Hi Chris,

>> Are there any rules of thumb for the percentage of USM (Photoshop CS) to
apply to digital images <<

No, this is very much dependent on image content, output method,
reproduction size, original size in pixels etc.


>> and is it best to add this after or prior to CMYK
conversion? <<

It all depends.

Sometimes in RGB with a fade or blend to luminance mode. Sometimes the L of
LAB mode produces better visual results for some images, despite the
arguments against using it.

Often sharpening the K channel once in CMYK, even if previous USM was
applied.

Sometimes no RGB sharpen, then CMYK - perhaps only with the CK sharpened for
a portrait where skintone artifacts were an issue in the MY plates.

And with many other methods and variations:

http://members.ozemail.com.au/~binaryfx/ST_USM1.html

http://members.ozemail.com.au/~binaryfx/PSTV_links.html#S (sharpening and
USM links)

http://members.ozemail.com.au/~binaryfx/PSTV_photocontrol.html (PC only)

http://members.ozemail.com.au/~binaryfx/PSTV_sharpcontrol.html (PC only)


Regards,

Stephen Marsh.

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