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In an answer to another question, a statement is made:

Sharpening is the only (? I can't think of any others ?) edit which needs to be done differently depending on the target medium. E.g. you would sharpen a 20"x10" print differently from a 640x480 pixel image for a web page.

What should be taken into account when sharpening for various output methods? How would one sharpen for a print vs. an onscreen image vs. other uses and how does the final image size affect sharpening?

Edit/added: with printing, does the surface/material of the print (paper, metal, canvas, etc) affect how much one should sharpen?

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up vote 11 down vote accepted

It's mostly the resolution of the image that affects the sharpening. When using unsharp mask, a radius of 0.1 mm is a good starting value. For an image displayed on a computer monitor at about 100 PPI, that translates to a radius of about 0.5 pixels.

Then there are also differences in the media that causes the sharpening to have varying effect. A printing processes may for example have a slightly blurring effect, so you would have to over-sharpen the image slightly to compensate for that. Some display screens are a bit blurrier than others (e.g. a CRT screen is blurrier than an LCD screen).

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This mostly describes 'why' but not much of the 'how'. Somethings like CRTs are actually blurry by accident (not by design), so it nearly impossible to compensate correctly, it will vary not only by the monitor but by resolution, age, etc. – Itai Mar 4 '11 at 18:38
@Itai you have two objections. The first one is a little unfair, given that @Guffa described how to compute the amount of sharpening (in pixels) based on the intended output size. Indeed, that--and his second paragraph--addresses your second objection: it's a starting value to be modified according to the inherent blurriness of the output medium. IMO, this answer deserves to be voted up. – whuber Mar 4 '11 at 20:48

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