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One of the most useful filters that you can use is one that doesn't actually alter the light that passes through your lens. A neutral density filter does nothing more than cut the amount of light that passes through it. An ND filter doesn't alter the light's qualities in any way. It doesn't change its color or diffuse it or anything like that, what they do is broaden your range of exposure options. For example, say I'm shooting some moving water in the daytime, and I'd like to use a slow shutter speed to create a blurry, silky look on the water.
If it's too bright out I might not be able to get a slow enough shutter speed to smear the water even if I slowdown my ISO as much as I can. With the Neutral Density filter I can cut a bunch of light out of my scene, which will allow me to use a longer shutter speed. Neutral density filters can also be used to buy yourself more aperture latitude. As you stop a lens down, you run the risk of softening your image because of something called diffraction. Many lenses get noticeably softer once you pass f/16. With a Neutral Density filter you can cut out some light and open your aperture back up to restore any sharpness lost to those diffraction artifacts.
ND filters come in different strengths, so you have some control over how much light you cut from your scene, and you can stack multiple ND filters to build up increasing amounts of darkening. In the old days, ND filters were measured and rated according to their optical density. A 0.3 ND filter would cut one stop of light, a 0.6 would cut two, a 0.9 three stops, and so on. Some vendors still rate their ND filters this way, but others have adopted an ND rating. An ND 2 filter, for example, cuts one stop of light making it equivalent to a 0.3 filter.
ND 4 cuts two stops making it equivalent to a 0.6, and so on. So when you're shopping for an ND filter, you'll see filters rated either way. Some vendors even use both ratings. This chart shows you the equivalent ND and optical density ratings. In general, it's good to have a range of ND densities. If you have some lighter ones and some heavier ones, you can stack them in different combinations to get different levels of light stoppage. If you're not sure exactly how much darker one stop is take a shot with your camera as metered, then dial in minus one stop of exposure compensation and shoot again.
This will show you how much darker the image would be if you had a one-stop neutral density filter on the front of your lens. In recent years, a new technology has come along, the Variable ND Filter. A Variable ND screws on to the front of your lens just like a normal filter, but it has this rotating element. And as I rotate it, I get more or less ND filtering. The image gets lighter or darker. This one gives me a range from 2-8 stops of ND. Now, what's great about the Variable ND is that it takes up much less space in your bag than a bunch of separate ND filters. That said, there are some caveats.
I find that with this particular ND I can't go too far to the extreme dark end without getting really bad variation in effect across my image. What's more, my camera doesn't meter properly through this filter. So to get good results, I do an initial shot and then manually adjust my metering until I get a better exposure. Now, here's the weird thing. If you work with a Nikon camera rather than a Canon you probably won't have this problem. The Nikon metering system more accurately handles the Variable ND. As convenient as a Variable ND is I'd still recommend sticking with traditional ND filters.
Yeah, they will take up more space and they are more of a hassle to work with, but they are more reliable. They yield cleaner results and might actually cost you less money especially if you don't need a full six-stop range. Here's another variation on the ND, the Graduated ND. So this filter has two stops of ND filtering at the top here, but the filtering ramps off to none by the middle of the frame. So with a Graduated ND I can even out an exposure when I'm shooting into something that's very bright at the top of the frame like a sunset.
With the Graduated ND I can expose for the foreground but still have detail on the sky. I won't blow out my nice sunset. Graduated ND's are tricky to recommend, not because they don't work but because they need very special conditions. If the ratio of dark to light that you want isn't the same as your filter, for example, if you want the dark to come down lower or end earlier, then you might not get all the darkening you need or might end up with a foreground that's too dark. What's more, if there's anything poking up from the bottom of the frame into the top, into this darker area like a tree or something, then that object will also be darkened.
Now, in the old days Graduated ND was the easiest way to get this type of effect, but nowadays you can easily create this effect in your image editor. That said, in the next movie you're going to see an ideal application for the Graduated ND filter. ND filters aren't everyday items, but for times when you want the kind of effect that they can yield, they are really your only options. So you might always want to keep at least a couple of them in your bag.
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