Soft Edged or Hard Edged?
As long as we're on the subject of graduated neutral density filters, another question that comes up often is whether to use soft-edged or hard-edged grads. Well, as with many things, it depends — in this case, on several different things.
For those unfamiliar with the difference, Singh-Ray and most other makers of graduated ND filters generally produce them both with a fairly abrupt or "hard" transition from clear to filtered, as well as a smoother, more gradual or "soft" transition. The most obvious reason to use one over the other is that not all subject matter is the same, and different subjects may call for different filters. If you are shooting towards a prominent, even horizon line, a hard-edged graduated filter is generally preferable. If instead the scene you are photographing lacks a good edge that you can line the filter transition up with, a soft-edged grad may be a better bet since you can more easily conceal the edge among the uneven terrain.
But there are other considerations. For instance, depth of field will affect how clearly you see the transition, making whichever edge you choose appear softer than it actually is. With a wide-open enough aperture or a long enough focal length telephoto, a hard-edged graduated filter will come out looking like a soft-edged filter, and a soft-edged grad will blur to the extent that the transition may fill the entire frame and become useless. I generally find myself using graduated ND filters more often with wide angle lenses stopped down to encompass a vast landscape. When you think about it, this makes sense. The wider the angle of view a lens sees, the more likely it is that its view will encompass changes of brightness sufficient to need filtering. With a narrow enough telephoto, you will be carving out such a small slice of whatever is in front of you that any changes of brightness will be more likely to be outside your view. From mountain peek to deep valley shadow, there will still be just as much of a brightness gradient, but you will only see a small slice of it so its full sweep doesn't matter. But if you do find yourself selecting a narrow slice that still has extremes of both bright and dark, a hard-edged filter will likely fit your needs better than a soft. With a long enough focal length though, you will reach the point where using a graduated ND filter just won't work at all. Even the hardest edged filter available will only serve to generally darken the entire frame. The narrow angle of view coupled with the shallow depth of field will mean the transition will be completely blurred beyond the point of being useful.
Speaking of angle of view, new digital shooters should stop to consider the effect their sensor size will have. If you use a digital body with a smaller than full frame sensor such as Nikon's DX system, the crop factor will change what your lens sees relative to a film body, and thus will make the transition of a GND appear softer than you may be accustomed to. Once you get used to things, you shouldn't have any problems but this difference may surprise some who are used to how grads work with film.
If you use a full frame lens on a DX body, the softening effect will be even more pronounced since you will only be looking through the central portion of the lens rather than the full surface area. If the filter transition covered an inch from clear to full density, this inch will be a greater percentage of the effective lens diameter. This added "DX effect" won't be an issue with an actual DX lens since the camera can then use the entire lens image circle rather than just the central portion of it.
By the way, "hard" and "soft" are relative terms and there is no universal standard for what these terms mean. not all manufacturers make their filters the same way. It's worth spending some time getting to know how yours affect a scene under various conditions and with various lenses so can be prepared to react quickly to changing lighting conditions common near sunrise and sunset. While you can see the effect of the filter through the viewfinder, the limited brightness range that can be captured photographically means your camera won't see the effect quite the same as your eyes do.