RAW Files

A Basic Outline and Guide to Understanding and Using 'Camera RAW'

Clive R. Haynes FRPS

Coping with an Incorrectly Exposed Image
Please note image sizes are fairly large to preserve detail, download times may therefore be slow.

From time to time things go wrong and the exposure is not what we would have wished.

Fortunately Camera RAW can recover an amount of 'lost detail', whether under-exposed or over-exposed.

The example below deals with an amount of over-exposure and the same basic principals may be applied for an underexposed scene.


Examining the Histogram for the image above we can readily see that the RH side of the Histogram - the 'white / max exposure end is running off the edge of the scale. If this was a JPEG or TIFF image then nothing could be done to rescue the missing detail. RAW files allows around six 'f-stops' of exposure recovery.

Working on the image above, the Exposure and Shadows 'sliders' have been adjusted to recover the 'lost' - overexposed detail. The 'Exposure has been set to 4 'stops' under-expose. The whole image will become darker by 4 'stops' but as you can see the detail in the bridge has been improved.

In the case illustrated above, the procedure would be to optimise the general exposure and any other 'corrections' for the scene using the RAW dialogue box and 'save' as a .PSD file, then re-open the original RAW file and make the exposure adjustment shown. Click 'OK' to open the adjusted (darker) image.

The two images would then be combined in Photoshop by 'copying' and 'pasting' with the' improved (darker) image' to a layer above the overexposed original.
Next, a Layer Mask' would be used to selective combine the best sections from each image. See picture below.
For more information about using 'Layer Masks' click on the link at the base of this page.

Above: The results of combining the overexposed original RAW image with the 'improved' (underexposed) / adjusted RAW image. The quality of the recovered detail is quite amazing.
To adjust for an underexposed image, the above procedure may adopted but this time lightening the dark areas of the picture.

Note 1:
Generally speaking it's better to slightly over-expose an image when shooting in RAW. This may at first glance appear to be strange advice, it's because of the nature of 'image capture' in RAW. In RAW the 'headroom available for highlight recovery is quite surprising. However as in all things, one should carry out exposure tests with the camera you are using.

Compensating for possible overexposure by deliberately under-exposing say by a stop or so - and yes, we've all done this, can result in the shadows becoming 'blocked up'. The 'blocking' of shadows can give rise to an excessive amount of 'digital noise' becoming visible when these areas are 'lifted' out of the gloom.

Note 2:
The preview histogram in your digital camera does note necessarily give a true indication of the actual range of tones recorded in RAW as the histogram display is derived from a JPEG version of the image - it may well therefore indicate 'clipping' where in fact RAW retains the detail.

Do experiment to discover how much latitude you camera allows in RAW.

Next we'll look at how we can Reveal the 'Highlights' (White) and 'Shadows' (Black) 'Clipping Points'
- click the 'Continue' link below.

Related Topics
Layer Masks
Camera RAW 'Home Page'

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