Most Important Filter?

If you had asked me a few months ago what the most important filter I own is, I would have probably said my circular polarizer.  I love the way they make skies blue and they're great for cutting down reflections.  I was so obsessed with them that I've easily spent over $1,000 on them.  It sounds extreme, but multicoated 72mm and 77mm filters from Heliopan or B+W are easily $160-200+ each.  

More recently however, I've discovered the genius of the graduated neutral density (Grad ND).  The advantage of the grad ND is the ability to get more dynamic range out of your photos.  For someone who hates processing photos digitally like I do, they save an incredible amount of time.  There's no need to try to recover highlights in RAW, use artificial "fill light" to recover shadow data, or worst of all, use some gimmicky HDR software.  I'd rather take one good shot than bracket 5 different exposures so I can sit in front of my computer and construct an unnatural scene.  

Grad NDs come in a variety of shapes and sizes and also differ by how much light the dark part of the filter allows to pass through.  Generally, you will see them marked 0.3 through 1.2.  A 0.3 grad ND reduces the amount of light by 1-stop, a 0.6 by 2-stops, a 0.9 by 3-stops, and a 1.2 by 4-stops.  A lot of companies make 0.3 and 0.6 grad NDs but the only one I've found with 0.9s and 1.2s is a British company called Formatt (available at B&H).  I own one of their 1.2 grad NDs and it is excellent.  However, it will vignette at the widest focal lengths on something like a 17-35 or a 24-70.  Grad NDs also come with soft and hard edges, referring to how gradual the transition is between the tinted and clear parts of the filter.  A hard edge is simply that; it appears to have a sharp cutoff.  Generally, I prefer soft-edges as the change is less noticeable in the final photo.  

Using a grad ND is fairly simple.  Ideally, you'd have one 0.3, one 0.6, one 0.9, and one 1.2, but the 0.9 and the 1.2 are really only for extreme cases.  To properly meter, fill the frame with the darkest part of the photo and meter, then fill the frame with the brightest spot and meter.  The difference between the two will allow you to determine what strength of grad ND you need.  Then, just put the camera in manual, put the filter on (I like the screw on ones for their ease of use), and set the exposure to the value you obtained for the dark part of the photo.  If you're shooting in rapidly changing light conditions, it is possible to meter with the filter on and it will give you more dynamic range, just not as much as it would if you have more time to compose.            

In the end, you'll have photos that appear the way you saw them with your eye, properly exposed everywhere.  And when everyone wonders how many hours it took you to manually blend the shot in Photoshop or use some cheap HDR program, you can think back to the extra 20 seconds you spent screwing on a piece of glass.