SHARPENING PICTURES IN PHOTOSHOP

Most of the digital camera users would have observed that their images tend to be soft when viewed at full size. Having migrated from film and having seen pretty sharp pictures even at slightly larger prints, one would expect a similar quality in digital output. But more often than not, the digital output will be fairly soft and will not have the sharpness desired, especially so if you are viewing raw images.

The solution is to sharpen the image after acquiring it. Most of the digital images are usually sharpened digitally at some point of their lifetime. It could happen as an in camera manipulation, in your raw processing software or in the image processing software. The sharpening filter is called unsharp mask or USM.

Applying sharpness on an image may seem to be a fairly simple task of running a command. But doing it right takes effort. In Photoshop, you need to adjust 3 parameters to sharpen an image. And there are no easy rules to find the exact values of these. The values could vary widely based on the image being processed. Only a proper understanding and experience helps to arrive at a proper setting.

HOW DOES IT WORK?

A picture is usually perceived as sharp if the edges in the image are well defined, reasonably crisp and have a good edge contrast. A soft picture would have a smooth transition between edges and does not have a clear line of distinction. When you apply sharpening on an image, an attempt is made to increase the edge contrast in the picture. This creates well-defined edges and hence making the image to appear sharper.

The USM filter detects the edges in the picture, and then creates halos around the edges. The halos are created such that darker side of the edge is made more dark and the lighter side of the edge more light. These halos increase the edge contrast and hence make the picture appear sharper.

In Photoshop, applying unsharp mask requires adjusting 3 different parameters. Good understanding of these parameters is essential to get good results in the sharpened image. Too little USM may not add much to the sharpness of the picture. Too much USM can add lot of grains to the picture and makes the edges brighter than rest of the picture making it look unreal.

UNSHARP MASK IN PHOTOSHOP EXPLAINED

Open Unsharp Mask Window by clicking Filter >> Sharpen >> Unsharp Mask. This opens the window shown below. It has 3 sliders - Amount, Radius and Threshold.

The Radius field is used to specify the size of the halo. The larger the specified radius, larger will be the halo produced at the edges. Keeping this in mind, it is good to use a small radius for a picture with lot of details - such as feathers of a bird. A larger radius could be useful for images that do not have sharp edges and needs a smooth transition. Pictures such as foggy mornings or sunsets do not have and do not require sharp edges or fine details and these are better off with slightly higher radius.

The threshold field indicates what part of the image should be considered an edge for applying USM. A threshold of 0 indicates every edge to be considered a candidate for USM. A higher threshold means that the edge contrast needs to be higher to consider that edge for applying the filter.

The Amount field indicated the amount of contrast to be applied on the halos created. A higher value amount means dark edge will be made darker and lighter edge is made to look lighter.

APPROACH FOR SHARPENING

First thing I do is to view the image at 100% and set the visible area in the screen to most prominent part of the picture. The USM window provides a 100% preview but it is not much help, since you don’t get to visualize the effect on the entire image. I turn on the preview checkbox so that I can see the changes while I adjust. Then, I adjust the 3 parameters to my satisfaction. Once done, I check all the areas of the picture for problems such as grains, prominent edges or soft areas that need further improvement. Then I zoom out the image to fit the window and look at the effect of USM on the entire picture. If I find any of the problems I mentioned above, I go back and re-work on the parameters.

The ideal sharpness parameters can vary based on the nature of the image. The first thing I would try to set on any image is the threshold parameter to decide which edges are suitable candidate for USM. I set the amount and radius to moderate values(say 125% and 1.25) and decide on a value for threshold. If the image has fine details like feathers of a bird or any such small patterns, it could mean that a good part of the image needs sharpening. So I set a low threshold to cover most of the edges. I first try to set it at 1 and increase or decrease a little to see the effect and continue to adjust accordingly. For a landscape picture such as picture of sky or setting sun which do not have sharp edges or small details, I start with a fairly high threshold - at around 4 to 7 and then adjust until I get the perfect sharpness desired. Too small threshold could mean attempting to sharpen the minor tonal difference in sky that could result in undesirable grains.

The next parameter to set is radius. This can be adjusted in a manner very similar to threshold. For pictures with fine details, I start with a small radius of 0.75 to 1.25 and continue to adjust till I get good output. For other images, I start with a radius of around 2. For amount, if the picture has high contrast, I start with a setting of 150 to 175. For images with low contrast I begin with 100 to 125. After setting all the values, I go back and see if any improvements can be made by slight changes in threshold again. If yes, I follow it up with changes in radius and amount. Two iterations should be sufficient to get best results. It is necessary to see that the final image is just sharp enough to please the eye, and does not induce grains in plain areas or edges do not look brittle, rigid and seem to have unusually high contrast. It is good to observe the image at full size as well as fit to window and ensure that the image looks fine in both cases.

Though this procedure works for me most of the time, it requires some experience, time and effort to get best results. While this approach has worked for me, some photographers prefer to start with setting the radius first. Choice of such methods may defer from person to person. It is to be kept in mind that USM is only useful to restore the sharpness lost in the digital workflow of capturing and processing the images but it can't be a work around to salvage out of focus images or images suffering from camera shake.

USM must be applied as the last step in image processing to offset any softness introduced during the workflow. Like in most post processing steps, it is normally recommended to use adjustment layers and lab colors when applying USM. It is also recommended that if you resize a picture, apply USM again on the resized image.

RESULTS

Finally to compare some results. Seen here is an image of a little grebe. The first picture you see is with no sharpness applied at any point of life of the image.



I applied USM on this image with amount 75, radius 1.25 and threshold of 2. I followed the procedure explained above. You can see that the picture sharpness has improved considerably. The feathers are now much more clearly visible.



The sharpness in the above image looks natural. But applying too much sharpness makes the image look like the one below. You can see that it looks unnatural and ugly. In some cases, it may also induce grain in areas that have mild contrast such as sky tapering off from bright to blue.