In these days of digital post-production software like Adobe Photoshop, is it really necessary to use filters?  After all, I can always add that effect in the computer later, right?  Well, the answer to that question is: On-camera filters are indispensable! First of all, a better image accomplished in-camera will always provide more latitude in the post-production digital darkroom. In this article I want to discuss just neutral density filters, in particular Graduated Neutral Density (GNDs) filters. That’s not to say that other filters like polarizers aren’t important--they certainly are, but that’s a subject for another day. The main purpose of a graduated neutral density filter is to control the dynamic range of light in the scene.  Ideally, in just one exposure, although a GND filter is still quite useful in bracketed shots, even those used in HDR imaging.  Whether capturing an image on film or on a digital sensor, photography is all about recording light. If the range of light is too great, and the highlights are blown out loosing detail, how can applying a digital filter to darken the sky (e.g., using Photoshop) going to bring back detail that was never recorded?  Getting the best image we can at time of shutter release will go a long way to getting the best possible results. Graduated Neutral Density filters come in a variety of sizes and densities (opaqueness), and are typically capable of controlling anywhere between 1 and 3 stops of light. Their light controlling ability is usually expressed as ND 0.3, 0.6, or 0.9, which translates as 1, 2, or 3 stops of light loss respectively.  GNDs come in a Soft or Hard Edge transitions of density to fully transparent.  The line of transition from dark to clear is placed over the horizon, thus darkening the sky and brining it into an acceptable dynamic range for the scene. The sky is often as much as 2 or more f-stops brighter than the foreground, so I prefer the 0.6 density with a soft edge.  However, it’s best to carry a variety. To get the best image possible in a given situation, you may need to go lighter or darker, or even stack more than one filter together.  Although GNDs come in the screw-in variety, its best to avoid these as you have no control of placement within the composition.  That is, the horizon would always have to be placed in the center of the image, which is usually not desirable. I prefer the 4x6 inch square filters that fit in a holder which screws onto the len's filter threads.  These holders allow you to move the filter up and down, as well as rotate, allowing you complete control of placement.  The larger filter size also allows you the flexibility to just hand hold the filter in front of the lens for quick and temporary use. Here is an example:

 Both pictures above were taken at dawn less than a minute apart.  Because the dynamic range of light was actually close to that of the foreground a GND filter would not normally be required. However, as you can see, the picture on the right was improved by the use of a soft edge 0.6 GND filter. Similar results could likely have been obtained in post editing since the dynamic range was not a problem here. Thirty minutes later and detail in the sky would have been lost altogether without the filter.

Besides using Graduated Neutral Density filters, I also frequently pull out one of my solid neutral density filters.  Although I generally prefer the large square type filters, when it comes to a solid ND filter, I'm partial to the screw-on 'fader' types like Singh-Ray's Vari-ND.  These filters allow you to dial-in the desired amount of density by rotating the outer ring of the filter, anywhere from 2 f-stops to as much as 8 or 10 f-stops.  The main reason I like to use a solid ND filter, is so I can slow down the shutter speed enough to let moving water blur.  There are times of course when freezing water action is desirable, but most of the time I prefer to let the water take on a silky smooth appearance, which I can only get with a slower shutter speed.  A variable ND filter gives me the ability to control the amount of light reaching the sensor and thus slow the shutter more than I could otherwise.