Global vs local process ing

Localised processing is very powerful, but use it with care...

■ In film-based photography, most of the ways of adjusting how a picture looks affect the entire image. Digital processing, in contrast, enables us to adjust increasingly specific attributes. Many of these methods are distinctly non-traditional, in that they can adjust brightness and colour according to what neighbouring pixels look like.

This is called tone mapping, a digital procedure for mapping one set of tones and colours to another. And there are two kinds of tone mapping: global operators and local operators. The first applies its calculations across a whole image, the second works only at a certain distance from each pixel. This kind of processing can alter the image – by teasing out shadow detail, for example – to a degree that would just be impossible using methods such as Curves. However, if used without care, the result will look less ‘photographic’. It pays to treat localised adjustments like Highlights, Shadows and Clarity with caution.

A detail of an image processed with Highlights and Shadows recovery, for a local effect. The sliders are highly effective, but traditionalists might not like the over-processed look