Digital Image Sharpening.
By Lauren Claire Pinches
www.laurenclairephotography.com 0
Photographic image sharpening is a subject often spoken about, but rarely understood. Most of today’s photographers’ don’t fully understand the editing process and refrain from sharpening their images. Ultimately, sharpening should be the final step in the editing process. In theory it states that all digital images require sharpening, as most images will benefit from the process. Sharpening consists of a three stage process in order to achieve a desired result. Digital in camera sharpening has said to be destructive to image quality and slows down the imaging process.
The three stages of image sharpening are: capture sharpening, selective sharpening and output sharpening. Capture sharpening enables an image to be sharpened during image capture; this is done at a minimal level and still allows the photographer to sharpen an image during the editing process. Selective sharpening is an aesthetic rather than a technical process. Creativity and the concept of the image lead this stage of the process. Output sharpening allows the photographer to sharpen an image according to a specific output device. This is applied to an image after all other post-production adjustments are made.
Unsharp mask is a popular sharpening process as it is sophisticated and controllable. It is used to sharpen the edges in a photographic image by increasing the contrast between different tonal ranges. Pixels on the edge of the lighter side are made lighter and the pixels on the edge of the darker side are made darker. It has been proved that the effects are more noticeable on-screen than when printed. The unsharp mask process originated in Germany in the 1930’s, as a way of increasing the sharpness of photographic images. The name of the unique process was derived from the way a blurred, or “unsharp”, positive image was used to create a mask of the original image. The unsharp mask was then combined with the negative, which created the illusion of a sharper image than the original.
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In history it states that a large format glass plate negative was contact copied onto a low contrast film to create a positive. The copy material however, was in contact with the back of the original which caused the blurred effect. Light was then directed through both the negative and positive images in an enlarger, this causes low intensity information to be removed and the dynamic range of the original negative to be reduced. The unsharp mask process allows the fine detail in the original negative to be emphasized, without the loss of highlight or shadow detail.
The discovery of digital photography changed the unsharp mask process to a large degree. One now relies on computer programs such as Adobe Photoshop to perform these tasks. It has been said that Photoshop applies a Gaussian blur to a copy of the digital image and then compares it to the original image. There are three setting controls in digital unsharp masking, namely:
1. Amount
ďƒ The specific control is listed as a percentage. It is able to control
how much lighter or darker the pixels can be adjusted. It has been said that anything between 80-180% are good figures to work with. 2. Radius
ďƒ The control specifies the width of the adjustment. It is advised to
not exceed a radius of 1 pixel when working with an image larger than an A4. A rule of thumb states that one should divide the image resolution by 200 in order to calculate the radius that should be used in the sharpening process. 3. Threshold
ďƒ The control specifies where the effect will take place. A low
threshold affects no pixels, where a high threshold affects the edges with a high tonal difference. The threshold is usually kept between 0-2, but a threshold of 3 has been suggested when using a 35mm camera.
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Example:
Figure 1: Pinches, L. 2010. McDonalds.
Figure 2: Pinches, L. 2010. McDonalds.
No sharpening techniques were performed on the image.
The unsharp mask sharpening technique was used on the image. The sliders mentioned above were set to: Amount: 120%, Radius: 1.5 pixels and the threshold: 3 levels.
There are a few important points to remember when sharpening an image. One must always remember to perform the sharpening technique at the end of the editing process. A sharpened and unsharpened version of the same image file should always be saved as backup files. The reason, mentioned previously, is that degree of sharpening differs when using different output devices. Always set the view to “actual pixels� when using the sharpening tools as it gives one complete control. As shown in figure 1 and 2, the slightest change to the unsharp mask sliders causes a dramatic sharpening effect. When used correctly, sharpening is one of the most impressive techniques one can use in the editing process. Sharpening has the ability to improve image detail by emphasizing the edges within the image.
There are many different sharpening methods that one can use to enhance photographic images. There is however drawbacks to these processes that one needs to take into consideration before the sharpening process begins. The biggest drawback is that of over-sharpening. When one over-sharpens an image the edges become unnaturally pronounced and the dark objects become outlined with a light halo and vice versa. The noise within an image becomes visible in areas that were smooth in the original image. In the case of extreme over-sharpening individual pixels tend to break up and stand out from each other, which don’t produce a successful over all images.
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Edge Sharpening is used mainly by portrait and nature photographers as this sharpening technique allows for only the detailed areas to be enhanced. It is created by placing a mask over only the edges in the image and only applying the sharpening technique to the areas under the mask. The sharpening brush is another selective sharpening tool in Photoshop which can be used to sharpen only the detailed parts within an image. Giving the photographer complete control over the areas sharpened will produce images of high quality. The high pass sharpening technique uses the high pass filter, as well as the unsharp mask function as part of the sharpening process. The biggest issue when sharpening images is the noise that becomes visible in the smooth areas of the image. This sharpening process has the tendency to not sharpen the noise in these areas because one can is able to select the edges that need sharpening.
In conclusion, one should always sharpen where one can have complete control of the amount of sharpening and the final product. Being in the photographic industry as a professional photographer one has to provide customers with good quality, sharp images. The secret with sharpening is to not overdo the process. If it is handled with precision and care it will produce a successfully edited image.
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Source and Reference List Child, J & Galer, M. 2008. Photographic Lighting. 4th Edition. Focal Press. Oxford. Galer, M & Horvat, L. 2005. Digital Imaging. 3rd Edition. Focal Press. Oxford. Langford, M. 2008. Langford’s Advanced Photography. 7th Edition. Focal Press. Oxford.
Evening, M. 2009. Adobe Photoshop CS4 for Photographers. Focal Press. Oxford.
Thom. 2003. Sharpening 101. [Online] Retrieved on the 25 August 2010 from: www.bythom.com/sharpening.htm.
Wikipedia. 2010. Unsharp Masking. [Online] Retrieved on the 25 August 2010 from: en.wikipedia.org/wiki/Unsharp_masking
Image Reference List
Figure 1:
Pinches, L. 2010. McDonalds. Summerstrand, Port Elizabeth.
Figure 2:
Pinches, L. 2010. McDonalds. Summerstrand, Port Elizabeth.
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