Digital Photography - Image ManipulationPosted on: 10 June 2008 by Gareth Hargreaves
How to create perfect pictures using software such as Photoshop.
Sharpness & Blur
When mathematicians learned how to simulate the effect of a darkroom technique by using numbers, the now-vital technique of image sharpening was born. While all physical processes tend to blur the image, sharpening helps reverse the appearance of blurred image
Sharp - Techniques
The darkroom trick involved creating an unsharp copy negative of the main image, then printing the negative held in register with the unsharp negative. This acted as a mask - the unsharp mask - which helped emphasize edge detail. While images can be improved with sharpening, they can equally be spoiled by oversharpening, which causes the appearance of artefacts such as haloes around objects, and leads to the creation of exaggerated noise.
The first rule of successful sharpening of the whole image is to use only the Unsharp Mask (USM) filter in your image manipulation software: this allows sharpening effects to be matched to the characteristics of your subject. USM works by applying small contrast increases at object boundaries. These contrast boosts applied over a larger area can enhance contrast in a way difficult to obtain by any other means. You simply set a high Radius but very low Amount.
When using USM, the control with the largest effect is the Radius setting. If your image is full of details, set a small Radius. If, on the other hand, it has large areas of smooth tone, set a larger Radius. A good starting point is an Amount setting of 111, a Radius of 1, and a Threshold of 11. To refine the effect of USM further, apply it to the Lightness channel in Lab mode. This concentrates sharpening on the channel carrying detail, thus helping to minimize artefacts and reduce the sharpening of colour noise. For this to be possible, you need program software that can work in Lab.
Sharpening - Lightness Channel
Assuming your software can work in Lab mode, change your image mode to Lab, choose the L channel in the channels palette, apply the USM to just this channel, and then return the image to RGB mode. A disadvantage of this method is that you have to judge sharpening effects with a black-and-white image as the L channel shows only brightness distribution.
USM should be applied only after you have completed all other manipulations. This is because USM sharpening, paradoxical as it may sound, actually destroys image data, which cannot afterwards be regained. This is why in-camera sharpening is not a good strategy for the highest-quality images. After you have applied USM sharpening, you should recheck Levels and adjust if necessary.
Positive Blurring - Useful Situations
While sharpening techniques are aimed at reducing blur, there are times when it may be useful to increase blur. For example, you may wish to achieve an apparent decrease in the depth of Field to make the subject stand out more from the background. To do this, select the whole area behind the subject and apply a small amount of blur. For parts of the background that are further away, select those parts and apply a stronger blur. Increasing the blur of the background can also be used to make softly focused subjects appear to be sharp by comparison.
Given that the camera will record everything in a scene, the ability to remove unwanted elements is very valuable. Making substantial changes to pictures has always been possible, but with film that meant a skilled retoucher working painstakingly, using single-hair brushes and a microscope.
Clone Stamp - & Remapping
In the digital era, the ability to change, remove, or replace objects that are spoiling the image is much easier and is taken for granted. The key tool for removing unwanted details is the Clone Stamp. This works by copying pixels from one part of the image onto another. The new pixels may also be placed with different blending modes and strengths - for example, to avoid applying light-coloured clone areas in dark areas, use Darken mode to improve the match.
However, replacing pixels is seldom the only task that's required. You may need to adjust tonality and ensure that the process of cloning onto an area doesn't smudge noise and other incidental detail. This is particularly difficult when you are working with scans from film, as the film grain is very easily destroyed when it is overlaid with film grain from other parts of the image. It may help to add noise to the cloned area to disguise the appearance of cloning.
Sometimes it's enough to make distracting details less obvious rather than removing them altogether. In portraiture, it may be appropriate to desaturate the background so that it loses some or all of its colour. Reducing local contrast, for example, in the case of bright lights in the background, may also help to keep the viewer's attention on the subject's face.
Another effective technique, which is particularly useful with digital cameras using small sensors, is to reduce the apparent depth of field by selective blurring of the background as shown in the images on the left.
For all these techniques, use a feathered Lasso tool to select the background - setting a narrow feathering band where there is much fine detail and a broader one where there is less detail - then apply the blur effect. Remember to check the appearance of the image both a high magnification of 100% as well as the overall appearance at print size.
This extract is taken from Digital Photography Masterclass, by Tom Ang, published by DK, available at all good bookshops RRP £20 or online at Amazon for £11.99.
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