essential skills: digital photography in available light
Optimizing a histogram in levels
The fi nal histogram should show that pixels have been allocated to most, if not all, of the 256 levels.
If the histogram indicates large gaps between the ends of the histogram and the sliders (indicating
a low-contrast subject photographed in fl at lighting) the relationship between the subject contrast
and light quality could be reconsidered.
Small gaps at either end of the histogram can, however, be corrected in Photoshop by dragging
the sliders in the ‘Levels’ adjustment dialog box to the start of the tonal information. Holding down
the Alt key (Option key on a Mac) when dragging these sliders will indicate what, if any, information
is being clipped. Note how the sliders have been moved beyond the short thin horizontal line at
either end of the histogram. These low levels of pixel data are often not representative of the
broader areas of shadows and highlights within the image and can usually be clipped (moved to
0 or 255).
Moving the ‘Gamma’ slider can modify the
brightness of the midtones. If you select a Red,
Green or Blue channel (from the Channel’s
pull-down menu) prior to moving the Gamma
slider you can remove a color cast present
in the image. For those unfamiliar with color
correction the adjustment feature ‘Variations’
in Photoshop gives a quick and easy solution
to the problem. After correcting the tonal range
using the sliders click ‘OK’ in the top right-hand
corner of the Levels dialog box.
Note > Variations is not available for Photoshop users working in 16 Bits/Channel mode.
Color
Neutral tones in the image should appear desaturated on the monitor.
Solution: Select the correct white balance for your lighting or use a manual white balance setting
to get the correct tone without any color cast or tints. Do not use the Auto white balance setting
- that is best for one-off snapshots.