Wednesday, September 2, 2009

Quick & Dirty Correction of Underexposed Images

Modern digital cameras are great but what they offer in convenience and ease of use (Point-n-Shoot) they often lack in control of the image exposure . . . relying on a weighed average of the light in a scene to determine exposure. That 'average' compromise works fine on evenly lit scenes and subjects but comes up short if part of the scene is in considerable shade while the rest is in bright sunlight. This is particularly prevalent when an integral part of the image is to the side and therefore outside of the center-weighing sampling grid of the camera viewfinder or screen (as in the example below). In this particular case the camera sets its' exposure based on the brighter creek bed in the center, and underexposes objects (like myself) in shade to the side.

Contrast between light and dark areas fool
many cameras into underexposing

The general work-around has been to set 'fill-in flash' to illuminate subjects nearby, but that presents another set of exposure problems . . . including the ambiance of the scene.

Some cameras offer the ability to select the exposure point . . . you typically aim a grid on the part of the scene that's to set your exposure, hold the shutter partway down while bringing the viewfinder back down as you'd like the picture composed before finishing pressing the shutter the rest of the way down. Still, you are stuck with an image that has been averaged for exposure. Parts may be over-exposed while others under-exposed. Also, it is rather difficult to carry out this dynamic composure and at the same time use the camera's self-timer.

Fortunately, the multipixel cameras now out offer a wide latitude of gamma . . . a measure of the minimum and maximum contrast available within the saved image. Post-shooting adjustment of the gamma and saturation of the image can often result in a salvageable picture.

After saving your images to your computer, fire up your favorite image editor and try the Automatic Fix first. Still unsatisfied, try adjusting the gamma up and down until the under-exposed portions come out of shadow but before you wash out the brighter areas.

Gamma Adjustment

Adjusting the gamma often results in 'duller' colors so try boosting saturation slightly. Now see your results. Save with a different file name just in case you want to experiment more.

Saturation Adjustment

The end result is a more natural, pleasing depiction of the scene as you experienced it.

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