Neutral Density Headaches
Neutral density filters allow for exposure control by blocking a percentage of the light entering the lens on a camera. But as useful as they are, nothing is perfect, and neutral density filters are certainly no exception. Indeed, they can be quite troublesome at times.
Outdoor lighting conditions can range from extremely bright in direct sun to quite dark indeed in those magic moments of twilight and beyond. Sometimes you can be faced with both extremes in the same scene with light falling over part of the frame but deep shade elsewhere. Cameras and photographers have long had a problem creating optimal images in mixed lighting such as this given the comparatively narrow range of exposures they can capture in a single frame. Even when the lighting isn't so variable and troublesome, setting the optimal aperture may be at cross purposes to the optimal shutter speed when both also influence more than just exposure. There's only so much you can do if you want both a wide aperture and a long shutter speed at the same time.
Mt. Saint Helens at sunrise. Guess where the transition line of the graduated neutral density filter was.
Modern digital cameras have improved things to a degree but the basic problem remains. Although we can now alter the camera's ISO sensitivity for each shot, there are limits. And while we can now digitally blend multiple shots made with progressively differing exposure settings, a simple gust of wind sufficient to cause something in the frame to move can make this strategy annoyingly complicated.
This is where neutral density filters come in. For the uninitiated, "density" means they darken the image. "Neutral" means they don't impart any color cast in doing so. Neutral density filters come in two basic types, solid and graduated. Solid filters cover the entire frame, blocking light equally in every part of an image, while graduated filters block light only on one side of the frame, and transition smoothly (gradually) to clear at the opposite side. Neutral density filters of both types also come in varying strengths, of degrees of density, to allow for just the right amount of light to still pass through for the desired exposure.
Or so the marketing materials for neutral density filters would like you to believe. In a perfect world, this all might be true, but as we all know, nothing is ever perfect.
To begin with, "neutral" is sometimes anything but. Neutral density filters that block only a small amount of light are relatively easy to manufacture, but as the strength increases so does the difficulty of making these things darken without also imparting some form of color cast. It's all too common for neutral density filters darker than a stop or two to add a magenta or perhaps brown tint that can potentially ruin an image. The only sure way to avoid color problems is to spend a lot of money on your neutral density filters. Singh-Ray is my preferred source for neural density filters, but their wares don't come cheap. The maxim that "you get what you pay for" applies to many things in the world of photography, so my advice is to not skimp when buying neutral density filters. Sure, a reasonably good neutral density filter that only blocks a stop or so of light can be had fairly cheaply, but then again you likely don't need such a thing these days when ISO can be adjusted so easily. But a good, strong filter will cost you. Many a photographer has become convinced that these things are simply worthless because they tried using only cheap neutral density filters.
If you're using a solid (non-graduated) neutral density filter though, you can often compensate for any color cast problems by adjusting the camera's white balance. But a graduated color cast can be quite troublesome to correct. If your white balance setting lends itself to a pleasing image on one side of the frame, the graduated nature of the imparted tint from the filter will leave you with an opposite cast on the other side of the frame. White balance affects the entire image, so if only part of the image needs correction, you have a problem. Photoshop and Lightroom both provide for graduated adjustments of their own that could potentially correct only what needs corrected, but making your adjustment in the digital darkroom match exactly the same graduation as your camera filter imparted can be a real headache. If you want to be able to use a graduated neutral density with enough strength to be useful, you're going to want to budget for a good one. Consider it an investment.
Then there's a whole host of problems associated with the graduation itself. The theory is that you can line it up along the horizon or some other natural edge within the image, but rarely does this work perfectly. A protruding tree or even a hill on the horizon can complicate things. Back in the film days, there were few good options other than to accept the fact that such trees or hills might end up somewhat oddly rendered in the final image. The bottom of the tree might look great, but above the graduation line the trunk would somehow get weirdly darker. The alternative of not using the filter at all was generally even worse with either a washed out sky or an overly dark foreground so we all learned to live with the issue. Thankfully today we have ways to address this problem.
First, there's a great deal of detail hidden in the shadows of digital captures from newer cameras. With a bit of patience and skill, a targeted adjustment brush can make such transition lines on trees and hills look much more natural. When digital cameras first hit the market, shadows filled with noise and were best left as shadows, but times have changed. It's remarkable how well you can selectively brighten up shadows to overcome problems such this. Surely, we'd all prefer not to have to spend time doing so, but at least we now can. Taking things further, you may find you don't even need a graduated neutral density filter if your camera yields sufficient shadow detail. Rather than brightening up just the tree trunks, do it to the entire foreground. There are still limits of course, but don't just assume you need a filter at all.
Another problem inherent in the use of neutral density filters is the simple fact that they darken the image, making it hard to see what you are doing to compose. A solid neutral density filter that blocks more than a couple of stops of light can make the viewfinder dark enough that you can't seem much of anything when you look through the lens. Just as your eyes will adjust upon entering a darkened room, matters will improve somewhat if you wait a bit. When need be, I've been known to throw my coat up over my head to block out stray light to allow my eyes to adjust better to just what is coming through the viewfinder. Doing so kind of makes me feel like an old-time large format shooter, but the technique works just as well for 35mm shooters.
A relatively new innovation in the world of neutral density filters is the advent of variable neutral density filters. These are essentially two polarizers stacked atop each other. By rotating the top half, the angle of polarization between the two can be varied, resulting in more or less light being cut. Turn the filter one way and everything goes black. Turn it back the other direction and the density decreases accordingly. This means you can carry just one filter rather than a whole range of neutral density filters with different densities. It also means you can compose with the filter rotated to the brightest view and then turn it back to the degree of density needed to allow the desired shutter speed or aperture. It's hard to convey just how handy this is in practice. Be aware though that variable neutral density filters aren't cheap. And with basically two filters stacked atop each other to create a variable neutral density, you have two layers that could impart a color cast if you try to save a few bucks. As I mentioned above, you do get what you may for.
With so many digital editing options available today, some photographers have dispensed entirely with neutral density filters, particularly the graduated variety. In my opinion, this is a mistake. While I do use them less and less as the years go by, I still find them to be indispensable tools, even with the inherent headaches that come along with their use.