Are Filters Still Needed for Digital?
Believing that most anything can be fixed later in Photoshop, it may be tempting to think you no longer need to use filters once you begin shooting digitally. In reality though, you're better off sticking with most of your filters.
The polarizer is the most widely used outdoor filter. Its most observable effect is that it darkens the blue of the sky, making clouds stand out more in contrast. While there is a commercial Photoshop equivalent of this effect made by Nik Software (formerly Nik Multimedia), darkening blue skies is one of the rare exceptions that can be done effectively on the computer. A polarizer also helps to reduce reflections from non-metallic objects such as puddles and tree leaves in the sun. If the camera sensor picks up nothing but glare in these reflections, insufficient information gets captured and you have a problem. Once the camera records nothing but pure white from overexposure, you have no information to work with and have lost all chance of fixing things later, no matter what Photoshop filter you employ.
The same problem exists with attempts to avoid using graduated neutral density filters. The effect of a grad ND can be achieved only within very narrow limits in Photoshop using the gradient tool, but only if the information is there in the first place. As with slide film, once you burn out the highlights in digital, nothing can restore them. Even if there is information to work with, it will likely exist only within the uppermost portion of the 0 to 255 range. Trying to expand that range downward in Photoshop will result in visible banding since there just aren't enough discreet tones to work with. My D100 is capable of recording detail in about seven stops of subject brightness range, wider than that of slide film for sure, but not enough to successfully capture both sky and foreground at sunset without help in the field.
I actually use solid neutral density filters more with my D100 than I used to with the F100. They are the only workable way to achieve slow shutter speeds given that the camera's slowest ISO speed is 200. If you've had a hard time blurring water with a digital camera, get a good neutral density filter.
Color conversion filters such as the 81 warming series and 82 cooling series are one type of filter you do no longer need. Their effect can be duplicated in-camera by simply adjusting the white balance. Filters and tone controls in Photoshop can sometimes be useful after the fact as well, within the limitations of the data you have captured.
Proficiency in the digital darkroom really can't substitute for good technique in the field. Just another reminder that digital photography is still photography.