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A variable neutral density (ND) filter is your best option when photographing near the sea, but not exactly for the reasons you might think. For my Atlantic Realm project, a series I’ve been photographing for some time covering a stretch of the Portuguese coast that is filled with legends of Atlantis, I’ve been, many times, photographing very close to the sea water, with all the problems you can imagine.
For my Atlantic Realm project I use to photograph in coastal areas in Portugal.
Now, many people will use ND filters to get some long exposures by day's end, and it works fine. They will, as I did before, stack filter upon filter, to get the right exposure and the silky water effect. Some will use a graduated ND filter, to compensate for the difference between sky and land or sea.
A variable ND filter will just be one surface to clean in front of your lens.
Long exposures at high-noon
That's what I usually do, with a little variant: I've used a set of ND filters to get the exposure right and the silky water effect in the middle of the day. Because of that option, that gives pictures a different look from those late afternoon seascapes people use to do (me included), I need to find ways to get a long exposure done. So my filter holder and three NDs have been in use a lot of time.
Photographing waves crashing on the cliffs meant cleaning my collection of ND filters over and over...
Working with filters close to sea water means a lot of cleaning. In fact, sometimes, depending on wind, how close I was to the water and the size of the waves, I would spend more time cleaning filters than photographing. Now do the math: three filters mean six surfaces to clean, plus the lens front element. And some days I would find that when I finished cleaning one filter the others were already showing marks of sea water spray.
More photography, less cleaning
It all changed once I started using a variable ND filter: I could use it to get the same effect (the flexibility of using one, two or three filters), sparing me the need to keep cleaning filters all the time. Now I just have to clean one surface and besides that I also discovered it is faster to get the desired exposure just rotating the ND filter. So, there you are: my photos are cleaner now, my microfiber cloth is less used and I get more keepers with less hassle all the way.
Now with just one filter it's a cleaner job to do this kind of photography.
I must admit that when I first looked at these variable ND filters, I felt they were nice but not something that would make me drop my set of ND filters to use in landscape photography. I felt comfortable with those. Now I am convinced. A variable ND filter lets you change exposure somewhere between 2 (ND4) and about 9 stops (ND500) and lets you play not only with speed but also with depth of field at times of day you would not even dream it was possible.
I can concentrate more into getting the right exposure at the right moment.
A variable ND filter is made of two opposing polarizers, the front element rotating over the non-rotating section. When you reach the maximum amount of effect, you get a black cross over the image and… have to back up a bit to get things right. It’s easy and works fine.
You'll find different options in the market, from the very expensive Singh Ray Vary-ND at 400 dollars to some cheap eBay filters that will probably introduce color shifts in your images. One does get what he pays for, so try to stay somewhere in the middle. A good variable ND filter (or fader ND filter or vary ND filter, names vary), will cost you around 100 dollars. Phottix just presented their own version. I am waiting for one to test.
Phottix has entered the market with their own VND filter for a reasonable price.
Some people will say that it’s cheaper to buy a set of ND filters with the conventional grades of 0.3, 0.6, and 0.9 (1, 2 and 3 stops) and stack them together on a filter holder when there’s need for so much loss of light, but one would still be short of the range of these type of filters and besides it's more precise and easy to control the results with a variable ND filter. And most importantly for coastal photography, your images and gear will be cleaner much cleaner!