WHEN TO USE ND FILTERS
Although ND filters are certainly not relegated to use only outdoors, I can safely say I can’t think of a single time I’ve ever used one indoors or under controlled lighting. So, in this article, I’ll focus on outdoor use only. Let’s start with the solid ND.
The ability to use the passage of time, motion, and light to stir emotion and project a message or mood is what I think makes photography a true art. It gives the photographer the ability to present imagery that escapes the bounds of the static capture, going beyond the documentarian style that can at times be limiting, stilted, and lifeless. With a solid ND filter at your disposal you can take a picture of a moving object and use effect to record that movement across the frame. Let’s say you are photographing a bicycle race and you are fortunate to be standing near the road or track. It is sunny outside and the light is so strong that you can’t set the camera exposure so that you can use a shutter speed that is slow enough to record the motion of the passing cyclist. You want to blur him to represent motion and without a filter to hold back the light you won’t be able to get this type of image. But if you attach an ND filter to the lens you can effectively reduce the light to the point where you can now utilize the slower shutter speeds that give you some blur. This usually works better, from an artistic perspective, than a static image where the cyclist seems to be frozen in time; just sitting on his bicycle with the tires stopped and no motion in his legs as he pedals on past you. With the ND filter you can represent the cyclist as a blurred, ghostly object, while the roadway and the background are frozen with sharp details. Or you can reverse this play on motion and pan your camera keeping the cyclist in the middle of the frame as you shoot. With the ND filter the motion effect would show the cyclist relatively sharp while the background would almost be a complete blur – again representing motion in a very effective way.
Another very popular use of the solid ND filter is to record moving water, like a stream or the surf of the ocean, with a slow shutter speed. Some people don’t care for this effect, but I find this to be one of my favorite techniques when shooting water. The slow shutter speed that is, again, provided by the light filtering capability of the ND filter will produce the moving water as a misty, ethereal , almost cloud-like feature. To me, it has a very soothing effect. In my view recording moving water as a frozen mass of churning froth just isn’t something that gives me a calming effect. I almost don’t desire to be there. By contrast, when I see a beach scene with some large rocks where the surf is coming in and out and it is recorded as a misty veil of softness amid the hard boulders I immediately want to be there.
A third way that a lot of outdoor photographers use to great effect is to record fast moving clouds with the ND filter. This is similar to the water effect except that in this case you have a sharp and detailed foreground with clouds that take on a rushing motion across the frame.
Finally, an ND filter can be used to access wider apertures rather than to lengthen shutter speeds in the exposure equation. Wide apertures (smaller f-stop numbers) will give you less depth of field. This is a desirable feature when you want to isolate a subject by blurring the foreground and background. The Japanese have a name for this out-of-focus effect; it’s called “bokeh”. A single flower isolated in a close up photograph with the background well out of focus giving a creamy background is a good example of this technique.
Now let’s move on to the graduated ND filter. This filter has a specialized purpose and is rarely used outside of landscape photography. Simply put, this rectangular filter is used to hold back the light intensity in the sky so that you can set an exposure value that will capture details in the darker terrain or foreground. It really is quite a simple filter to use but the key to success is placing the transition of this split filter so that it is hidden in the scene. This takes a little practice. The other important issue with this type of filter is choosing the correct strength so that the overall exposure remains balanced, a true and natural looking interpretation of the scene you are photographing. I’ve seen many images where the filter is not placed precisely and it either darkens the upper region of the foreground, or it is placed too high in the scene resulting in an unnatural bright halo just above the horizon. Also, if you don’t have the correct amount of density you can get weird looking results. If the filter is not dark enough for the situation, or by contrast, you might have a filter that is too strong the sky will just not look right. For this very reason, and in order to use these filters effectively, you must have a set of filters spanning several densities. There are also two types of transitions – a hard transition for use when the horizon is well defined and level across the frame, and a soft transition where the terrain’s edge is jagged, like a mountain range in the distance. Here is a good place to explain my little rant in the previous section about ditching your circular graduated ND filter. These filters have the transition from dark to light right in the middle of the filter. This means that the only possible way to use this filter is to frame your shot where the horizon runs exactly in the middle of the frame. In almost all cases and situations you want to avoid this placement of the horizon. It is one of the most unpleasing compositions I can think of. Good landscape photography depends on compositional decisions that create pleasing imbalances between light and dark often referred to as “tension” or “positive versus negative space”. Placing the horizon smack-dab in the middle of the frame will never give you this tension. So, like I said, take that circular graduated ND filter and run over it with your car.
Many photographers, including myself, often stack two or three, rectangular ND grads to get the strength of filtration we need. This is another big advantage of using rectangular filters.
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