Pup wrote:


This is good advice for images that are to be seen onscreen.

However there are other ways to use unsharp mask, to achieve other purposes.

One: when making large prints, often a larger Radius (1.5-3 pixels) produces a better print. Radius controls the 'halo' effect: how far the sharpening of a pixel boundary goes. The higher Radius halo may look a bit strange on a screen, but once through the mechanical process of printing, often produces a sharper looking print.

Two: you can use Unsharp Mask to improve image contrast and dynamics. Again you're using the Radius halo effect to advantage. With this technique you use less sharpening Amount - ie. 10% or thereabouts, experiment with this. But you set Radius to a large value: 20-100 pixels or even higher (experiment). The effect of this is to add contrast (that halo effect) locally across say 30 pixels. The result can be subtler -and more image-related- than the curves adjustments. When it works well the image takes on a luminous quality.

I haven't found a clever way to use the Threshold adjustment for other purposes but it can be useful to control texture smoothness and to limit where sharpening takes place. Basically, a higher Thershold the greater the contrast between pixels has to be before it's sharpened.

A higher Radius / lower Amount is also useful with low contrast images, for adding a bit more definition... the lower Amount is to keep the halos from becoming overy bright.

The Threshold adjustment is classically used to retain smoothness in areas where too much sharpening isn't wanted, ie. skin tones. Adjust upwards till the skin is as smooth and the blemishes aren't accentuated...

I really like the idea of using USM to control contrast, Ness. 1.5-3 radius for print is a completely new one to me though: I'm intrigued - if it looks odd on screen, how do you judge what the optimum radius for printing is?

Heh, proofs! I think in part what's happening is inkjet spread tends to hide sharpening, so a bit of an extra keeps the pop. There are software sharpening packages out there that advertise optimization to printer/size/paper automatically. I got the tip from that book I've mentioned, Mastering Digital Printing.

Would you believe, I don't have a printer? :-) Figured I could never get close to the kind of quality that good on-line labs with their £15,000 Fuji Frontier printers do... If I ever see a cheap Epson R2400 though, I'll know what to do! :-)