Adobe Photoshop CS5 for Photographers - Martin Evening [125]
Detail panel noise reduction sliders
The Noise Reduction controls are shown in Figure 4.11. The Luminance slider is used to smooth out the speckled noise artifacts. The default setting is zero, but I think you'll find that even with low ISO captures it is helpful to apply just a little Luminance noise reduction; in fact my colleague Jeff Schewe likes to describe this as the fifth sharpening slider and suggests you always include adding at least a little Luminance noise reduction as part of a normal sharpening process. Excessive Luminance slider adjustments can lead to a softening of edge detail and to help counter this, the new Luminance Detail slider acts like a threshold control for the main Luminance slider, where the default setting is 50. When this slider is dragged to the left you will see increased noise smoothing. However, as you do so, important image detail may get treated as noise and also become smoothed. Dragging the slider to the right reduces the amount of smoothing used and so allows you to dial back in any missing edge sharpness. With the Luminance Contrast slider the default setting is 50. The smoothest results are achieved by dragging the slider all the way to the left. However, doing so can sometimes make the noise reduction look unnatural and plastic-looking. Dragging the slider to the right therefore preserves more of the texture contrast in the image, but at the same time this can lead to increased mottling in some high ISO images. It is also worth pointing out here that the Luminance Contrast slider has the greatest effect when the Luminance Detail slider is set to a low value. As you increase the Luminance Detail, the Luminance Contrast slider has less impact on the overall luminance noise reduction.
Figure 4.11 The Detail panel showing the Noise Reduction sliders.
Is the noise really visible?
Some of the latest digital cameras, such as Nikon and Canon digital SLRs, are capable of capturing images at extremely high ISO settings and although reviewers talk about seeing noise in these captures, I've yet to see much in the way of noticeable noise appear in print, unless you were to print an enlarged detail of what can be seen on the screen at, say, 2:1. The main point I am trying to make here is not to over-obsess when analyzing your images in close-up.
The Color slider works by separating the color from the luminance information and smooths out the color noise artifacts such as the magenta/green speckles you commonly see in noisy high ISO captures. For the most part you can safely crank the Color noise reduction slider up towards the maximum setting. However, increasing the Color noise reduction slider can also result in color bleeding, which will result in the fine color details in an image becoming desaturated. This kind of problem is one that you are only likely to see with really noisy images that contain fine color edge detail, so it's not something you need to worry about most of the time. The Color Detail slider allows you to help preserve color detail in such images and as you increase the Color Detail slider beyond the default 50 setting you'll notice how it preserves more detail in the color edges. Take care when you use this slider, because as you increase the amount that is applied this can lead to some color speckles reappearing along the preserved edges. In areas that have a strong color contrast you can see an over-sharpening of the color boundary edges. To understand more clearly the effect all of these sliders are having, you may want to zoom in to look at a 400% view.
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Here is a photograph that was shot at 3200 ISO