Sharpening Intro

(I will go over this in class so if this sounds confusing, wait)

Why?: To counteract the blurries in both input and output stages.

With What?: Unsharp Mask
It sounds like the last thing you would want to do but it actually makes sense; it has its origins in a traditional photographic technique for enhancing sharpness.
The things we see as edges are areas of high contrast between adjacent pixels. The higher the contrast, the sharper the edges appear. So to increase sharpness you need to increase the contrast along the edges.
What happens is we make the lights lighter and the darks darker along the edges thus creating a halo. The halo is the secret to successful sharpening and the Achille’s heel depending on the size and intensity of the halo, and where it appears on the image. Photoshop lets you precisely control the size of the halo but there is no single magic setting that works for all images: so you need to know how he control works, but also what you are trying to achieve in the image.

Amount
High amounts produce intense halos with many pixels driven to pure white or deep solid black. Amount has no effect on the width of the halos.
As you increase Amount settings, the blips around big tonal shifts (edges) can be pushed all the way to black and white. At that point, increasing Amount has no effect whatsoever – you can’t get more white than white. Worse, the all white halos often stand out as artifacts and can look really dumb.
Photoshop experts recommend starting by setting Amount much higher than you eventually want it – between 400 and 500 – until you see the Radius. Then adjust down from there, depending on the image.

Radius
Radius is the first thing to consider when you are setting up sharpening: it sets the width of the halo that the filter creates around edges. The wider the halo, the more obvious the sharpening effect. Choosing the correct Radius is probably the most important choice in avoiding unnaturally oversharpened look, and there are several factors to take into account when you choose, starting with the content of the image itself, the output method, the intended size.
Note that a Radius value of 1.0 does not result in a single pixel radius. In fact, the halo is often between four and six pixels wide for the whole light and dark cycle – two or three pixels on each side of the tonal shift. However, it varies in width depending on the content of the image.

Threshold
Unsharp Mask only evaluates contrast differences: It does not know whether those differences represent real edges you want to sharpen, or areas of texture that you don’t want to sharpen. The threshold control lets you specify how far apart two pixels’ tonal values have to be (on a scale from 0 to 255) before the filter affects them. For example, if a Threshold is set to 3, and two adjacent pixels have values of 122 and 124 (a difference of two), they’re unaffected.
You can use Threshold to make the filter ignore relatively slight differences between pixels in smooth, low contrast areas while still creating a halo around details that have high-contrast edges. And, to some extent at least, you can use it to avoid exaggerating noisy pixels in shadow areas. Low Threshold values (0 to 4) result in a sharper looking image overall (because fewer areas are excluded). High values (above 10) result in less sharpening, but often produce unnatural looking transitions between the sharpened and unsharpened areas. Typically start out with a zero Threshold value, and increase only if necessary.