One of the most asked questions of me is one concerning lens filters. So let’s talk about filters for a minute.
Filters are round glass elements that screw onto the end of your lens, or in some cases glass or resin panels that are placed on front of the lens using a fixture. The purpose of these filters is to affect several different things when you’re taking a photo.
During the era of film photography many colored filters were used, mostly used with black and white film. These colored filters would block or cancel certain colors of light causing corresponding areas of color to respond in different ways. An orange or red filter will darken blue tones and lighten reds, while a blue one will darken reds and lighten blues. In digital photography these colored filters are not needed as the sensor has the ability to filter Red, Green and Blue light.
During that time we also used UV filters to cut the effect that ultraviolet light had on film. Today the digital sensors have built in UV filtration so they are no longer necessary. Today they're used to protect the lens element from scratches. The only caution that you need to consider is that you should remove them at night. Even though they're clear, they can cause light to reflect between the filter glass and the lens glass. This can cause a double image of any light in the scene.
In digital photography the most commonly used filters are a circular polarizer and neutral density filters.
A circular polarizer, or a CP filter, will do a couple of things to your photo according to how it’s used. The primary purpose is to reduce glare and reflections on things such as the surface of water or even wet leaves. It will also turn the sky a deeper blue. It is made with two elements, one which you can turn to adjust the amount or place of polarization. The filter glass will be somewhat dark so it will stop light, the amount of which varies depending on the darkness of the particular filter but a typical CP filter will stop about 2 f/stops.
The next filter that is most commonly used in digital photography is a neutral density filter. A neutral density filter modifies the intensity of all wavelengths of color. In short its purpose is to block or stop bright light. The purpose typically is to extend or lengthen one’s shutter speed during bright light such as a sunny day. When a photographer mentions neutral density filters, they typically call them ND’s or ND filters. ND filters come in a variety of “darknesses” stopping different levels of light. They can vary in optical density from almost clear to nearly solid dark. The most common ND’s are ND2, ND4 and ND8 with a corresponding 1, 2, and 3 f/stop reduction. Another common ND used for extreme stops of light some choose an 8 or 10 stop ND filter.
Neutral density filters also come in what is called a graduated neutral density filter. This filter is just as it describes. It has a graduation from top to bottom making half of the filter dark and the other half clear. This is used in neutralizing the exposure when you have an extremely bright sky and a dark foreground, It stops the light of the sky making the exposure more even.
As mentioned previously I use my circular polarizer to affect the blueness of the sky, to remove glare and reflections from water surfaces and wet foliage which will allow the color and texture to show. I love using it for creeks and waterfalls, especially on a rainy day or a day where it’s recently rained as the water will typically reflect the bright light from the sky. So too will the leaves and plants reflect this light from the sky. Once you polarize them the shine goes away and color and textures start to show through. An important thing to remember is that a CP filter works best when the light is coming from 90 degrees from the direction that you’re shooting. As the angle changes so does the amount of affect that the filter has on the photo. Also, the filter will allow me to extend my shutter speed to smooth the water a little more to give it a feeling of movement or flow.
My primary purpose for ND filters is to allow me to extend my shutter even more than I could without them under extremely bright light. They come in handy if you show up to a creek or a waterfall during mid day sun.
As for graduated ND filters, I use them as little as possible as they tend to darken areas that don’t necessarily don't need to be. A good example is if you want to darken the sky but there are trees or buildings that extend into this area. The most ideal case for the use of one would be at the coast in a photo of the ocean with an even horizon line.
Whew. This can all sound a bit complicated, but once you use them it will become easy to understand. If you use your camera on the Manual setting it’s also easier to understand as you probably have encountered some of these problems while trying to get that shot at less than an ideal time. I do my best to show up at a scene in good light. If I want to extend my shutter at a creek or a waterfall I find it best to show up when the light is right. Good light from a creek or a waterfall is subdued light with little or no glare or reflection on the surfaces in your photo. I find it best early in the morning or later in the afternoon, but I love it best when it’s drizzling or an even overcast cloudy sky. Bright light is not your friend in these cases. Surprisingly, the CP works under cloudy skies too.
In the photo included in this blog post you can notice a swirly in the water. This little eddy was caused by a current in the water that held bubbles that were caused by the little pour off in the scene. It took awhile for the bubbles to make a circle so I wanted to extend my shutter as long as I could so I had to block light in any way possible to me. I lowered my ISO, stopped down (narrowed) my aperture and applied my CP for two more stops of light. By doing this I was able to get some decent shots at up to 6 seconds exposure time.
I hope that this helps clear up this subject a little. If you’re serious about your photography put a CP and some ND’s in your bag.