Sharpening is one of the most- un-glamorous, yet totally essential, digital imaging edits you can make to your photographs. Sharpening is a must if you want to make your images look their best; RAW files require sharpening to make them look like JPEGs, and JPEGs (as well as RAW, TIFFs and every other image file format) require specific sharpening based on the output of the finished image. Not to mention the creative sharpening that dramatically affects the style of a photograph, which is all subjective based on creative license
But this post isn't about how to sharpen. It's about the downside of sharpening—the artifacts and problems that you can create in your digital image files by sharpening too much. So keep reading to find out how to minimize sharpening problems and keep from oversharpening in the first place.
The best way to eliminate sharpening artifacts is to keep from oversharpening in the first place. How do you do this? You sharpen deliberately. The Threshold slider in Photoshop's Unsharp Mask filter (a favorite sharpening tool for many photographers, myself included) is a powerful tool. With it you can dictate what types of edges are sharpened in a photograph. To understand this fully, I'll back up for a second and explain how sharpening works.
Digital image sharpening is essentially a function of accentuating the contrast between edges of light and dark within a scene. A portrait subject's dark shoulder against a light background, for instance, is an example of a large edge that can be accentuated (or artificially "sharpened") in a digital image. But so is the contrast edge between a highlight in the eye and the dark part of the iris, and the edge between a hair and skin, and the edge between a pore's highlight and shadow. All of those edges of contrast can be sharpened, so it's up to you to dictate which ones and how much. That brings us back to Threshold.
The Amount and Radius controls in sharpening tools like Unsharp Mask dictate, as you can tell, the amount of sharpening and the radius (measured in pixels from the contrast edge) of the affect. But it's the Threshold that determines which edges will be affected. A higher threshold limits sharpening to the biggest, boldest edges in a scene (like shoulders in a portrait), whereas a lower threshold allows every edge to be affected—including all those hairs and pores. The problem this illustrates is the added sharpness in things like pores and hairs (not to mention blemishes and flaws) is almost never desirable. The faux sharpness exaggerates problems and creates one type of sharpening artifact: the added appearance of noise-like texture that comes from giving more pixels more defined edges. To eliminate this, start with a higher threshold and lessen the amount of the sharpening. Concentrate on sharpening more significant contrast edges to minimize this grainy, noisy, hypertextured variety of oversharpening.
The other telltale sign of oversharpening is a halo effect around major contrast edges. This appears most often as a dark shadowed outline that spills into bright areas, and it can be very distracting. To eliminate it, of course you could start by sharpening a lesser amount, or adjusting the Radius to be slightly smaller, but this type of sharpening artifact is usually a side effect from creative sharpening that is desirable elsewhere in the frame. I find it happens a lot when I'm fairly aggressive with the Clarity slider in Lightroom; it crops up if I'm really accentuating the sharpness of a scene to give it an edgier look. But I want to eliminate this telltale sign of oversharpening without eliminating the rest of the "good" sharpness in the scene. To do that in Lightroom I use the Adjustment Brush in the Develop module and adjust the clarity slider way down toward -100. Then I simply paint away the oversharpened halo wherever it's evident in the frame. In Photoshop, a simple layer mask on the oversharpened layer allows you to selectively paint away artifacts. This level of selective control, whether you wield it in Photoshop or Lightroom, is a great way to get the benefits of dynamic sharpening without the drawbacks.