Neutral Density Filters: The Optical Option

April 01, 2012 —

Imaging software has lessened the need for on-camera filtration, but it simply can’t do the job of a bona fide neutral density filter. Here’s why.

For all its fancy algorithms, Photoshop has lulled photographers into a false sense of security. It has led a lot of us to believe that nearly any shooting error can be corrected—and any desired effect achieved—in the digital darkroom. True, Photoshop and its image-editing ilk can do much of what on-lens filters used to do, after the fact. You can add any color your eye desires, or apply softening for poetic or cosmetic purposes. And since RAW capture is in color by default, black-and-white photographers can let software sift channels to achieve tonal control at least as sophisticated as in-camera filtration used to give them.

Yet there are drawbacks to digital filtering. Some of these are subtle, such as the risk of increased grain and other artifacts when you base a black-and-white image largely or entirely on a single color channel. Some of them are obvious, such as the inability to preview a digital filter’s effect in-camera. And from an optical perspective, there are physical filters—whether glass or resin, metal or plastic—that simply defy Photoshop’s abilities altogether.

Neutral Density to the Rescue
The most obvious of these is the polarizing filter, which deconstructs light down to the very angle of its vibration. But there’s another kind of camera filter that imaging software simply can’t touch: the neutral density (ND) filter. Like the polarizing filter, the ND filter acts directly on the light entering the camera.

All it is, really, is a translucent filter that introduces colorless gray density into the light path—colorless, that is, if it’s manufactured properly. ND filters do come in a variety of densities, meaning the degree to which they reduce light transmission. These are indicated with a decimal system that represents a progression of “stops.” Each one-tenth of optical density is the equivalent of one-third stop of light blocking. For example, an 0.1 ND filter blocks one-third of a stop of light, an 0.3 blocks a full stop, and an 0.9 ND blocks three stops.

ND filters can be bought in circular, screw-in form, but these don’t come in nearly as great a range, nor in as many increments of density, as “system” filters: square or rectangular filters that slide into slots in a holder that mounts on the end of the lens. The holder usually comes in versions with one, two or three filter slots, and is attached to the lens with adapter rings that come in a variety of thread sizes. This means you can mount the same holder on different lenses, and use the same set of ND filters, simply by swapping out the rings. (ND filters come in graduated versions, but here we’ll focus on the no-less-useful solid NDs.)

All that said, not all system-based ND filters are created equal, as you’ll learn in the sidebar included here titled “Vision Quest” (page 100). In the past few years, issues of quality and availability have hampered and complicated their use. This makes it all the more important to understand why, and when, you need an ND filter in the first place.

Neutral Density for a Slower Shutter Speed
Imagine that you’re shooting in fairly bright light but want to set a slow shutter speed to blur the subject’s movement—maybe to create a more expressionistic photograph of a runner or a more naturalistic photograph of flowing water. To get the correct exposure in brighter light, you’d ordinarily have to set a faster shutter speed—probably too fast to achieve the blur you want. In order to slow the shutter speed down, you might first set your DSLR to as low an ISO as it can go. The reduced sensitivity to light would require a slower shutter speed for sufficient exposure.

But maybe not slow enough. The light could be so bright that even if you were to stop the lens all the way down (to f/16 or f/22), you’d still get an overexposed image at the slow shutter speed you want to use. You might also have an aesthetic issue with stopping all the way down—not wanting the increased near-to-far sharpness that such a small aperture would produce.
If you were to mount an ND filter on the lens, though, you’d solve this problem. The ND filter would evenly reduce the amount of light entering the camera and striking your image sensor. And this, in turn, would allow you to set as slow a shutter speed as your wished-for blur required, without having to stop the lens down needlessly. Indeed, with access to a full range of ND filters that can be swapped out on the fly, you can achieve—in virtually any light level—the exact amount of light reduction needed for the specific f-stop and shutter speed you want to set.

Neutral Density for a Wider Lens Aperture
Let’s say you want shallow depth of field in your photograph—necessary to produce a soft background in a portrait, for example. To get it you need to set a wide lens aperture. But if the existing light is fairly bright, it may be difficult to set a wide f-stop without causing overexposure. So again, you start by lowering your ISO. The lower ISO means more light is needed for a proper exposure, which might allow you to set that wider aperture.

Or not. The light may still be too bright to get the correct exposure at that wide lens aperture—in which case you’d probably find yourself using a progressively higher shutter speed to reduce the light entering the camera. Eventually you’d hit the wall, though. The ISO is as low as it can go, the shutter speed is as fast as it can go, and the image will still be overexposed at the wide f-stop you want to set.

Neutral density to the rescue: By reducing the light reaching your image sensor or film, an ND filter lets you obtain the correct exposure at a wider aperture. (This is why neutral density is also so valuable in DSLR video, though that’s a subject for another story!) In fact, with enough neutral density, you don’t even have to resort to such a high shutter speed—making it possible to achieve both subject blur and shallow depth of field in the same image. With that kind of creative freedom, how can photographers remain neutral on the value of neutral density filters?!

Vision Quest: A Good ND Filter is Hard to Find. Or is it?
Time are tough if you’re searching for neutral density filters that are optically excellent and available in a wide, incremental range. Screw-in glass ND filters really aren’t a good option unless you’re willing to settle for a small number of densities and, sometimes, a trade-off in quality. And now that holder-based Cokin is back in business, its P-series filters cost about 70 percent more than before. This, despite the fact that the resin used to make them continues to be outsourced, is a practice that can result in inconsistent quality. (Note that the same resin is used in Cokin’s Pro lines.)

Photographers looking for a high-quality alternative to Cokin usually turn to the resin-based Lee filter system. Despite their high price, Lee filters are in such demand that the company has been unable to keep up with its orders. It can take as long as half a year to get a Lee filter.

Fortunately, problems with quality and supply don’t plague the little-known Hitech filter line. Hitech is the only maker other than Cokin and Lee to offer a dedicated holder, and (contrary to some Internet blather) Hitech filters are on par in optical quality with Lee’s. Unlike Lee filters, though, if Hitech filters aren’t already in retailers’ stock they can be ordered to arrive within a week or two. And unlike Cokin filters, Hitech’s much-harder resin is produced entirely in-house, allowing a higher level of quality control.

Hitech’s 85mm-square filters fit the popular Cokin P holder, while its 100mm (4 x 4-inch) filters fit Lee’s holder. And the Hitech line has recently been re-priced: Its 100mm filters are now a third less expensive than the Lee equivalents, while its 85mm (Cokin P-sized) filters now cost 20 percent less than Cokin’s. If you’re starting a system from scratch, Hitech’s own anodized-aluminum holders have also dropped substantially in price, and the company has recently introduced a modular holder to compete with the more expensive Lee version.

The Hitech line also offers greater incremental precision than Lee in its ND range. While Lee offers ND filters in one- (0.3), two- (0.6), three- (0.9), and 10-stop (3.0) densities only, Hitech ND filters are available in every third-stop of density up to a full five stops, then in six-, seven-, eight-, nine- and 10-stop densities. As part of the new Hitech ProStop line, six- through 10-stop ND filters are even available in a special version with a thicker light gasket to fit the somewhat wider slots of the Lee holder. If you’re vested in the Lee system, you don’t have to wait in line for the filters you want.

For details on Hitech’s filter line, visit VisualDepartures.com.