camera RAW
Choosing a bit depth - Step 4
The only step that remains is to select the ‘Depth’ in the lower left-hand corner of the dialog box
and then click ‘OK’. If the user selects the 16 Bits/Channel option, the 12 bits per channel data
from the image sensor is rounded up - each channel is now capable of supporting 32,769 levels.
In order to produce the best quality digital image, it is essential to preserve as much information
about the tonality and color of the subject as possible. If the digital image is edited further in 8
bits per channel the fi nal quality will be compromised. If the digital image has been corrected
suffi ciently for the requirements of the output device in the RAW dialog box the fi le can be edited in
8-bit mode with no apparent loss in quality. Sixteen-bit editing, however, is invaluable if maximum
quality is required from an original image fi le that requires further or localized editing of tonality and
color after leaving the camera RAW dialog box.
Limitations with 8-bit editing
As an image fi le is edited extensively in 8 bits per
channel mode (24-bit RGB) the histogram starts
to ‘break up’, or become weaker. ‘Spikes’ or
‘comb lines’ may become evident in the resulting
histogram after the fi le has been fl attened.
Note > The least destructive 8-bit editing
techniques make use of adjustment layers so
that pixel values are altered only once, when
the layers are fl attened prior to printing.
The problem with editing extensively in 8-bit mode is that there are only 256 levels or tones per
channel to describe the full color range of the image. This is usually suffi cient if the histogram
looks healthy (few gaps) when we begin the editing process and the amount of editing required is
limited. If many gaps start to appear in the histogram as a result of extensive adjustment of pixel
values this can result in ‘banding’. The smooth change between dark and light, or one color and
another, may no longer be possible with the data supplied from a weak histogram. The result is a
transition between color or tone that is visible as a series of steps in the fi nal image.
Final histograms after editing the same image in 8 and 16 bits per channel mode