Photoshop has replaced a lot of the gadgets we used to depend on as photographers. Colored and soft focus filters are now no longer needed because their effects can be duplicated easily in post-processing. Warming/cooling filters for different types of light are also no longer useful because most digital cameras have a white balance setting that makes them completely obsolete. But some filters can't be easily replaced with a simple Photoshop command, and one of the most practical is the neutral density filter. In this article, I'll explain what a Neutral Density filter is, and when it's helpful to use one.
So what is a neutral density filter?
A neutral density (ND) filter reduces the amount of light that passes through the lens of your DSLR, which allows you to use a slower shutter speed or larger aperture to obtain the same exposure you would have gotten without the filter. Why is this useful? Because it gives you more control over your final image.
ND filters work by blocking a precise and uniform amount of incoming light in any given scene. Because an ND filter blocks an equal amount of light across the whole visible spectrum, it doesn't impact other elements such as contrast or sharpness. ND filters look tinted (like a pair of sunglasses), but the camera "sees" through them in pretty much the same way your eyes see through a pair of sunglasses, so the tint doesn't show up in the final image. One drawback, though, is that it may be hard to see what you're shooting at through your viewfinder, so you may need to compose first and then add the filter.
Types of neutral density filters: standard, variable and graduated
A standard neutral density filter has a uniform tint across its entire surface, which means it will block an equal amount of light in all parts of the scene. A standard ND filter blocks the same amount of light regardless of conditions, while a "variable" neutral density filter gives you more flexibility. Variable ND filters allow you to adjust the amount of light the filter blocks, which means you have more control over how the final photograph will look. Variable ND filters are pricey, though (a good one may run you up to $200 US compared to $50 for a standard one).
Standard ND filters are available in different "strengths", usually classified by how much light they block (referred to as the optical density or equivalent f-stop reduction). For example, a .6ND filter is equal to 2 f-stops, with a light reduction of 4X. A .9ND filter is equal to 3 f-stops, with a light reduction of 8X. You can get ND filters that go all the way up to 20 stops, which will allow you to capture motion blur even on the brightest days. (Tip: you can "stack" several ND filters to increase your light reduction, which may eliminate the need for one of the more expensive variable ND filters).
A graduated ND filter is used for photographing scenes with a broad dynamic range, which means that there is a lot of difference between the brightest and darkest parts of a scene. The human eye is actually about twice as good at picking out details in scenes with a broad dynamic range than even the best digital camera--which is why we don't see blown out highlights and black shadows on bright sunny days, even though that's what our cameras see. A graduated ND filter can help make a scene with a broad dynamic range look much closer to what the human eye sees.
When do you need a neutral density filter?
Let's say you want to capture some motion in a river, but you're shooting on a fairly bright day and there are no trees to help filter the light. If you add a neutral density filter, you can keep your aperture where it is but use a slower shutter speed than you'd be able to use without the filter. Alternately, you can use a neutral density filter to decrease your depth of field (by changing the aperture to a smaller F-stop number) while maintaining a faster shutter speed. This can be useful for taking portraits or macro images on brighter days, or in other situations where you want your background to fall out of focus or where you want to add some motion blur.
A graduated ND filter is good for brightly lit scenes where you want to maintain detail in the brightest and darkest regions. When photographing a scene with a bright sky, for example, you can position the graduated ND filter so that the darkest portion is above the horizon, and the lighter part is below the horizon. This will help maintain color in the sky and/or bring out detail and definition in the clouds while also maintaining detail below the horizon.
With the help of a tripod, you can also use a ND filter to get rid of those pesky tourists. No need to upset anyone by shooing them out of the frame; instead you can use your ND filter to slow down the shutter speed so that anyone who carelessly wanders in front of your camera will be blurred out of existence. There's also another trick for this effect using Photoshop.
Can you simulate a ND filter in Photoshop?
Well, sort of. But the effect is much more easily obtained with the filter. To achieve the same effect of a graduated filter in Photoshop you need to take two identical shots of the same scene (using a tripod), with one shot exposed for the highlights and the other for the shadows. Then the two shots can be combined in Photoshop. This will give you a similar effect as what you would get with the ND filter, but with much more effort. And you can always clone away those annoying tourists, too, but who has that kind of time?
A neutral density filter is one of those tools that every amateur photographer should have, but most seem to do without. Carrying one in your camera bag will give you far greater flexibility in a lot of different situations, so it's worth picking one up if you have the means. As always, experimentation will give you the broadest possible range of results, so don't limit yourself to the on-board controls of your particular camera. You have a lot of options, and the neutral density filter might become one of the most important tools in your photographic arsenal.
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