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Wednesday, April 22, 2009

The High Pass Filter

In the ongoing search for new techniques that will give your images a unique look and set them apart from the competition, the High Pass Filter can be another valuable tool for your business

This Article Features Photo Zoom

In every instance, however, shooting RAW allows far greater control over an image: the only on-camera settings that affect the image are aperture, shutter speed and ISO—everything else can be manipulated, including white balance. It’s not just coincidence that one meaning of raw is “untried.” RAW files are an open door for High Pass work.

What exactly is High Pass Filtration? It’s a means of blocking lower-wave frequencies while permitting only higher ones to pass. Such filters are used in acoustics, optics and electronics and physics, among other fields. For Photoshop users, it’s useful to think of the opposite of the High Pass Filter as Gaussian Blur. Other adjectives may help draw this distinction:



For Photoshop purposes, High Pass Filtration reveals where the greatest contrasts (i.e., the sharpest angles and edges) appear in a photo, then intensifies and boosts their saturation. It’s a layering tool that permits higher frequencies to pass through while blocking lower frequencies. In order to understand the High Pass function, think in terms of layers, rather than effects created by a single pass. Sound waves are a reasonable comparison. If a stereo is adjusted to all treble, one could argue that this is High Pass Filtration at 100% opacity and saturation.

Sound waves are a reasonable comparison. If a stereo is adjusted to all treble, one could argue that this is High Pass Filtration at 100% opacity and saturation.
Conversely, all bass would be Gaussian Blur. The original sound would either be brassy or blurry at the extremes. The magic is in adjusting the balance—and that’s what tweaking settings and using the High Pass Filter can do for a good image. To take this comparison one step further, adjusting the bass and treble won’t make a good song out of a bad one.