ND Filters-- Built-in or Add-on

In light control you have two tools, one of which is fairly open-ended and the other of which is fairly limited. The latter is the aperture, where there is a range of perhaps 6 or 7 stops, with each stop representing a halving or doubling of the amount of light coming through. The shutter speed offers a much wider range, and may be, with a system that allows you to shoot from 30 seconds to 1/8000 second automatically, a range of 18 stops (!) So, when adjusting for the light at hand you have more leeway when setting shutter speed than when setting aperture.

For example, let’s say you are working in dim light. You open the aperture on your lens to its widest setting, say f/4. That’s as much light as the aperture will bring to the table. You have maxed out on the quality limit of your ISO, say that's at ISO 800, and you know that going with more ISO will diminish picture quality to the point where it's not worth it. 

But the shutter speed can be set at a very slow speed, even as slow as a few seconds. Of course you need a tripod to keep the picture steady, but shutter speed will not limit you in making the shot, even though aperture “quit” on the deal at f/4 and ISO above 800 might just have been too noisy.

However, let’s say you want to create the image effect of a slow shutter speed in bright light. You might want to capture a “flow motion” effect on a waterfall or a stream, or pan, follow a subject in motion as you make the shot, a runner or a speeding car at a race. That is more challenging, as the aperture may not allow you to create that effect because it can only limit the light coming through even when closed down to its minimum aperture. And when shooting for that effect only a certain range of shutter speeds will do—from about 1/15 to ¼ sec.

For example, say you take a reading of a waterfall in bright light and know that to gain the effect of flowing water you need a shutter speed of about 1/8 to ¼ second. You set the ISO at its lowest, since you do not want more sensitivity on the sensor, you want less. And even at the minimum aperture on your lens, say f/16, the shutter speed comes up at 1/60 sec. How can you “lose” three steps to 1/8 sec or four steps to ¼ sec.

This photo was made using the minimum aperture of the lens, f/22, at ISO 100, with a 4X ND filter over the lens, at 1/8 second, yielding a flow effect on this waterfall. Photo copyright George Schaub, all rights reserved.

You solve this problem with a neutral-density (ND) filter over your lens to diminish the amount of light coming through the lens. An ND filter is “neutral” in that it does not change the color of the light, and offers a “density” or degree of darkening effect. The filter comes in various strengths, in multiples of 2, where a 2X gives you a one-EV gain, a 4X, 2 EV and so forth. Thus, using a 4X ND filter on the example above, you could shoot the scene in the same light and at the same ISO at f/16 at 1/15 second and with an 8X filter it would be f/16 at ¼ sec, yielding the flow effect you desire and the shutter speed used here.

A number of cameras, including Canon's G series digicams, now offer this as a built-in, and using the menu you can choose the power of the ND as needed. Also note that this is one control—the shutter speed effect-- you can’t apply after exposure in any image processing software.

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