Saturday, May 7, 2011
The ND filter
Ever wonder how photographers do those slow speed waterfall shots with smooth flowing water on broad daylight? Of course they do it with really slow shutter speeds, but a slow shutter speed means letting more light in, which means if you are photographing a waterfall at noon when the sun is high the light levels are so high that you end up overexposing your photo. Sometimes even when you close your aperture to like f18 or more, you still get parts of your photo overexposed. Although closing the aperture that much or more with a non full frame camera causes diffraction, it is not really advisable.
The solution to this is using a filter called a Neutral density filter. A neutral density filter is a filter that limits the light coming in to your cameras sensor. It's like sunglasses for your eyes. Neutral density filters are supposed to be neutral in color. They are not supposed to shift color or put a color cast in your images, that's why they are called neutral.
The degree of light transmission is determined by the number following the ND label. For example, ND2, ND4, ND8 and so on. An ND2 means the light coming through is 50%, while an ND4 means the light coming through is 25%, an ND8 by 12.5%...and so on. There are also ND filters that are adjustable...although those cost around like $100 or more. They basically work like circular polarizers.
So basically go up to a waterfall, snap your nd filter into your lens, set up your camera on a tripod, then tweak your settings. Meter your exposure with your ND filter on. Set your shutter speed to a second or slower and then adjust your aperture without overexposing your image. Better yet, set your camera to shutter priority to make things easier. Bracket your shots if necessary. Remember, the slower the shutter speed, the smoother the water flow will look.
ND filters can also be used in a studio to achieve maximum depth of field from a wider aperture with very powerful lights. It also allows you to minimize your shutter speed outdoors if you are using a flash so you stay below the flash sync speed limit.
Here are some of my shots using an ND filter: