If you read my last two blogs on sharpening (see Older Blogs), you will have learnt that packing more pixels into an image does not make it look sharper. The perception of sharpness is about how well edges or boundaries are defined in the picture. You learnt also that the manipulation of edge definition is not new; it has been used in nature for advertisement and deception since the time animals developed organs that could make sense out of light.
You know when you take a shot that looks fabulous through the viewfinder but it looks crap when you see it on your monitor. It's because the way we see is selective and is coloured by our preconceptions and what we want. Cameras don't see the way we do and neither does editing software. In fact they can't 'see' at all. They just convert captured light patterns into numbers, and then crunch the numbers according to your will.
So if it can't see, how does an editing program find the edges in the image? Quite simply, it compares the tonal values of adjacent pixels (how light or dark they are) over the whole of the image. Where the difference in tone is bigger than a threshold amount, it calls it an 'edge'.
Once the edges have been identified like this, they can be made more prominent by darkening the darker pixels and lightening the lighter pixels in these adjacent pixels. Edge contrast is increased in just the same way as illustrated in the examples I gave from nature in my last blog.
The amount the tonal values are altered is under your control.
The other parameter under your control is how far from the 'edge' the contrast adjustment is applied. This is measured in the number of pixels from the edge or radius.
For technical reasons, digital cameras generate a soft image and sharpening nearly always has to be applied to counteract this. Cameras add some sharpening by default, so if you want to take full control of how your pictures are sharpened, go into your camera's menu and turn sharpening off or to a minimum.
If you are using a simple editing program like Picasa, you will find a slider that controls the amount of sharpening applied. If you are happy with what you get, that's cool. Carry on with it.
The most commonly used sharpening tool in more advanced editing programs is called Unsharp Mask. Sounds a funny term to use, but it harks back to a technique used in the wet darkroom to make prints appear sharper.
In Photoshop and Photoshop Elements, Unsharp Mask has three sliders: Amount, Radius and Threshold which allows independent control of the three parameters I described above.
Let's illustrate how this works in practice.
An image with no sharpening applied. The picture on the left has been expanded to show what is going on at the level of the pixels - not a lot in this case.
Sharpening in this image has been racked up to 300%. Radius has been increased to two pixels and you can see that the zones of lightened and darkened pixels extends to about two pixels from the edges.
But notice that the sky is beginning to show graininess. This is because the Threshold is set so low that Photoshop is finding edges in smooth tonal areas. We don't normally want this.
By tweaking up the Threshold, we have restored smoothness in the sky but retained sharpening of the other edges.
How to view images on your monitor to apply sharpening. The sample of the image shown in the small inset above the sliders is always at 100%. This means that each pixel of your image is represented by one pixel on your monitor screen, and this is the best screen magnification to use when adjusting sharpening.
You can set your main image to this. Choose the Zoom tool, right-click the image and choose 'Actual pixels'. With a high resolution file, this will make the image too big to fit on your screen. Try reducing screen magnification to 50%. Don't reduce it to 66.7% or 33.3% as the screen resampling corrupts the image too much to assess it properly.
If you are using Unsharp Mask in Lightroom, you will find four sliders instead of three. Amount and Radius are the same as in Photoshop.
One of the sliders is called Masking. This is the same as Threshold in Photoshop. They both, in effect, apply an adjustable mask over the parts of the image with less distinct 'edges'.
The fourth slider in Lightroom is called Detail. As I understand it, this is a different kind of mask that restricts the sharpening effect to those parts of an image having the most detail. If anyone has a better understanding of what the Detail slider in Lightroom does, please add a comment to this Blog.
Sharpening after resizing images. As the Radius is measured in pixels, be aware that if you reduce the size of an image by resampling (e.g. for digital projection), the zone of sharpening will be reduced and you will probably have to add a little extra sharpening. Try Amount 50%, radius 0.3 to 0.5, viewing the image at 100% on your monitor.
For a high resolution image, you will probably need settings of Amount 50 to 300, Radius 1 to 3. Adjust Threshold to just remove unwanted noise in smooth tones.
Take care not to oversharpen. You should not be able to see white halos around edges when looking at the picture at normal viewing distance.
Finally, think of sharpening as seasoning. It helps improve the flavour of a dish without making itself obvious!
A common man marvels at uncommon things; a wise man marvels at the commonplace. Confucius