Have you worked on an image only to find that grasses and hair resembles dried straw and that parts in your picture are surrounded by white halos. If so, then it is probable that at some stage you have oversharpened your picture. Although some degree of sharpening is essential in any digitally printed image it is easy to overdo it and some understanding of the sharpening process is useful to obtain the best from your print.
Digital sharpening is an illusion. The computer can degrade an image to simulate an out of focus picture but it cannot refocus the original image taking lens to produce a sharp image where an unsharp image existed previously. Digital sharpening increases the contrast at contrast boundaries in an image. For example, in a picture of a lamp post taken against the sky, you will see a contrast boundary where the post meets the sky. The post will appear darker than its surroundings and there will be an abrupt change to a light sky at the outline of the post. Digital sharpening will make some of the pixels on the sky side of the boundary lighter and those on the post side darker. The eye perceives this as an increase in sharpness. In reality you have produced adjacent dark and light lines around the lamp post. If this is taken to its limit, the light lines become white and the dark lines become black. At this stage, the black and white lines [called artifacts) are visible to the eye and the picture is over sharpened.
There are a number of steps that you can make to avoid over sharpening.
1) Always apply your chosen sharpening method with "actual pixels" selected and move around the image to check all parts. By clicking on the hand tool found in the tool bar you can click on the actual pixels tool button in the top menu bar and use the hand to move around the image. In this way you can view every pixel in the image that will be printed and will notice any black or white artifacts.
2) A method to give you slight control of your sharpening is to save your layered image and then flatten by selecting layer and flatten image Make a copy of the flattened image and apply your chosen sharpening method to the copy. You can now adjust the opacity of the sharpened layer until you are satisfied with the effect.
3) If you chose to use the "high pass filter" sharpening method described elsewhere on the web page under Digital Tips you can adjust the opacity of the top high pass layer to modify the level of sharpening. As an alternative, you can make a selection of the areas that you wish to sharpen and by right mouse clicking select "layer by copy". If you now apply the high pass filter, only the selection will receive the sharpening effect .
In my opinion, a better way to sharpen an image is to work only on the edge details. The method described is favoured by Davis Rowley and with a little effort enables you to sharpen an image without introducing noise into the smooth areas such as a sky or areas of featureless colour. Before starting, turn on the layers and history palettes . If you do not normally have these active, then select them from windows drop down menu.
A) Reduce your image to a single layer or background
B) Make 2 copies of the image by either selecting layer>duplicate layer twice or by opening the layers palette and dragging the active layer into the icon next to the dustbin at the extreme bottom right of the palette. Do this twice. In the layers palette you should now see 3 identical layers.
C) With the top layer active select Image>Adjustments>Desaturate to remove all colour from the visible image.
D) Select Filter>Stylise>Find edges. This will reduce the visible image to black lines on white or dirty white.
E) Select Image>Adjustments>Levels and drag the white slider [the small white triangle at right hand side) until you produce distinct lines on white. It is only the black areas that you are going to sharpen so if there are any areas that you do not want to sharpen then paint out the relevant lines using a small brush size and colour white selected. Examples of this would be distant wavelets in a water scene or unflattering facial lines in a portrait.
F) When you are happy that only areas of detail that you want sharpened show black detail lines, open the Channels palette . Hold down the control key and left click in the RGB icon at the top of the channels palette. By methods known only to photoshop you have now made a selection [ seen as marching ants) of all areas from white to mid grey.
G) Goto Select>Inverse to make the marching ants selection the mid grey to black lines ie marching ants showing the areas that you want to sharpen.
H) Open the layers palette and drag the top layer to the dustbin icon at the bottom right. The original colour image will re appear but with the marching ants still showing
J) Select Filter>Sharpen> Unsharp mask. You can safely increase the sharpening amount to 175 or above since you are only working on edge details
K) Select OK and then Select>Deselect.
L) You should be viewing the image at actual pixels. Open the layers palette and turn off the top layer by clicking on the eye. You will now see the unsharpened image for comparison.
M) Click back on the top layer and adjust the opacity of the layer to achieve the desired level of sharpening
Any Questions then please speak to Bruce on a club night.