Double Pass Sharpening
Results will differ if you filter the same image layer twice. Why? First, either the technique or the settings can be varied. Second, having been filtered once, the state of the pixels will have changed before a second pass is applied, generating a different final effect. Consequently, not only the type and amount of filtration matters, but also the order in which the filtration is applied.
One classic double pass sharpening technique involves filtering first with an Unsharp Mask setting using a low Radius (under 1.0) and a high Amount (300% or more) and second with an Unsharp Mask setting using a high Radius (approximately 1.5) at an Amount of 100%. A variant of this technique adds a third pass of High Pass sharpening. Both methods use the first pass of Unsharp Mask to give the second pass of filtration more to bite into. The key to making any multi-pass sharpening technique successful is to produce a strong, yet still convincing effect with as few, if any, unwanted arti-facts as possible, either with or without masking.
Some routines will repeat filtration at a lower Amount multiple times; for instance, a sharpening setting may be applied 10 times at 10% instead of one time at 100%. The idea behind this approach is that you can achieve a more intense effect (crisper edges) with fewer artifacts (accentuated noise/texture). As it’s inefficient to perform these routines by hand more than one time, this type of approach is best handled by recording an Action that you can play for future uses, which may need to be modified if resolution varies substantially.
Are there benefits to filtering more than twice on the same layer? Maybe. Maybe not. You get diminishing returns with each additional pass of filtration. You may also run the risk of producing more unintended artifacts. Furthermore, as complexity rises, your ability to both predict and interact with the final effect diminishes. In general, I recommend you to be cautious of highly complex routines and urge you to ask yourself if you derive significant benefit from them.
Sharpening results also will differ if you apply varied filtration techniques to separate layers. Here, the order of the layers in the layer stack matters.
To combine the effects of the different layers, use Blend modes. Darken will display the only values on a sharpening layer that are darker than values on layers below it, such as the dark line. Lighten will display the only values on a sharpening layer that are lighter than values on layers below it, such as the halo. Luminosity will display any values that change in brightness, but not hue or saturation, and may override any sharpening effects below them, so consider separating one Luminosity layer into two layers, one on Lighten and the other on Darken, as their cumulative effect will enhance rather than override underlying effects.
High Pass sharpening layers (or any technique that reduces an image layer largely to gray values) combine easily with other layers using Blend modes (typically, Overlay); they do this so well that many times it doesn’t matter whether they’re placed above or below other sharpening layers.
To reduce file size, you may decide to merge multiple sharpening layers into a single layer. While this makes a file easier to manage now, it reduces your ability to modify the sharpening effect in the future and to clearly track any effects or artifacts that were produced. Weigh the pros and cons of this option carefully.
By keeping sharpening effects on separate layers, you not only preserve the future flexibility of the effects you create, but you’re also able to selectively control the effects and target specific areas of an image more precisely. There are three primary ways to do this: Blend modes; Blend If sliders; and masks. A layer’s Blend mode controls the way its values combine with values in layers below it; access a layer’s Blend mode at the top of the layer stack. A layer’s Blend If sliders let you quickly remove effects from highlights and/or shadows. Activate a layer’s Blend If sliders by double-clicking on it; split the sliders for smoother transitions. A layer mask allows you to target different areas of an image. Add a mask to any layer by clicking the Mask icon in the Layers palette and fill (with a selection or brush) areas where you want to reduce an effect with varying shades of gray; darker values reduce effects more.
When you combine different sharpening techniques, you’ll find that when it comes to the appearance of detail, you’ll have a wider variety of choices to choose from. This can affect more than just the look and feel of your images. You also can use it to guide the eye to specific image areas in different ways, producing a qualitatively different visual journey. Sharpening can make the world look different. Master sharpening, and you may even see the world differently. People who view your images certainly will.
John Paul Caponigro, author of Adobe Photoshop Master Class and the DVD series R/Evolution, is an internationally renowned fine artist, an authority on digital printing, and a respected lecturer and workshop leader. Get access to hundreds of lessons and his enews Insights free at www.johnpaulcaponigro.com.