Images from digital cameras usually need a little sharpening. That’s because digital cameras place a filter above the sensor which softens the photo. This is to hide the grid pattern of the sensor array which would otherwise show up in the final picture. It is possible to add some sharpness in the camera menu, but it is usually far better to do it once you have the file in the camera. The in-camera option treats every photo the same way, no matter how different they might be. Another reason to do your sharpening in the computer rather than in-camera is that it is reversible... apply too much and you can undo it.
Setting aside one-step photo fixes that all editing programs offer, the main tool used to sharpening photos is a filter called the “Unsharp Mask” tool. Odd name for a tool to sharpen images... it uses digital technology to simulate something that film photographers used to do to make blurry pictures look sharper. They took a slightly out-of-focus duplicate negative and sandwiched it with the picture they wanted to sharpen.
That in itself was based on a CMYK printing technique: when a colour print or transparency is separated into Cyan, Yellow, Magenta, and black dot patterns, and a slightly unsharp positive image of the original is sandwiched with it, the apparent sharpness of the reproduction increases.
Unsharp masking (USM) increases the contrast between different tones and colours in an image, but only in a very narrow band on either side of edges in the photo. The darker side is made a bit lighter and the light side a bit darker. It’s an optical illusion, but a very useful one that makes us think the image is sharper. By the way, since it is actually contrast that is boosted rather than sharpness, you might like to try increasing contrast before you try sharpening.
USM is not an all or nothing approach. When it was introduced to drum scanners, the scanners were programmed to detect tonal transitions and then add a bit more or less halftone dot size to emphasize the transition lines (edges). The term "radius" was used to describe how many dots on each side of the transition the sharpening was applied to, with a radius of 1 equalto half the width of a halftone dot. The term "threshold" was used to describe how much contrast between tones was necessary before sharpening action was applied.
With Photoshop, Adobe adopted and carried forward the names "unsharp masking", "Radius" and "Threshold" and added a third value "Amount" to control how aggressively the USM was applied. PaintShopPro uses the terms “Radius”, “Clipping” and “Strength”.
Radius specifies how close together pixels have to be before they are compared for sharpening (the radius within they have to coexist)
Threshold/Clipping controls how much contrast there has to be before sharpening is applied. The greater the threshold the more contrast or difference there must be before sharpening is applied to that area (0 = everything is sharpened 500= nothing is sharpened). You might use 5 to sharpen black eye lashes without affecting surrounding skin tones.
Amount/Strength controls the overall strength of the command (obviously).
In practice, you have to choose between using a large amount with a small radius, or a moderate amount with a large radius, otherwise the image will be degraded (over sharpened). The small radius/large amount method doesn’t work so well for printing because of the way inkjet printers layer ink, but it is excellent for screen images. The large radius / moderate amount approach works better for printing because it provides wider “white” areas where tone has been artificially reduced to add contrast, and these are less likely to be lost to ink bleed into porous paper.
There are a few problems with USM the first being that it doesn’t work with Adjustment Layers, and the second is that there are some images that USM is just not suitable for:
Images that have any visible noise, which, unless you have a top-line camera means anything shot at ISO 400 or above
Photos that needed a lot of exposure correction, histogram or brightness or contrast adjustments.
Pictures that have been highly compressed (jpeg)
Well exposed pictures that contain expanses of flat colour, like featureless skies.
No matter what settings you use with pictures like these, if USM works on the areas you want it to, it will oversharpen the pixels you don’t want. Visible noise will be multiplied, jpeg artifacts with become gross and the plain sky will become a textural mess. In the last example, you could duplicate the layer, apply USM to the top layer and erase the sky, but for the other examples, you need an alternative method.
The key to sharpening in these problem pictures is called “High Pass” Sharpening. In PaintShopPro it is the default method and works in the same way as USM: select it from the Adjust Menu and you are offered 3 controls: Radius and Strength just as with USM, and a drop menu labelled “BLEND MODE”. The first two controls have the same meaning as in USM. The Blend modes are a subset of the Blend modes associated with Layers: three choices are offered: Overlay which removes the neutral tones in the image and makes edge details stand out; Hard Light which produces greater contrast than Overlay; and Soft Light to produce a softer-looking photo.
High pass sharpening in Photoshop is a bit more complicated. First, you'll need to duplicate your image layer. You want the new layer to be the top one. Set the Blend Mode of the layer to Overlay (after the last step, you should try the other options offered in PSP: Hard Light and Soft Light). Don’t worry when the image turns darker and the contrast goes through the roof - it is supposed to at this stage.
Now go to Filter / Other / High Pass and set the radius slider to a value that makes the image look just a bit too sharp. The final step will fine tune the effect depending on whether the image is being sharpened for the screen or print. Your image preview in the filter dialog should be mostly grey with detail only showing where the image has edges; if not, you have the radius set too high. Click on "OK" when the main image looks just a bit too sharp.
Finally, use the Opacity slider on the High Pass layer in the Layers Pallet to fine tune the effect. If you've set the radius correctly, an opacity of about 50%, should work, but 25% to 75% is not uncommon.
In Overlay, Hard Light and Soft Light blend modes, medium grey has no effect on the underlying image. Since the High Pass filter converts the image mainly to flat grey, these blending modes concentrate the sharpening effect in the areas where detail does show which is the edges.
In Photoshop the difference in outcome between these method for most images is slight and perhaps not worth the extra effort for all but the most troublesome photos, although if it appeals to you, you can record it as an Action and just play it back when you need to). In PaintShopPro however, it is so easy to apply that the choice is easy, and High Pass Sharpening quickly becomes the norm for any image.
INTEL CONTRIBUTOR
SIGN UP
