Essential Filters: Neutral Density and Graduated ND
Neutral Density filters come in two basic varieties: solid and graduated. Both function by simply cutting down the light that is transmitted to the film, just like someone turned off the lights. The key difference is between the two types is, of course, the graduation.
Solid neutral density filters typically come as traditional screw-on filters in a variety of sizes and strengths. Their main purpose is to allow longer exposures when you can't or don't want to stop the lens down any further. This is usually to afford longer exposure times when photographing moving water to get that silky effect, but can also be used as another means to control depth-of-field. Owners of digital cameras may find these filters especially handy if their slowest film speed is no better than ISO 200.
Like everything that has an impact on exposure, these filters are measured in stops, but manufacturers seem to have a hard time agreeing on how to label them. While some are politely labeled with the number of stops of light they block, others such as Tiffen and Lee use the filter grades 0.3, 0.6, and 0.9 for one, two and three stops. Then there's B+W and Hoya who prefer calling the same thing 2x, 4x, and 8x (known as the filter factor). In addition to these, I also have a B+W 64x which translates to a full 6 stops of neutral density. This is the ticket to great waterfall photos.
Graduated Neutral Density is my favorite category of filter. As I've said before, I'm not a big filter user, by my Grad ND's go everywhere with me. The idea with GND's is that one end is clear and the other is neutral density. In between the two ends, the filter transitions through either a soft-edged or a harder edged graduation which allows you minimize the likelihood that the transition will be visible within the picture frame. The aim is to position the filter within the image by sliding it up and down in order to equalize the brightness across the frame. The trick to getting the graduation in the right place is practice. Practice and the depth-of-field preview button that is. By stopping the lens down to your shooting aperture and then moving the filter around, it becomes much easier to see the edge of the graduation. Bracketing your compositions slightly with respect to how the filter is positioned in order to make sure you've gotten it in the right place can help if you are unsure. Don't pay any attention to where the graduation appears to be when you look at the filter sitting there on the end of your lens. It may seem like it covers a third of it, but if the lens is stopped down at all, it may not even cover any of the actual image; it is only what you can see through the stopped-down diaphragm that counts, not the entire lens diameter.
Graduated filters come in strengths of one, two, or three stops with transitions either soft-edged or hard-edged. The most useful are the two-stop soft-edged for general use, and the three-stop hard-edged for the extreme brightness differences when the sun is near the horizon. On occasion, I've been known to use more than one GND at the same time in situations with even wider exposure differences between foreground and background. Sizes for Graduated ND's vary also, but the standard format is 84mm wide, the size of the Cokin system holder. Cokin makes good holder, but avoid their filters as they aren't truly neutral in color.
Oh, and don't even bother with screw-on graduated filters.
A useful variation on the Graduated ND is the "Daryl Benson" Reverse Graduated ND marketed by Singh-Ray. This filter starts off clear at one end, then has a hard edged two or three-stop graduation in the middle that then tapers off to only one or two stops at the opposite edge. For shots right at sunrise or sunset, this can be just the ticket.
Metering with Grad ND's can be somewhat tricky. The easiest way I know of goes something like this: spot meter various portions of the frame in order to determine if you need a filter at all and if so how many stops it should block. If you need to cut down the brighter end of the frame (typically the sky and background) by two stops to get it in line with the darker end (typically the foreground), then choose a two-stop GND, and so forth. Adjust your exposure for the portion that will be covered by the clear end and would thus be unaffected. Before adding the filter, the brighter areas would therefore be overexposed. Finally, slide the filter into place (holding down the depth-of-field preview button of course) and you should be set.
When buying neutral density filters, quality matters. What you want is true neutral color, but many cheaper brands will impart a magenta or brown color to your image. The brands I use are B+W for solid ND's and Singh-Ray for Grads; they aren't cheap, but if well cared for will last a long time. Resin filters are cheaper but may not be perfectly flat and can scratch more easily than glass.
Update 02/21/2005 - I put the following image together for Nikonians, but felt like it would be a good idea to post here as well. This is my graduated neutral density filter collection. While it probably proves I have too many of these things, it should also give you an idea of how they differ.