Understanding USM

Sharpening is a critical part of a digital workflow. Having a good understanding the USM filter is essential to enable you to produce the best results possible.

An Introduction

If your picture is muzzy or blurred USM isn’t really going to help. If an image is out of focus, applying some sharpening to it can help improve the overall appearance, but it won’t make it a sharp image. The sharpening tools in Photoshop will not replace good technique. Firstly lets deal with why it is called unsharp mask and what its origins are; I am not old enough to remember the process being used in the commercial darkrooms that I have been involved with, but I know someone who does.

To improve the apparent sharpness of a weak negative (typically the old glass plate negs from yesteryear) Darkroom technicians used to create a copy image by exposing another glass negative in contact with the original image. This would create a slightly soft and slightly out phase positive of the original negative. When the two were then placed together and printed areas where the images coincided there would be an apparent increase in edge contrast, increasing the apparent sharpness.

So by merging a soft image with a slightly out of focus (Unsharp) image you could improve the apparent sharpness. USM works in the same way hence it’s name

Sharpening should be the last process that you go through with your images, after less destructive processes such as levels, balancing, cropping etc. If you are resizing an image (either up or down) this should be carried out before USM is applied.

Any process of digitising an image will lead to a degree of softness, irrespective of whether you have scanned a negative/positive or captured the image using a digital imaging sensor, and the degree of sharpening required is dependent upon several factors, including but not limited to: the size of the image, the detail of the image, the type of imaging sensor, etc.

There are very, very few occasions when sharpening may not be required, usually images that have large areas of continuous tones, commonly sunsets and sunrises, however they often can benefit from a little judicious sharpening in specific areas.

For example the image (above) any sharpening is likely to increase the appearance of noise or grain, however it did require sharpening where the sea meets the sand and where the footprints are. So only the lower half of the image was sharpened to save the distant sea and sky turning to a noisy mush.
Whilst sharpening essential for many images it is important to realise just how destructive the process is, so the best advice is a little at a time. It is also important to work with your images either at 100%, 75%, 50% or 25%, preferably at 100%.

If you are not sure why I recommend this simply open an image that contains a lot of detail on your laptop twice and look at one at 33% and one at 50% the difference between the two is incredible, I am not entirely certain why this should be the case, possibly down to the way that the graphics card handles rendering images but give yourself a head start and work at 100% where possible.

Getting Started
To get started with sharpening using the Unsharp Mask filter, make sure your image layer is selected on the Layers palette and then choose Filter > Sharpen > Unsharp Mask from the menu. This will bring up the Unsharp Mask dialog box, which includes several settings that allow you to control the way sharpening is applied.
There are three areas that can be controlled from the Unsharp Mask Dialogue Box, I shall give a brief explanation of what they do and the range of adjustments available.

Amount: Controls how weak or strong the mask image that is blended in will be. Thus, it controls how much edge contrast will be added and how much apparent sharpness we will get. Range of adjustment from 0% to 500%. The higher the number, the more contrast is added along whatever pixels are determined to be edge pixels in your image.

If set too low, you won’t see any change at all, but if set too high, the inverted halos from the mask will be visible in the final image and it will look artificial.

Radius controls the degree of un sharpness the mask image will have. Thus, it determines the width of the halos the mask will produce. The higher the radius the more evident the sharpening effect will be. Set too low, the effect will be invisible or too subtle to be effective. The appearance of the halo will also vary with the content of the image since the amount of contrast already present will play a role. Range from 0.1 to 250. While large halos are a serious problem in the image, appropriately sized halos add to the perceived sharpness of the image. The radius setting is one of the most critical settings when using the Unsharp Mask filter. Just remember that a little goes a long way.

Threshold It determines how much contrast must exist in order for a given pixel to be considered an edge pixel. Thus, it prevents sharpening of unimportant or incidental detail while reserving the effect for detail that should stand out. It separates signal from noise, if you will. Set too low, sharpening will be applied to every edge or change of tone throughout the image including such things as film grain. If set too high, no transitions will be viewed as being edges and no sharpening will result. Range from 0 to 255. The higher the number, the more contrast must exist in order for a pixel to be considered an edge. In other words, a higher setting means more areas of the image will not have sharpening applied to them.

The settings that you will use will depend on a variety of factors, depending upon what you finally want to do with the image. If you intend to print the image then you will require to sharpen it more than if you are just viewing on screen. The larger the image file (I am referring to the number of pixels rather than the amount of hard disk space it requires) the greater the amount of sharpening that it require.

I personally prefer to start with the radius slider and set that depending upon the nature of the image that I am sharpening, the more detailed the image the smaller the radius setting that I will use, I will generally use the radius set somewhere between 0.3 and 2.5 and the sharpening between 100-200%.

The image shown left is a 100% crop from a image captured using an EOS1 D Mk II N, original file size 3504 by 2336
pixels (equates to approximately 23mb when opened in Photoshop). As this image consists of a high level of detail, I decided that the sharpening radius required to get the best results would be
small radius

This is the image that I believe shows the best level of sharpening for viewing on screen. If I was to print this image I would probably
look to increase the level of sharpening slightly further. However as I supply newspapers with images, they will have a better idea of the levels of sharpening best suited to their printing process.

This image shows a clear sign of being over sharpened, we are probably just short of seeing signs of halos appearing (probably the most obvious sign of over sharpening) but the increase in contrast between tones is starting to be over powering.


Here you can clearly see that the sharpening has been over done increasing the threshold will reduce the level of sharpening however it is far better to get the radius correct to start with.


Pushing the point to the very limit here you can see the Halo effect caused by over sharpening.

It is important to evaluate the effect of sharpening by viewing an “actual pixels” display for the image. This requires that the image be zoomed to a 100% scale, so that one pixel on your monitor represents one pixel in the image. Any other zoom setting requires that the image be displayed with more or fewer pixels on the monitor than exist within the image, which results in a less than accurate preview of the sharpening effect.

With images that contain less detail, or smooth textures, transition from bright to dark at contrast edges is smoother and covers a wider range of pixels. Therefore, a larger greater effect is required
to bring out those edges and increase perceived sharpness. I usually use a Radius setting in the range from 2 to 3.

Because the halos will now be relatively large, it is important that they are not too bright. Correspondingly the Amount setting needs to be lower than for detailed images. I’d suggest between 80% and 150%

For low detail images you’ll more likely have smooth textures you want to preserve. So a higher Threshold setting should be used. With portrait you don’t want to over do the sharpening of skin detail, so raise the Threshold to a value that the minimal contrast within the skin isn’t enough to cause sharpening of those areas. For low detail images, a Threshold of between 8 and 12 is usually best.


Starting with an image, straight out of the camera (shot on an EOS1 D) with a 70-200mm F2.8L IS USM we can see that the image is slightly soft and will require some sharpening. The question is how much?

Using similar settings to the images above of St Georges Hospital the sharpening is very unflattering, revealing every small defect in the skin, the pores are accentuated and the small creases under the eyes are emphasised. These settings are far too harsh.

Changing the settings to use a much larger radius and a higher threshold setting means that we do not need to sharpen by the same amount, it also leads to a much more pleasing image.

LEARN TO USE YOUR EYES

This page lists guidelines only, do not take these settings as gospel, you will need to experiment, depending on the equipment that you are using. The best settings for a given image depend on a number of factors and each image will call for different settings. The rules of thumb provided in the previous sections simply provide a good starting point.


PRACTICE MAKES PERFECT


The Unsharp Mask filter is a useful tool for improving the appearance of your images. Understanding the settings available and how they should be adjusted will make it easier for you to find the best settings for your images. Within no time at all you will be a master-sharpener

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