I was enthralled with long exposure photography long before I took up the practice myself. There’s something about seeing movement represented in a still image that I’ve always found compelling. In everyday life, we observe stillness in more or less the same way we do in a photo. Movement, however, is represented differently in photography than we see it in everyday life, and long exposure photography allows us to explore and study movement in a unique, abstract way.
In the months after I purchased my first DSLR, I quickly realized there was more to taking a long exposure photograph than simply setting the exposure and tripping the shutter. Sure, you can stop down to f/22, and turn your ISO to 100, but unless it’s really, really dark, you’re going to have a hard time reaching shutter speeds long enough to achieve a great deal of movement represented in your photo. Plus, when a lens is stopped down to its minimum aperture, or set within a stop or two of that value, it can often suffer from diffraction, which means that lenses often don’t yield their best image quality at very small apertures. Anyways, after a few frustrating attempts, and a whole lot of online research, I purchased my first six-stop neutral density filter before a trip to California a few years back. There was a bit of a learning curve, but by the end of the trip I got comfortable enough (and lucky enough) to capture the photo at the very top of the page, which shows a motorcycle whizzing through Joshua Tree National Park in bright moonlight. I’ve been hooked ever since.
Neutral density filters reduce all of the light coming into the camera evenly, allowing for longer shutter speeds . If you’re wondering whether this will cause any color shifts, this largely depends on the quality of filter you’re using, with higher quality ones being less susceptible to color shifts when compared to “budget” filters. It’s also important to differentiate between color shifts with film vs. digital sensors, as you’re more likely to see color shifts with color negative or color reversal film than with digital sensors. The photo below, shot across from Grand Central Terminal, was a 30-second exposure taken with a 10-stop ND filter, on a tripod. This long exposure time allowed me to record the colorful light streaks of the moving traffic, while use of the tripod helped to ensure that the still elements of the scene, like the buildings, remained sharp.
In long exposure photography, and by “long exposure” I mean photography that uses a shutter speed longer than a second or so, it is absolutely imperative to use a tripod. If you want to get anything close to resembling a sharp image when shooting at slow shutter speeds, the camera must be held stable, and the best way to do that is to use a tripod. Aside from using a tripod, other tips for getting a sharp image while taking long exposures are engage mirror lockup (for cameras with a mirror), use the self-timer, and use a physical or wireless remote trigger.
Like traffic lights, water is another common moving element in landscape photography. In the black-and-white image below, I was able to achieve the serene yet defined look of the water with a shutter speed of 8 seconds using a six-stop ND filter. As you may have guessed by now, the strength of a neutral density filter is measured in “stops of light.” Each stop cuts the amount of light being let into the camera by half, allowing you to slow your shutter speed without overexposing the shot. Had this image been shot without a six-stop ND filter, the shutter speed would’ve been 1/8 sec.
So, how do I decide how many stops of ND I need to take a great long exposure of a given scene? Well, it depends. It depends on the amount of natural light available, and it depends on the look I’m going for.
In brighter scenes, you may be able to use your camera’s internal meter through your various NDs to find the correct filter to fit your desired exposure time. But in darker scenes, or when using stronger ND filters, there is often too little light entering the camera for it to meter properly. In cases like this, you’re better off metering the proper exposure with the ND filter detached from the lens, and using one of the many great long exposure calculators available in the Apple App Store or Google Play Store to determine the best filter in your arsenal to achieve your desired shutter speed. The more you practice with ND filters, and tools like long exposure calculators, the more you’ll develop a “feel” for the appropriate filter and shutter speed to use for a particular scene.
There are two main varieties of ND filters that we can attach to our lenses, circular and square. Circular-type ND filters screw onto the front of your lens, and tend to be quicker and easier to put on and take off than square filters. Square filters are more versatile, and can be stacked on top of one another, but require the use of a filter holder, which can be a bit cumbersome versus the self-sufficient circular filters.
In terms of variety, there are a few different kinds of filters. First, there are solid ND filters, where the filter factor is fixed and cannot be adjusted. But if you need some flexibility and don’t want to carry several ND filters with you, there are variable ND filters, where the density of the filter may be adjusted using rotating rings to block out more or less light.
Also available are graduated ND filters, which are typically used to reduce the exposure in the brighter parts of a shot. One half of the filter is clear, while the other half is of neutral density, and these two halves blend together, either gradually or abruptly. Lastly, center ND filters are graduated ND filters that are slightly opaque in the center and clear at the edges. These may be used for special effects or to compensate for light falloff that is seen with large optics.
I hope you enjoyed this primer on neutral density filters. Please leave your questions in the Comments section, below, and I’ll be back to answer them as soon as I can.
