sharpening

[the_incubus]

When ypur doing PP in any program and you sharpen a pic what does it do? I obviously know it makes the pic look sharper (i may be dumb but im not stupid :)) But what does it do to add that sharpness. Also, when you just keep sharpening and heavily over-sharpen you see it get all pixel-y. Why does this happen (just curious).

thanks

[Anke]

Top of my head, it increases the contrast between light and dark edges. I'm sure someone will fill you in more.

[Tee Why]

I believe you are right.

[StewartR]

This Ron Bigelow tutorial (http://www.ronbigelow.com/articles/sharpen1/sharpen1.htm) will tell you everything you want to know about sharpening, and more.

[Quad]

It increases contrast by adding little halos of dark/light on the areas where two different brightness values occur. Increasing sharpening can take the form of increasing halo size or increasing the contrast between the dark/light halos, also how small an area is before sharpening kicks in. It can emphasis unpleasantries such as noise and pimples.

The light halos seem to stand out more and it is possible with masks to control the light halos separate from the dark halos and all sorts of other things like protecting uniform color areas like skies from any sharpening (thus controlling noise in those areas).

Most digital captures need some sharpening since the image is blurred slightly by the AA filter to deal with other problems so don't fear it but be gentle in its use as it does not make handshake and lack of focus go away. Maybe it should not be called sharpening at all but anti-anti-alias filter filtering.

I probably got something wrong here but that will bring out the experts to correct me and explain it to you better.

[Doug Pardee]

When ypur doing PP in any program and you sharpen a pic what does it do? I obviously know it makes the pic look sharper (i may be dumb but im not stupid :)) But what does it do to add that sharpness.

There are all kinds of sharpening, but the general effect is to increase the modulation transfer function (MTF) at higher resolutions.

There, aren't you glad you asked? :rolleyes:

The MTF is a theoretical function that describes how contrast that was in the original scene is lost on its way to the print. Of interest for this purpose is the resolution, or how fine the detail is. As a general rule, the higher the resolution (the finer the detail, that is) the more contrast is lost. At some resolution the contrast shrinks to where white and black both look the same shade of gray.

Most sharpening techniques are aimed at increasing MTF at high resolutions by increasing the contrast in fine details. Overdoing it—increasing the MTF beyond 100% so that the contrast in the picture is higher than the contrast in the original scene—can introduce halos and other objectionable artifacts.

A related technique, local contrast enhancement (http://www.lonestardigital.com/photoshop_quicktips.htm), is performed by increasing the MTF at low resolutions.