Sharpening

Vincent Bockaert, 123di.com

There are two types of sharpness and it is important not to mix them up. Optical sharpness is defined by the quality of the lens and the sensor. Software sharpness will create an "optical illusion" of sharpness by making the edges more contrasty. Software sharpening is of course unable to create detail beyond the camera's resolution, it will only help to bring out captured detail.

Original
Magnified crop (2X)
Comment Soft edges before sharpening Sharper edges after sharpening Over sharpening results in halos

This simple example shows that normal sharpening creates cleaner edges than the original. Over sharpening makes the circle look artificially sharp. This is achieved by creating a white external halo (making the light gray of the background brighter around the circle's edge) and an internal black halo (making the darker gray of the circle darker around the circle's edge). Because the difference between the white and black halos is larger than between the gray of the circle and the background, the edge contrast has been increased, creating the illusion of enhanced sharpness. But the halos are undesirable in photographic images and are extremely hard to undo, unless you shoot in RAW (see below).

In-camera Sharpening

Digital cameras will, as a part of their default image processing, apply some level of sharpening, to counteract the effects of the interpolation of colors during the color filter array decoding process (which will soften detail slightly). Note however that too much in-camera sharpening will create sharpening halos and increase the visibility of jaggies, noise, and other image artifacts. Prosumer digital cameras and digital SLRs allow users to control the amount of sharpening applied to an image, or even disable it completely.

Sharpening with Software

If the camera allows you to shoot in RAW, the in-camera sharpening can be undone via software afterwards on your computer. You can then decide the level of sharpening you want to apply in order to avoid the above sharpening halos and depending on the purpose. For instance for web or monitor viewing purposes you may want to apply some sharpening to "pull out" fine details of downsampled images. For printing, sharpening should be applied with caution to avoid the image looking fake and over-processed.

If you shoot in JPEG it is recommended to apply some in-camera sharpening (e.g. "Low" or "Normal") because with regular software, it is not so easy to achieve the same sharpening quality level of in-camera sharpening. One of the reasons is that in-camera sharpening is applied before JPEG compression, while sharpening on your computer is done after JPEG compression, thereby making the edges of the JPEG compression squares more visible. If the in-camera sharpening was insufficient, you can still apply some additional sharpening with software. This is much easier than to undo the effects of over sharpening.

This article is written by Vincent Bockaert,
author of The 123 of digital imaging Interactive Learning Suite
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