Common questions about shooting raw files
What are blown highlights and clipping?
If you darken shadow pixels to pure black, that image information is said to be ‘clipped’. Imagine a subtle shadow containing many variations of tone; turn all those tones pure black, and you’ll see how this equates to a loss of detail in those areas.
In the same way, if you have an area of highlights containing tones of subtly varying brightness and you lighten all those pixels to pure white, lose detail in that area – ‘blowing’ the highlights.
Clipping can occur when you take a shot or during editing, and image information can be clipped in one, two or all three of the RGB colour channels.
Should I still use the histogram?
All digital SLRs can display a histogram for saved images so that you can tell how well they’re exposed and whether any shadow or highlight detail is ‘clipped’. This is fine for JPEGs, where what you see is the image file the camera has saved.
With raw files, though, you’re only seeing the camera’s representation of the image. It’s purely a rendition of the image for viewing purposes – after all, the actual processing parameters have yet to be decided, by you. So, what you see isn’t what you get.
Camera manufacturers are not clear about how these playback ‘previews’ are generated. Experience suggests that they’re generated using the white balance and other settings applied to the camera at the time.
In other words, they represent the JPEG the camera would have produced in those conditions with those camera settings.
They may not, therefore, be a terribly good guide to the tonal range actually available in the raw file.
Generally, there’s a little more shadow and highlight detail available than the histogram suggests, which you can reveal in raw software.
How does raw capture work?
It’s useful to understand the point at which the RAW data is saved in the image-making process. First, the image is formed on the camera’s sensor.
How sharp it is will depend on the lens quality, the focus accuracy and the lens aperture (and hence depth of field). None of this can be changed later.
Next, the camera ‘reads’ the sensor data based on your ISO setting. This is used to assign brightness values to the pixels using amplification circuitry. This can’t be changed later either.
It’s only at this point that the data is recorded as raw. If you’re shooting a JPEG file, the camera’s internal processor will now choose the white balance setting, the contrast and brightness, the colour saturation and sharpening (because of the way digital sensors form images, sharpening is always required at some point).
If you’ve saved the raw file, though, you can carry out all these adjustments later instead of relying on the camera to make them.
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