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Neutral density filters are specialized filters that increase the number of stops necessary to correctly expose your image. Why would you want to added extra stops? Lowering the amount of light entering your camera will allow you to use longer shutter speeds, and longer shutter speeds increase your ability to smooth out moving water, remove people wandering through your scene, and even cause clouds to have a dreamy motion effect. No matter what effect you are looking to achieve, if you want longer shutter speeds, especially in bright situations, adding a neutral density filter is the best solution. The strength of the ND filter you need depends on a few things including your ISO speed, your aperture setting, and how long you want your shutter to remain open. Let's first explore how changing your ISO affects your camera's exposure. By lowering your ISO setting to say 100, your camera's sensor will be much less sensitive to light. This in turn will allow you to drag your shutter longer compared to an equivalent exposure with a higher ISO setting. Next we can stop down our lens's aperture to reduce the light entering the sensor even more. If you shoot at f/8 instead of f/5.6, you can drag your shutter twice as long. Most lenses can stop down to f/16 or even f/22 but this comes at a cost. Your lens will actually become less sharp due to diffraction as you stop down past a lens's 'sweet spot' which typically is around f/5.6 — f/11. So while you can definitely slow your shutter down by stopping down all the way to f/22, the image quality in your photograph might actually be worse than if you shot the same photo at f/8. |
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