While a photographer would want every photograph to be crystal clear and pinpoint sharp, as a concept, sharpness is often misunderstood and frequently misused.
Rather than being a measurable aspect of a photograph, it is in fact a subjective impression.
To define sharpness, one would have to say it is “the perception of how hard and distinct edges are”. As a result, these are subjective and there are several ways of influencing it. A high-contrast image, for example, tends to look sharper at a glance than a photograph with muted tones. There is also no fixed measurement of sharpness. Any evaluation of sharpness is often “too sharp” or “not enough”. That’s totally subjective and depends on how those seeing the photos like their photos.
The factors that give an impression of sharpness are acutance, contrast, resolution, and noise:
Acutance is how abruptly one tone changes to another across the image. The more abrupt the edge between one area of pixels and another, the higher the acutance.
Contrast has an effect in that an edge between black and white looks sharper than an edge between grays.
Resolution is the degree of detail and more detail looks sharper than less.
Noise breaks up the image and lessens detail, although sometimes it can have the opposite effect and add to the sharpness simply by being itself sharp and definite.
Because sharpness and sharpening are such perceptual issues, we have to consider how the photograph is going to be seen. This would include the viewing conditions, distance and the eventual size of the photograph. Because some of the sharpening controls are measured in pixels rather than percentage, the size and resolution of the digital image do matter – a one-pixel radius has much greater effect on a screen-sized 640 x 480 image than on a high-resolution 3,000 x 2,000 image. The conditions of the original image are also a factor.
A detailed, high-quality image, for example, needs a different kind of sharpening technique (finer, more detailed) than does an image that suffers from, say, low resolution, slightly soft focus, or noise.
Ultimately, two things stand out. One is that each photograph needs to be sharpened in a way that is appropriate to it; judge every photograph – or at least every set of similar photographs – on its own merits. The other, important point, is that sharpening should be the last action you perform on an image before it is displayed – whether as repro, fine art print, or on the Web. Sharpening can only be judged on its appearance as intended. And never sharpen an original, only a copy. If you are shooting Raw, then the Raw files are your true originals, but their optimization represents an investment of time and skill, and you may consider saving the finished TIFF in its unsharpened form.
Sharpened versions for various purposes can be given a slightly amended file-name (such as adding “s1” depending on versions). Note that repairing focus blur and motion blur involves sharpening, but this should not be confused with the sharpening discussed here.
What affects digital sharpening
- Image size
- Quality of image detail
- Reproduction size
- Printer quality and settings
- Viewing distance
- Your taste
Delivering sharpened or unsharpened?
When it comes to printing your photos, you have to remember that most of your larger prints will be made by someone else. It is therefore absolutely essential that these people know whether or not an image has been sharpened. If it has not, both should agree upon how it will be sharpened – and whether it needs sharpening. Because of all the variables it is clearly better not to sharpen before delivery, but take no chances that this is understood. In most cases, it is wise to listen to the print guy since he knows his machines and has also seen a greater number of images being printed – more so if you haven’t printed too many before.
Varying degrees of sharpness
Sharpening an image is aimed ideally to add to its overall impact and not make it appear unnatural or make it appear pixilated. Obvious signs of over-sharpening are high levels of what appears to be grain, even in areas outside the image’s main focused areas, and pixilation around detailed areas. Watch out for these and you should be okay.
