When you listen to music, noise equals unwanted sound that interferes with your ability to hear. When you view photos in Adobe Photoshop, noise's unwanted pixel-level detail degrades image clarity. You may not see small flecks and specks until you magnify a high-resolution image. On a low-resolution photo or in images with high levels of "pollution," you see noise clearly at first glance. Suit your correction techniques to the types of pixelation you identify, first eliminating resolution-based distortion as the cause.
How Low Can You Go
Magnify an image in a favorite image-editing application to the limits of its zoom feature's capabilities, and you'll see the individual pixels that make up a continuous-tone bitmap in a matrix of rows and columns of tiny colored squares. Enlarge a small image or low-resolution photo to large dimensions, and those pixels become visible even without zooming in. You can use sharpening techniques, including the Unsharp Mask filter, to overcome small amounts of the softening that accompanies enlargements in the 125-to-150-percent range, but once you see pixels instead of image detail, even a Photoshop master can't eliminate the distortion. To create jumbo versions of images, start with a large, high-resolution master photo.
Images pick up spots and blots from numerous sources. If you digitize prints or transparencies on a dusty scanner or the images themselves accrue dust, it forms an assortment of random dots and tendrils. Capture an image on a digital camera with dust on or behind the lens, and the same types of problems emerge. You can distinguish these mechanically introduced annoyances from flaws in an image itself because dust always stays in focus, courtesy of its position on the surface of a print or piece of equipment. Other forms of pixelation result from image capture and manipulation, including high ISO settings chosen to overcome dark scenes or aggressive lossy compression aimed at producing smaller photo files.
Turning Down the Noise
Most noisy images suffer from one or both of two types of unwanted pixel detail. Color noise tints small areas in shades that don't appear in the original scene or source. Luminance noise introduces unexpected bright or dark pixels. In many cases, noise appears only in one channel of an RGB image, often the B, or blue, channel. If you treat the entire image instead of limiting your corrections to the problematic channel, you unnecessarily soften other aspects of the image. Photoshop's Reduce Noise filter addresses both types of noise in an interface that gives you control over the strength of the correction, how much it limits its effects to retain more image detail, and whether it addresses the quilted, blocky look of high JPEG compression levels. The filter's advanced settings limit its operation to specific image channels. Reduce Noise can apply sharpening to compensate for the softening effects of noise reduction, or you can use your favorite sharpening procedures and features in a separate procedure.
Speckles, Dust and Scratches
Like Reduce Noise, two other Adobe Photoshop mainstays of pixelation control operate on 8- or 16-bit images. Despeckle, which includes no onscreen controls, looks for the edges of image detail -- defined by abrupt transitions between colors or luminance levels -- and applies blur to the rest of the document or to a selection or individual image channel. Dust and Scratches includes two sliders that raise or lower the radius and threshold values to which the filter applies. Radius defines how far Photoshop looks beyond each pixel to find image areas it considers different from their surroundings and therefore suspect. Higher radius settings increase the strength of the filter and can yield unrecognizable mush at extreme levels. Threshold establishes the extent to which pixel values must differ before the filter applies to them. Lower settings yield more blur; settings below 129 provide finer incremental control than those above. To limit Despeckle and Dust and Scratches to specific color channels, open the "Channels" panel and deselect the channels you don't want to filter.
It's All a Blur
Many image-correction operations reduce pixelation at the expense of image detail, either as part or as the intent of their operation. Adobe Photoshop's Gaussian, Smart and Surface Blur filters dominate the retoucher's roster of adjustable, "clinical" image softeners that go beyond special-effect treatments. Apply Gaussian blur in radius increments of 0.1 pixels, with greater effects at higher values. The result resembles a near-sighted person's uncorrected view of distant scenes. The filter's drawback lies in its tendency to operate beyond selection boundaries and its inability to distinguish details. Smart Blur includes radius, threshold, quality and mode settings. The first two options operate on the same basis as their equivalents in Dust and Scratches. The Mode drop-down menu switches among Normal, Edge Only and Overlay Edge. Edge Only and Overlay Edge turn sharp contrasts into what look like black-and-white or white line art. The Surface Blur filter locates and retains edge details, with radius and threshold settings that determine blur extents, and can clear up noise in solid-color areas.
Information in this article applies to Adobe Photoshop CC 2014, Adobe Photoshop CC and Adobe Photoshop CS6. It may differ slightly or significantly with other versions or products.