The Raw photography facts revealed

Tackling poor exposure
Raw processing provides greater scope for correcting poor exposure, and particularly for recovering ‘blown’ highlight detail.

Unlike the human eye, the camera sensor responds to light in a linear fashion. One of the functions of raw conversion is to map the camera’s linear response to a gamma more closely resembling the visual response of the human eye (and film).

One consequence of this is that a relatively larger proportion of the available bits are allocated to the highlights than the shadows.

The implications for correcting exposure problems may not be immediately obvious, but it amounts to the fact that, when shooting raw, it’s better to overexpose than underexpose.

There’s not much you can do with an overexposed jpeg; if there’s no detail in the highlights, no amount of messing with levels or curves will help.

But with Raw files it’s different, and Adobe Camera Raw (ACR) and some other Raw converters can help you recover highlight detail in images that have been overexposed by as much as a stop.

If you’re unsure where to start with the exposure and other controls in ACR, try selecting Use Auto Adjustments from the Settings menu (Ctrl & U). Most of the time this does a pretty good job. You can also check the Auto boxes to auto adjust each of the tone controls individually. 

Due to the linear nature of camera sensors, six of the 12bits of image data per pixel are used to record the brightest stop of tonal detail in your image. That means that if you underexpose by one stop, you’re effectively only using half the available bits to record your image.

To compound the problem you also have to drag the Exposure slider to the right, effectively stretching the histogram across the full tonal range. There’s unlikely to be enough information in the shadow end of the histogram to enable you to do this without introducing a very visible noise element.

This is going to be the case whether you’re working with a 12bit Raw file or an 8bit jpeg.

However, with the Raw file, depending on the ISO setting and the degree of underexposure, you might just get away with it, though the resulting file won’t stand up to further tonal adjustments.

With an 8bit jpeg the noise problem will be compounded by posterisation which you’ll see as gaps in the histogram.