Wednesday, November 17, 2010

Noise reduction in photos

In another post I showed how to lighten shadow detail in photos with strong lighting contrasts (Coping with harsh sunlight). The price one pays for lightening dark areas is making noise much more noticeable. Although there is no free lunch, it is possible to reduce the price. “Noise Reduction” to the rescue.

Here is an example photo, one that was taken in direct sunlight, with deep shadows in the subjects face. The strip shows a section of the original with the same section from heavily post-processed versions.

The left-most image is a crop from the original – clearly too dark to even recognize the child. Processing was done using Windows Live Photo Gallery 2011. In the second image the Adjust ExposureShadows slider was set full right, the edit mode closed to save the file and the process repeated. This just about eliminated the shadows. You can see the effect better by clicking on the image strip for a larger view. Noise became very prominent in the areas that had been dark.

Next I aggressively reduced noise, see third image. You can see that the image histogramis “smoother” but has some ugly artifacts. In the “lighten shadows” operation the dark area was lightened by a factor of about six. See the histogram of a small area (original on top). The noise was also increased by this factor and was too high to be effectively eliminated by the noise reduction step.

How about doing the noise reduction first then the lighten shadows operation? The result is shown in the fourth image. Smoother, but notice how small detail (hair in front of hat) has been blurred.

The fifth, right-most, image was produced by reducing noise, but less aggressively, then lightening the shadows, followed by another noise reduction step. This last procedure produced the most acceptable result.

Still no free lunch, but using all the editing tools available in Photo Gallery can go a long way to getting results you can share with pride.

Please also visit my blog on computer tips: This ‘n That