There are many things Adobe Camera Raw can do for you, but one of its most fundamental functions is to correct the exposure of your raw files.
The key controls are grouped under the Basic tab, which is common to all versions of Adobe Camera Raw whether it’s running under Photoshop, Elements or Bridge. Under Photoshop there are a few other tonal adjustment tools, including the Tone Curve tab.
The Exposure slider is your main weapon for major adjustments, but exposure is almost always a more complex thing than a single slider can control.
True, if a shot is suffering from general over- or underexposure, it should be pretty straightforward to simply lighten it or darken it down, and indeed simply moving Adobe Camera Raw’s Exposure slider to the right or the left respectively will do the job in a moment.
In an overexposed shot, for example, the tonal information is usually ‘clumped’ towards the highlights end of the histogram, with less data in the midtones and the graph falling away before it reaches the shadows end of the scale.
Moving Adobe Camera Raw’s Exposure slider to the left quite intuitively shifts all this tonal information to the left, darkening the shot down.
As you work in ACR, you’ll be guided largely by what looks right (assuming that your monitor is trustworthy). The histogram at the top of the slider panel is an invaluable cross-check, showing you what’s happening to the image data so you can assess the effects that your edits are having and identify what more may be needed.
The histogram is a graph of the brightness or tonal intensity values of pixels in the image, from shadows at the left to highlights at the right. The height of the graph at any point indicates the relative number of pixels with that intensity value.
The combined channels are represented as a white histogram, which is your foremost indication of exposure. In a tonally well-balanced image, the graph should taper off to virtually nothing before hitting either end.
A cut-off spike in the graph at either end indicates that some data is being ‘clipped’ (forced to solid black or pure white), meaning that fine tonal detail is being lost, typically in burnt-out highlights or underexposed shadows.