Sharpening and Noise

I once sat through a talk where the speaker repeatedly criticised digital photography for its lack of sharpness. The truth is that digital images are usually two-thirds guesswork anyway, because every pixel only records one colour out of red, green or blue, and the other two colours have to be estimated (based on surrounding pixels). Due to all the estimation, the raw image appears soft. The alternative would be to produce one set of RGB values for every four pixels, making your 16MP camera effectively only 4MP.

The first stage in sharpening up your image is called Capture Sharpening, where complex software tries to reduce the softness of your digital image by increasing the contrast in any patterns it finds. Capture Sharpening is done in-camera, or in your raw processing software. It’s also important at this stage to correctly align the red, green and blue components of your image, which can become displaced by Chromatic Aberration, which also softens the image.

Resizing/resampling your digital images will increase its softness further, and this needs to be corrected. Photoshop has a “Bicubic Sharper” option for resampling images, which works very well when reducing images for the web, digital frames, or projection.

Printing will soften detail, as the image is interpolated by the software, and so you may benefit from a bit more sharpening, the type and strength of which varies with medium, size, and viewing distance.

This all comes under the umbrella name of Output Sharpening. Many people have their own techniques for output sharpening, but lazy people, like me, prefer to use Photoshop plug-ins designed especially for the job, like Nik Software’s “Sharpener Pro”.

Sharpening is all well and good, but it has its downsides. Any noise in your image will also be sharpened—effectively emphasising it. Consequently, it’s a good idea to apply noise reduction to your images before sharpening. Unfortunately, noise reduction has a tendency to “throw the baby out with the bath water”, and reduce image detail too. However, there is another way…

Assuming you have been careful to not capture too much noise in your image in the first place, you can selectively sharpen it, emphasizing detail, and avoiding less detailed areas where noise is usually more apparent.

This is easy in raw conversion software. You can adjust masking controls to reducing sharpening in large plain areas of surfaces and sky, whilst increasing sharpening in more detailed areas. You can also control the size and texture of details which are sharpened. The benefit of this selective capture sharpening is the need for less noise reduction, if any.

If you don’t shoot raw, all is not lost! There are clever methods you can employ to selectively sharpen your JPEGs in Photoshop. One Photoshop method recommended by the enthusiasts simulates raw capture sharpening. First, view your image at 100% zoom. Duplicate your image to a new layer and run it through the High Pass filter (Filter > Other > High Pass), being careful to adjust the Radius slider to emphasise detail only. When you’re happy, press OK, and change the new layer’s blend mode to Overlay, Soft Light, or Hard Light, depending on the results, and reduce its Opacity to taste. This effectively increases contrast, but only in the details.

For a more advanced technique, try this:
www.earthboundlight.com/phototips/image-masking-with-itse…

Keith Nuttall, 2011

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