Wednesday, January 23, 2008

More on "extending" exposure range

The discussions on the net about extending exposure range often get involved in tone mapping (al la HDRi), and suspect that many people see more contrast are doing the same on a single image (ie expanding the contrast, using exposure adjustment) and it is unfortunately commonly also called tone mapping, but perhaps it is better called adjusting tonal scale.


An histogram adjustment slider is such a tool and window live photo just happens to have one, if you know where to look. Firstly select the photo (yes single image) and click on the Fix tool on the tool bar. A list of common fix techniques pops up on the right hand side. Select the first item Auto Adjust, the image will be adjusted and a small panel of sliders will drop down. A histogram graph of the individual pixels tone (from dark to light) is displayed at the bottom imageof these tools. Under it are two sliders (one with a black dot, representing the darkest shade on the adjusted image, the other a open square which is the lightest tone) . As you move these sliders you will see the image "tone stretched" as you go. For the image below, base on my stitched panorama from a few days ago, I have chosen to slide the dark slider up, which appears to darken the image, but all that has happened is the tones are moved to fill a space between dark and light. The three sliders above can also adjust the histogram but they compact or stretch individual parts of the histogram. I thought the image required stronger highlights and moving that to the right stretch the upper part of the histogram a little but left the rest unchanged.

IMGP0514 Stitch-1

Does this look closer to what I remember photographing? YES, with a slight colour shift. Is it tone mapping? NO, but it is an easy way to liven up a "flat" result.

A lot of the auto fix tools in camera bundled software, popular photo album programs and even photo processing kiosks, use tone scale adjustment as a significant part of their "magic".

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