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Tuesday, August 10, 2010

Reduce Noise With Multiple Shots

Use Photoshop Layer Stacks to clean up an image

This Article Features Photo Zoom

3) Choose Stack Mode

So what is Photoshop doing? Photoshop’s Stacks feature was designed for analytical work in various scientific fields, like astro-photography and forensics.

Photoshop first aligns a series of images as separate layers, converts them into a Smart Object and then blends them, reducing or amplifying the differences between the layers with a variety of rendering modes. You can choose one of 11 rendering modes: Entropy, Kurtosis, Maximum, Mean, Median, Minimum, Range, Skewness, Standard Deviation, Summation and Variance. Few people will ever use all of them, and most won’t use any of them, but I recommend that you try two—Median
and Mean.

Median and Mean select values in between the highest and lowest values, smoothing out the differences between aligned layers in a Stack. Median works best for images with some motion, either subject or camera, to remove moving objects or noise. Mean works best for processing exposures without motion. (Astro-photographers typically make many exposures, sometimes dozens or more, of the same subject and use Mean to reduce noise.)

4) Before Noise Reduction

5) After Noise Reduction

The more exposures you make and combine, the better the noise reduction. Only practical limits apply. How many exposures can you make? How many exposures can Photoshop process on your computer? While you can stack and process as few as two images, three is my recommended minimum. Combining as few as three exposures can make a substantial difference in the quality of your final image. (Try using your camera in burst mode more frequently.) The most challenging part of this technique is identifying situations where it’s helpful and remembering to make multiple exposures.

Combine the recent advances in digital cameras that offer exceptionally low noise at high ISOs, with new exposure techniques, with new post-processing techniques with the latest software, and you have a profound paradigm shift in photography.

Learn these techniques, and you’ll find that your photographic options will expand dramatically.

The most challenging thing isn’t learning the techniques, it’s redefining what’s possible and practical. You have to experience it to truly understand it.

John Paul Caponigro, author of Adobe Photoshop Master Class and the DVD series R/Evolution, is an internationally renowned fine artist, an authority on digital printing, and a respected lecturer and workshop leader. You can get more than 200 Lessons and his e-news Insights free at his website, www.johnpaulcaponigro.com.