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Tuesday, December 11, 2012

Solutions: Try The High Pass Filter

Give this tool a look for sharpening your images

Labels: Gear

This scene is an ideal candidate for the High Pass filter.

The High Pass sharpening technique isn't what I would call a "true" sharpening process, but rather what I generally refer to as a local contrast-enhancement adjustment. It's similar in overall concept to sharpening, but the contrast being added to create a sharpening effect occurs across a larger range of pixels. Instead, you can think of the High Pass sharpening technique as being very similar to the Clarity setting available in Photoshop's Adobe Camera Raw and Lightroom.

You'll find the filter under Filter > Other > High Pass.

High Pass filter controls.

Use Opacity to fine-tune the effect.
As with sharpening settings, when using the High Pass approach, the settings will vary not only based on image content, but also on overall image size. Let's assume an image where a particular object is 100 pixels across, and where you determine that a Radius setting of 10 pixels for High Pass provides a good result. If you then enlarge the image so the object is 200 pixels across, you'll need to use a larger setting for High Pass to achieve a similar effect.

The seemingly contradictory statement in the example above relates to the fact that when you print an image larger, any evidence of sharpening will be easier to see. From my perspective, this means that for a larger image, you need to use a higher Radius setting for High Pass because the image is larger. However, you also need to keep in mind that with a large print, any exaggerated effect will be more visible. Therefore, mitigate your increase. If the image has doubled in width, that doesn't mean you double the High Pass setting. But it still will be higher than it would have been for a smaller image.

Viewing distance does, obviously, play a role, as well. I've often said that if the viewing distance is great enough, the image quality doesn't matter at all. Think about billboards along the highway, and if you ever get the chance, take a very close look at one. So, if you know the print will be viewed from a relatively large distance, you can get away with a little more sharpening. But in my experience, it's pretty rare to have an image that's always viewed from a distance, so I avoid any adjustments that will cause visible problems upon close examination.

For those not familiar with the process, the technique is simple. Make a copy of the background image layer by dragging the thumbnail for that layer to the Create New Layer button (the Blank Sheet of Paper icon) at the bottom of the Layers panel. Change the Blend mode for this Background Copy layer to Overlay. From the menu, choose Filter > Other > High Pass. Start with a Radius of about 10, but adjust as needed for the image.

Go to www.timgrey.com to sign up for Tim Grey's newsletters, and check out his blog at timgrey.com/blog/.

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