In the last posting we wrapped up the discussion about lens selection and ended with some comments about filters that are used with your lenses to create special effects or solve difficult lighting problems. We will pick up on that topic in this posting.
In the ancient days of 35mm film, many filters were considered to be essential for good photography. In the pre-digital age tinted filters were needed to adjust color temperature for different films but today these adjustments can be made easily in post-processing software so it is rare to need color adjustment (often called warming or cooling) filters.
UV / haze filters are often sold to buyers of new cameras and lenses to protect the front of the lens from physical damage. These filters are said to help clear haze from the sky on landscape shots but really have no appreciable benefit over adjustments to the image that can be made in post production software. Low quality filters can degrade image quality and induce flare from back lit subjects and should never be used with high quality lenses. High quality filters may not harm the image quality but may add $100 to the cost of each lens and while they keep dust and water off the front lens, their ability to protect from physical damage is questionable. I use them on some of my lenses but am not sure that the $500 cost was worth the price. I personally believe that a lens hood is better protection and will not degrade your images.
A circular polarizing filter (sometimes called lens or CPL) is probably the most important filter for outdoor photographers. The polarizing filter will cut through haze, reduce reflections from water or shiny objects, saturate the sky, and enhance clouds better than any post-processing software and produces just about the only image changes that cannot be achieved in software. The CPL is actually two filters mounted together in a rotating ring that can be turned for the desired effect. The CPL works by aligning light waves and thus reducing glare. The filter has its greatest effect when it is 90 degrees to the light source so it must be rotated to the proper position before the image is captured. The filter should be used in almost all landscape photography and shots of still waters. The CPL has two side-effects. First, it reduces exposure by about two stops meaning that aperture or ISO will need to be increased or a larger f-stop will be needed compared to a shot without the filter. Next, the filter can cause some saturation abnormalities when used with wide-angle lenses. Because the filter is most effective when turned 90 degrees to the light, a wide-angle lens may 'see' variations in saturation of the sky. High quality CPLs are quite expensive and should be sized for your largest diameter lens. Adapters are available to match the CPL with lenses of smaller diameter. High quality CPLs are very thin and do not induce vignetting at the corners of you shots as may occur with cheaper, thicker filters. High quality CPLs are coated to reduce lens flare.
Sometimes, nature photographers need to reduce the amount of light reaching the sensor to achieve a proper exposure in bright daylight. A classic effect is to use a very long exposure 1/2 to 4 seconds to create a dreamy smoothness to a waterfall or other moving water. When the ISO is reduced to the minimal sensitivity and the aperture is reduced to the smallest opening, sometimes in bright daylight, the slowest shutter speed to properly expose the image is 1/60 - 1/100 second - too fast to get the desired effect of smoothing the water. Here is where the neutral density filter comes to play. A neutral density filter is a neutral gray filter that reduces light to the lens without altering the tint or hue of the light. The filters are rated by the number of stops (halving) of the light caused by the filter. Typically a 2-4 stop filter will allow the light reduction that is needed for the exposure time you want to use but sometimes the light needs to be reduced by 6-8 stops. Neutral density filters can be stacked to achieve an additive effect on exposure reduction. Stacking more than two filter may cause vignetting of the corners of the image. Another solution is a variable neutral density filter that can be adjusted from 2 to 9 stops of light reduction. These seem to work well as a cost-effective alternative to having multiple fixed neutral density filters. A third solution is a rectangular external gel or glass filter added to the end of your lens. Multiple filters can be stacked in the holder to achieve the desired effect.
Another very useful filter for the landscape photographer is a graduated neutral density filter. These filters are usually rectangular glass or resin mounted in a holder on the end of the lens. The filter is graduated in that the bottom is clear (usually) and the top is progressively more dense. The purpose of the GND filter is to reduce light from a bright sky while maintaining the light in the foreground. The filter is simply moved up or down after the shot is composed so the light reduction occurs above the horizon in the bright sky. These filters tend to be pricey and are available with soft or hard edges meaning the darkening occurs abruptly or gradually. I prefer the hard edge set on the horizon. They are also available in a 'reversed' form for certain shots with a bright foreground and may be stacked to achieve the desired effect. GND filters are rated at values of 0.3 (1 stop), 0.6 (two stops), and 0.9 (three stops) of light reduction. The two stop reduction is most useful but at times needs to be stacked with a second 1 stop filter. They may also be stacked with a polarizing filter or plain solid, neutral density filter. The glass filters are easily broken and the resin (plastic) filters are easily scratched so while very useful, GND filters can quickly become very expensive. The screw-on circular GNDs should never be used. Although cheaper than the rectangular filters, they drastically limit your ability to properly compose your shots since the graduation is at a fixed point on the lens.
The list of specialty filters goes on and on. These filters can be used to help tone black and white images, induce color (hue) shifts, achieve white balance, or produce unusual light effects such as star or other shape effects at points of light. None are necessary to the nature photographer but they can produce interesting a visually unusual effects.
So by all means obtain CPL and ND filters as you will definitely find a need for them. As for the others, experiment with their use and when you have some more money to blow on photography gear, why not?