stop Neutral Density Filter
Kase Wolverine Shockproof 100mm ND64000 16-Stop Neutral Density Filter