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SHARPENING IMAGES WITH UNSHARP MASK
This tutorial was prepared by Roy Killen and is copyright. It is part of the training materials provided for members of the Belmont 16 Ft Photographic Club and is not intended as a complete, stand-alone instruction package.
Comments and suggestions on this tutorial are welcome - they should be emailed to Roy Killen.
Before reading this tutorial you should read the tutorial on IMAGE SHARPENING - BASIC PRINCIPLES.
WHAT SHARPENING FILTERS ARE AVAILABLE IN PHOTOSHOP?
If you have an image open in Photoshop and select Filter>Sharpen you will be presented with five sharpening options:
Only two of these filters are of use to most photographers: Unsharp Mask and Smart Sharpen.
In this tutorial we consider just the UNSHARP MASK option. [Don't be confused by the strange name - it comes from a darkroom technique that was used to sharpen images.]
Unsharp Mask is oftern referred to simply as USM.
THE UNSHARP MASK
This filter allows you to increase the apparent sharpness of an image by increasing the contrast along tonal edges in the image. This effect was explained in a previous tutorial and is illustrated here:
The Unsharp Mask dialogue provides three sliders that allow you to adjust the strength (intensity) of the sharpening effect.
The AMOUNT slider varies the intensity of the sharpening by altering the amount by which the pixels either side of the tonal edges are lightened or darkened.
The RADIUS slider controls the width of the sharpening effect - the width of the area that is lightened/darkened on each side of the tonal transitions.
The THRESHOLD slider determines how much tonal difference there needs to be between adjacent pixels before the difference is identified as an edge to be sharpened.
EFFECTS OF MOVING THE AMOUNT SLIDER
As the "amount" slider is moved to the right, the intensity of the sharpening increases because the pixels on either side of the "edges" are being lightened/darkened more. If it is taken too far you will start to get obvious halos along the tonal edges and sharpening will be obvious in areas of the image that should not be sharpened (the skin in this example).
EFFECTS OF MOVING THE RADIUS SLIDER
As the "radius" slider is moved to the right, the intensity of the sharpening increases because the width of the area being darkened/lightened near each edge increases. If it is taken too far, you will start to see very "harsh" edges and colour shifts in the image. For very fine details (and for low resolution images) you need to use a small radius.
EFFECTS OF MOVING THE THRESHOLD SLIDER
As the "threshold" slider is moved to the right, the intensity of the sharpening decreases because there needs to be greater tonal variation before an "edge" is identified. If it is taken too far, no sharpening will be applied.
HOW DO YOU FIND THE RIGHT SETTINGS?
There are no "correct" values for the slider settings. You have to adjust them to suit each image.
Over time, you will become familiar with the USM settings that give you the best results for images from your camera when they are printed on your printer. But as a starting point you could try this approach:
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Display your image at 100% so you are viewing "actual pixels".
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Set the Amount slider to a high value - say 300%.
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Set the Threshold slider to 0.
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The above two settings will probably give an exaggerated (and undesirable sharpening effect) but they will make it easy to see the effect of moving the Radius slider.
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Move the Radius slider to the right until edge halos start to appear or the image is obviously degrading. Then back off the Radius adjustment by 10% to 20%.
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Reduce the Amount of sharpening until the image looks just a little over-sharpened. You might fiind it useful here to have the image displayed at 50%. Do NOT use other values (such as 66.7%) because Photoshop has to do some interpolation to produce these views and what you see may not be what you get when you print the image (particularly with versions of Photoshop earlier than CS4).
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Adjust the Threshold slider to limit the sharpening to the areas that you want sharpened (and to prevent areas such as skin being sharpened).
- If necessary, do some fine tuning of the sliders, then make a test print.
You need to keep in mind that sharpening is achieved by creating "halos" of increased contrast at tonal edges, but we don't want those halos to be visible in the print at "normal viewing distances". For A4 to A3 prints, normal viewing distances are usually 0.5 to 1.0m. At those distances, sharpening halos that are less than about 0.25mm will not be visible. Just what sharpening radius you need to achieve this will depend on the print resolution of the image, on the resolution of the printer and on the type of paper yu are using. So you will need to experiment to determine the optimum sharpening radius to use for your final sharpening before printing.
The type of printer you are using makes a difference. For best results on injet printers you generally need more sharpening than you do for commercial lab printers.
MINIMISING COLOUR PROBLEMS
If your image contains bold colours (high levels of colour saturation), you might find that the sharpening halos appear to be a different colour from their surroundings. In fact the halos are likely to be more saturated and brighter than their surroundings. This is evident in the following image where the version on the right has been sharpened.
This colour shift can be minimised if, immediately after applying the unsharp mask, you select Image>Fade Unsharp Mask from the menu and then select Luminosity blend mode and reduce the opacity to a suitable level.
The resulting image has a sharp transition from blue to yellow, but the sharpening halos are not so obvious.
This process will confine the sharpening to the luminance information and leave the original colour information substantially intact.
OH DEAR!
There are two serious problems with the USM approach to sharpening described above:
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It is destructive and non-reversible - it changes the image pixels forever.
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It applies the same degree of sharpening to the entire image - whether it needs it or not.
There are various ways around these problems, some of which are described in the tutorials listed below.
ALTERNATIVE SHARPENING TECHNIQUES
All images and tutorials on this site are copyright.
This page last updated 25th August 2009 (RK)