Sharpening in Elements part 2

Let's return to our Unsharp Mask dialogue box and to the matter of the parameter values. There are several considerations which, for me, influence the Unsharp Mask parameter values. They are:

1. The overall condition of the image
2. The amount of noise in the image
3. The subject style of the image
4. The final intended image display format

IMAGE CONDITION

If we are confronted by a genuinely POOR image, unless the content is important and unable to be re-shot, I am inclined to reject it altogether. Experience has taught me that, beyond a certain point, it will always be a matter of rubbish in - rubbish out.

On the other hand it seems clear that if a sooc (straight out of camera) image happens to be well nigh perfect (a somewhat rare event) SOME sharpening will always be a good idea, but not very much. When I say well nigh perfect I mean:

• No discernable camera movement or out of focus softness
• No readily discernable noise
• No significant adjustment of exposure required
• No white balance adjustment required

There is ample evidence that Amount values over 100 and Radius values over 1.0 can allow some sharpening halos to become visible under close examination. Sharpening halos (i.e. previously described highlight lines along edges) are always present, but in some parts of an image they will be more noticeable than others, particularly as halo lines become brighter and/or wider.

For a full sized near-perfect sooc image which will only ever be viewed on a computer screen and likely to be scrutinised by a pixel peeper, the best Unsharp Mask values could easily be as follows

Amount: 100
Radius: 0.5
Threshold: 0

IMAGE NOISE

If an image has been shot at high ISO values, it will probably show noise, especially in shadow areas. If the image is underexposed and has to be brightened, the noise will be enhanced in the process. If colour saturation and/or white balance has to be adjusted, chrominance (i.e. colour) noise will be further elevated. In sharpening the image, we have to avoid making the problem still worse.

If the noise is only really visible under pixel peeping and the final form of the image will be (say) a 6x4 inch print, noise reduction may do more harm than good. During sharpening we should set the Threshold value to about 5. When the Threshold is 0, virtually everything in an image will be regarded as large enough to have an "edge" around it and thus be deserving of a sharpening halo. Because of this, masses of tiny noise artifacts will effectively be enlarged and made more noticeable. As we increase the Threshold value, progressively larger objects (most especially the noise) are omitted from the sharpening process. Thus important features of the image receive sharpening while existing noise artifacts should be made no worse.

Unfortunately the subjective effect of increasing Threshold values is to reduce the overall impact of the sharpening process. Accordingly it becomes necessary to bump up the Amount and Radius values to retain the visual impact of having sharpened an image while avoiding noise enhancement. So for a somewhat noisy but otherwise acceptable image, Unsharp Mask parameters might well be:

Amount: 180
Radius: 0.9
Threshold: 5

IMAGE SUBJECT STYLE

Once again, let us assume that our sooc image is basically excellent but we still want to overcome its anti-aliasing filtering and incomplete camera sharpening. The parameter values may be influenced by the subject style of the image itself.

Let us first imagine a truly complex mega-edge image such as a garden with masses of shrubs and trees with many many thousands of leaves, petals, twigs, compost bits, pebbles, blades of grass etc etc. A lot of these features start to assume the size of the noise particles in our last example. If we set our Threshold value too low, the image is made more muzzy by the thousands of tiny halos. It becomes actually harder to identify objects in the image than it may have been before. Unless there are some features of the image that are less edge dense, there may be a sound argument for not sharpening at all. However it is usually true that some features of such an image - such as people or statues etc (which are indeed less edge dense) may indeed benefit from sharpening.

It may be best to retain a high Threshold value so as to leave the millions of leaves and gravel pieces in relative peace while still sharpening the people. A low Radius value might also be a good notion, while to compensate, one might consider a slightly higher Amount value.

Amount: 200
Radius: 0.5
Threshold: 5

Some image styles are inherently short of prominent edges and the edges which DO appear, might well be low contrast in nature. Such images might include close ups of faces with large expanses of fairly feautureless skin or close ups of flower heads in which the main features are delicate fibrous patterns in petals and large leaves.

In such cases we may WANT to highlight fine texture and may WANT to emphasise subtle edges. You may like to try parameter values such as:

Amount: 200
Radius: 1.5
Threshold: 1

INTENDED IMAGE DISPLAY FORMAT

Once upon a time, the vast majority of images captured by cameras were either intended for privately viewed prints or commercially published transparencies ... and that was basically that.

These days the situation has changed. Vast numbers of images may never reach paper. Most pictures taken for private/family consumption will never be published, never even be printed for display in frames and albums. Indeed most will never be anything other than digital images for display on computers, electronic display frames and/or television sets. Even then, many will stay full size while vast numbers will be resized down for use in emails or on web galleries.

Small prints, large prints, published prints, slideshow images, resized images. All of these intended uses embody different considerations when post processing. The issue often requiring the most thought will be sharpening. Indeed many photographers will not sharpen their processed images AT ALL until final intended display formats and applications have been determined. They will then sharpen particular versions for particular applications

Leaving aside the considerations already discussed, these are some VERY GENERAL suggested parameters for particular display applications:

Prints in general will benefit from extra sharpening as the process of printing will tend to rob many images of their sharpness, while noise seems generally less prominent in prints than it might be on a computer screen:

Amount: 200
Radius: 0.7
Threshold: 0

Images resized down for email or web gallery display will benefit from increased radius. Halos which might have been visible full size, may not be when everything has been proportionally reduced. Assuming you sharpen an image BEFORE scaling it down, the parameter values might be:

Amount: 200
Radius: 1.5
Threshhold: 3

And so on and so on. In the end, each of us has to make a decision based on many considerations. There will never be ONE correct set of values in every case. Experiment for yourself based on the suggested values. In the next article we'll examine an approach for more accurately customising our sharpening for individual images.

See my work at: www.pbase.com/davidhobbs