I am presently trying to cope with the sharpening strategy and came across the interesting discussion started last January by Colin (When/How to Best Sharpen a Digital Photograph).
My present workflow consists in developing raw files with Capture One and post processing them in Lightzone. My general strategy is to develop 'flat' (checking the 'linear' option instead of keeping the 'Standard Film' option) and to apply a tonal curve later in Lightzone, together with local contrast enhancements using an USM tool at various radius - say, 8, 24, 64 as an example. Of course, there is a capture sharpening in C1 of the 300 /.7 / 4 type. I always apply small radius before large radius, to avoid strengthening halos.
My understanding of the creative sharpening mentioned in the discussion is that it has more to do with local contrast enhancement than with actual sharpening, starting with a radius of 2.3 for example at an amount of 53/500. Whether it is sharpening or contrast enhancement depends actually on the subject. If the latter consists on large homogeneous surfaces, it will be a sharpening. If you have a lot of grass in the foreground, it will be local contrast enhancement...
The issue I am struggling with concerns the necessity of being selective, when sharpening or enhancing contrast. I usually exclude the sky, at least at small radius USM, but sometimes i am too lazy to do this and there it happens, I get strong dark and light borders at the frontier between that rock and a brilliant haze in the sky. Selective sharpening was also mentioned in the discussion, this is actually a critical point (time factor multiplied by the limits of the computing resources at your disposal...)
But I certainly will be lazy again in the future and wonder why the USM algorithms did not evolve to cope with that difficulty. I came across various masking techniques, applied either to sharpen selectively borders or on the contrary to exclude identifiable borders from sharpening. Would it not be easier to add a parameter, namely an upper threshold, avoiding sharpening where the differences in luminance excess a certain value, say 100 or 120? It would be much easier to avoid sharpening artefacts where they appear first, namely at luminance frontiers, which are clearly identified anyway? Lightzone does not have the flexibility of PS, but you know, limitations sometimes also makes one to be creative.
Yesterday, I was confronted to an output sharpening issue. I print using QImage, a specialized program, so I let it output sharpen my pictures to its taste; the default setting is actually 'moderate'. Now, I had a picture, which I re-edited several times to moderate those sharpening artefacts at the transition between rock and sky. They did not go totally. At last I made an additional print setting the output sharpening by QImage to zero and there it happened: the double halo vanished. So, even a moderate output sharpening can have devastating effects if it is not closely controlled. Actually, QImage tells nothing about what values are applied when you select that 'moderate' sharpening...
Most of my printing from a 10 Mp camera is on A4 at 360 or on A5 at 720 PPI, meaning native resolution. If I understand correctly Colin's comment on January 21st, I don't actually have to output sharpen. Output sharpening is important after downscaling.
Is it also after upscaling, say to 250 PPI?
P.S. I already ordered Fraser's gold mine, thanks for the reference!