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When I started out in photography shooting slide film, I was trained to carry two "mandatory" filters: a polarizer and a set of graduated neutral density filters (ND-grads). Polarizing filters not only increase contrast in skies, but they are indispensible in removing glare and reflections from water and foliage. ND-grad filters offer a way of compressing tones in scenes with a large dynamic range.
Do you need a set of ND-grads for digital photography?
While the effect of a polarizer is nearly impossible to recreate in image editing software, there are all kinds of ways of replicating the effect of ND-grads in post. In fact, many of the newer software packages offer built-in ND-grad "effects" that can easily be applied to your images. In addition, modern HDR software techniques have made it possible to create fantastic, natural-looking images that capture a tremendous amount of highlight and shadow detail. If that's the case, why do I still carry a set of ND-grads with me in the field?
I'll be the first to admit that even when I bring my ND-grads along in the field, I don't always use them. ND-grads can be tricky to align, especially in the semi-darkness of twilight. You also need to choose the right filtration strength as to not over-filter the scene. Reflections, for example, should not be brighter than the actual object, but that can be something hard to spot in the field. On the plus side, having a DSLR makes using ND-grads a little easier. First, I can use live view to help me position the filter in the frame. Second, I can review my images immediately and decide if they are OK or need to be retaken. But more than that, there are times when a filter is actually preferrable to using software or HDR techniques in a landscape shot.
This scene is not a good choice for using a ND-grad filter, as it would unnaturally darken the tops of the foreground rocks.
This subject is ideal for ND-grads because there is a distinct horizon line between bright and shaded areas, and no foreground objects protruding into the sky.
The traditonal advice we get for exposing outdoor images is "expose for the highlights and recover the shadows." What this means is that we'll often deliberatly underexpose the scene to preserve hard to recover highlight details and then bring back the shadow details in post. This technique works fairly well most of the time, but it does have some drawbacks, namely noise and color. Any time you recover shadow or highlight detail from your image file, you run the risk of adding noise and losing color saturation. The resulting image can often look flat or lifeless, because the shadow contrast just isnt' there. On the other hand, if you use an ND-grad in your shot, you're likely to have a better overall exposure and still have clean shadows. If you do need to use shadow recovery techniques, you're less likely to need extreme adjustments if you've used a grad filter for the original shot.
There is a huge trend right now in making high dynamic range (HDR) images. In fact, it's a technique that I absolutely love. When done properly, you can create amazing and yet, still natural images with modern HDR software. With HDR, you need to take multiple images and combine them in post. The main drawback with this technique (other than hideous processing), is that if objects are moving you'll get motion or "ghost" artifacts. In landscapes, this can be moving water, leaves, or a person walking through the scene. Sometimes, no matter what you try, conditions don't allow for a good bracketed HDR exposure series. In these cases, it's good to have a set of ND-grads handy to allow you to get the best possible exposure on a single frame.
So, given that it's good to have a set of ND-grads, how do you choose which ones to use? First, you'll need to decide what shape and size filter to get. Next, you'll have to choose the strength and edge characteristics of the filters you get.
ND-grad filters are available in both round (screw-in) and rectangular form factors. To use a round filter, you just get the size that fits your lens and screw it on, and you can rotate it if you want. While this seems convenient enough, you'll need to get multiple filters if you have different lens diameters. Moreover, the screw-in grads are terrible for creative control. The transition zone is in the center of the filter, forcing you to compose around the filter position.
On the other hand, rectangular filters are quite versatile. They can be mounted on a holder and the holder can accept adapter rings of different sizes; meaning that you can use rectangular filters on just about any lens in your kit. Secondly, not only can you rotate rectangular filters, but you can also slide them up and down to position the gradient exactly where your composition dictates. That's important if you want to have your horizon line somewhere other than the middle of the frame.
Rectangular filters come in two primary size classes: "Cokin-P" sized and "Lee" (4x6") sized. Many years ago, I started with the least expensive set I could get in Cokin P format. I have since decided from experience that I should have purchased 4x6" filters from the outset, despite their higher price. Having larger filters means that I can hand-hold them without getting my fingers in the frame. Hand-holding a grad filter allows you to not only use your lens hood (something you can't do with the filter holder), but also move the filter during a long exposure to further soften the transition zone. 4x6" ND-grads will also offer a wider transition zone, meaning you'll have better placement opportunities when using them. I currently use a set of Singh-Ray 4x6" filters.
The other two factors in choosing ND-grad filters are strength and edge type. You'll see 1, 2, 3, and even 4-stop ND-grad filters. The strength you choose will depend on the amount of shooting that you do and where you shoot. For most landscape photographers, you'll get by with 2 or 3 stops most of the time. If you're shooting out west or in alpine elevations, you'll want to add a 4-stop filter because the tonal range tends to be much wider in the thin, dry air of the mountains. You can get either soft or hard-edged filters. A hard-edge filter has a relatively abrupt transition zone between clear and maximum filtration, while the soft-edge filters offer a wide transition zone. Hard-edged filters work best with telephoto lenses and for scenes with well-defined horizons. With a wide-angle lens, you'll want to use a soft-edged filter to mask the transition zone due to the very large depth of field these lenses create.
You might not need to use ND-grads often, but they sure do come in handy. I keep my set in my bag whenever landscape photos are on the agenda.