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IMAGE SHARPENING - BASIC PRINCIPLES
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.
WHAT IS SHARPNESS?
Sharpness describes the clarity of the detail in an image. Images appear to be sharp when they have distinct "edges" in them - that is, definite transitions from one tonal value to another.
Sharp images appear to contain a lot of clear detail. Soft images have blurred or indistinct edges (gradual or indistinct tonal changes) and, therefore, do not show much detail.
"Edges" in an image are created by transitions from one tonal value to another - not necessarily one colour to another. Sharp images have abrupt tonal changes at edges, soft images have gradual tonal changes at edges. This is the same thing as saying that sharp images have high contrast edges and soft images have low contrast edges. The technical term for how quickly image information changes at an edge is "acutance". A high acutance image has rapid changes in tonal values at edges.
There are actually two factors that determine how sharp an image appears - acutance and resolution. The camera's resolution - its ability to distinguish between elements of fine detail - is determined primarily by the camera sensor. You have no control over this after the image is captured.
Acutance - the ability to reproduce sharp transitions and, therefore, details with clearly defined edges - is influenced by the camera lens and by the post-capture processing of the image.
"Sharpening" is the process of trying to make the "tonal edges" in an image more distinct by increasing the contrast at those edges. That is, sharpening is the process of increasing the acutance of the image.
In addition to acutance and resolution, our perception of the sharpness of an image is influenced by the distance at which we view the image and by noise (grain) in the image. The greater the viewing distance for a particular image the sharper it will seem. Small amounts of grain (noise) in an image can also make it appear sharper.
WHY DO WE NEED TO SHARPEN?
In the process of transferring an original scene to a print made from a digital photograph of that scene, sharpness (and detail) is lost in each of the following stages:
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In the camera lens. Poor quality lenses are not capable of producing sharp images.
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In the camera sensor. The size and quality of the pixels on the sensor limit the amount of detail that can be recorded.
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Processing of the image signal in the camera. The algorithms used to convert the signal from the sensor into an image file that is recorded on the memory card can degrade the image information, particularly if JPEG files are recorded.
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Processing of the image file in the computer. Inappropriate processing techniques can destroy image detail, as can saving the file as a JPEG.
- Printing the image. Only very high quality printers will produce sharp, fine-detailed prints.
If you do not apply any sharpening to your digital images, the majority of them will appear very soft when printed. Sometimes you will want a soft image, but generally you will want your images to appear sharp (at least in some critical areas).
WHEN SHOULD YOU SHARPEN?
To compensate for the loss of sharpness in an image (due to any or all of the above five causes) you can apply sharpening at one or more of the following stages:
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In the camera. If your camera is set to save JPEG files it will apply some level of sharpening during the JPEG conversion process. On most cameras you can select from two or three different levels of sharpening. However, you will have very little control over the sharpening and it will be applied to the whole image. It is generally better to set your camera to save RAW files so that no sharpening is applied in the camera (or set the sharpening of JPEG files to a minimum).
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During RAW conversion (sometimes called input sharpening). RAW conversion programs such as Adobe Camera Raw provide the option of applying sharpening. In some programs you have considerable control over the amount of sharpening and the regions of the image to which it is applied. Refer to this tutorial for details about sharpening in Adobe Camera Raw.
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During image editing. In Photoshop, or similar image editing programs, you have numerous options for sharpening images. One of the most popular techniques is described in this tutorial on sharpening with the unsharp mask.
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Immediately before printing (sometimes called output sharpening). You might use the same technique as you used during editing, or a different one.
It is not very often that you can achieve the best possible results with just one application of sharpening. You are more likely to get consistenty good results if you take this approach:
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Apply some initial sharpening during RAW conversion to compensate for the loss of sharpness that occurs in the lens and camera.
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During your image editing, apply an appropriate method of sharpening so that the image looks good on your computer screen. This will usually involve "selective" sharpening - applying different amounts of sharpening to different parts of the image.
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Apply extra sharpening immediatley before printing the image, taking into account the size of the print, the type of printer and the type of paper you are using. This is the stage that will require the greatest amount of experimentation.
WHAT HAPPENS WHEN YOU SHARPEN AN IMAGE?
When you sharpen an image you create an optical illusion. You create an image that APPEARS to have more detail and APPEARS to be in better focus, even though you have not actually changed the focus or created any more detail or changed the resolution of the image.
Regardless of the method you use to sharpen an image, the result will be an increase in contrast along "edges" in the image, and this makes the edges more pronounced. "Edges" in an image are simply tonal transitions and when the tonal transition is abrupt (high acutance) the edge appears "crisp" or "sharp". This effect is illustrated below:
The above two images contain the same four patches of grey. In the image on the left the transitions between the different shades appear "soft" whereas the transitions in the right-hand image appear crisp or sharp.
This was achieved by "sharpening" the right-hand image. At each of the three transitions, the pixels on the "light" side of the transition were lightened and the pixels on the "dark" side of the transition were darkened.
In the sharpened image, the transition that appears most "natural" to me (i.e., crisp without being too exaggerated) is the centre one, even though the same amount of sharpening was applied to all three edges. You might disagree and think that one of the other edges looks more "natural". It doesn't really matter - but the fact that you might consider any one of the transistions more "natural" than the others simply illustrates the point that "global" sharpening of an image will not produce the same effect in all areas of the image.
You have to be careful with sharpening - however you do it, if you take it too far it will become obvious in some places and the image will appear "over-sharpened". By applying more subtle sharpening to the above example, the following effect can be obtained:
The transitions are now sharp but the "halos" along the edges are less obvious. Later tutorials will explain how to achieve this effect.
Creating an appropriate level of sharpness in an image is always a bit of a balancing act - you have to try to emphasise the edges without making it obvious that you have done so.
SOME MISCONCEPTIONS ABOUT SHARPENING
Misconception 1: More is better. This is definitely not true. If you sharpen an image to the point where is becomes obvious, you have probably taken it too far. If you apply too much sharpening you may get overly contrasty images, halos along edges, or speckled areas.
Misconception 2: Unsharp Mask is always the best way to sharpen. This particular sharpening technique will often produce very good results; but better results can be achieved on some images using alternative techiques.
Misconception 3: There is some magic set of values in Unsharp Mask (or in the adjustments for whatever sharpening technique you are using) that will always produce the best results. Definitely not true. As you become familiar with the various sharpening techniques you will get a feel for what "starting" values you should apply to various types of images, but they are just that - starting values.
Misconception 4: Some images don't need sharpening. For the reasons outlined earlier, all images lose sharpness as they pass through the camera, computer and printer. That's why sharpening is automatically applied in-camera if you are saving JPEG files and why there is a defauit amount of sharpening applied in programs such as Adobe Camera Raw. Unless you are deliberately trying to produce an image that is soft all over, some sharpening (perhaps just selective sharpening) will be necessary.
HOW TO SHARPEN
Tutorials on the following sharpening techniques are available at these links:
All images on this site and all documents and tutorials linked to this site are copyright.
This page last updated 24th August 2009 (RK)