essential skills: digital photography in available light
Reading exposure levels
The color information in a 24-bit RGB digital image fi le is separated into three ‘channels’ (8 bits per
channel). Each color channel is capable of storing 256 levels of brightness between black (level 0)
and white (level 255). When viewing all three color channels simultaneously each pixel is able to
be rendered in any one of 16.7 million colors (256 x 256 x 256). These brightness levels can often
be displayed as a simple graph or ‘histogram’ in both the capture device and the image editing
software. The horizontal axis displays the brightness values from left (darkest) to right (lightest). The
vertical axis of the graph shows how much of the image is found at any particular brightness level. If
the subject contrast or ‘brightness range’ exceeds the latitude of the capture device or the exposure
is either too high or too low then tonality will be ‘clipped’ (shadow or highlight detail will be lost).
Histograms
During the capture stage it is usually possible to check how the device has handled or interpreted
the tonality and color of the subject. This useful information can often be displayed as a histogram
on the LCD screen immediately after capture when using a DSLR or as part of the live preview
when using a high quality prosumer digicam. The histogram displayed shows the brightness range
of the subject in relation to the latitude or ‘dynamic range’ of your capture device’s image sensor.
Most digital camera sensors have a dynamic range similar to color transparency fi lm (around fi ve
stops) when using the JPEG or TIFF fi le format and greater than seven stops when using the
camera RAW format on a DSLR camera.
Note > When using the JPEG or TIFF fi le formats you should attempt to modify the
brightness, contrast and color balance at the capture stage to obtain the best possible
histogram before editing begins in the software.
A
C
Shadow
adjustment
Highlight
adjustment
Midtone
adjustmentA
BC
B