Tuesday, October 5, 2010
Dark slides for hot pixels
3) The images combined to eliminate noise
1. Open an appropriate “dark slide” and an image you wish to use it with in Photoshop.
2. Holding the Shift key, drag and drop the dark slide Background layer on top of the Background layer of an image. To ensure precise registration, don’t crop, rotate or scale either the dark slide or the image until after this process is complete.
3. Set Blend Mode to Difference. The noise will drop away. To confirm this, make sure to examine the combined effect at high magnification—100% or greater.
4. Optionally, reduce the opacity of the dark-slide layer.
5. Optionally, use a Curves adjustment layer to fine-tune the contrast of the dark slide. If you do this and if you use either the black point or white point slider, be particularly vigilant that you don’t clip important information. Remember, the goal is to eliminate artifacts, not introduce them.
4) The layer stack in Photoshop
While you’re making one dark slide, why not make them all? Make a series of dark slides at varying durations (up to the maximum exposure time you’ve used) for each ISO rating. In less than an hour, you’ll have dark slides for your camera. You may or may not need to make dark slides for significantly higher temperatures; leave these tests for a time when you’ve photographed in very warm temperatures and have discovered a need for a dark slide. ISO and exposure time are recorded in a file’s metadata, so the only factor you need to add to the file is temperature, either in the metadata or in the file name. Dark slides are unit-specific (your individual camera), not model-specific (the make and model of your camera), so dark slides can’t be shared. Dark slides may need to be remade if your camera ages significantly (many years or thousands of exposures); as your camera ages, its hot pixels will become hotter.
This technique is most useful when using high ISOs, when making extremely long exposures or when photographing in extremely hot conditions (avoid storing your camera in very hot conditions). It won’t eliminate all noise in an image, but it’s excellent for eliminating this kind of noise in an image. It can make a significant difference in your final image quality.
This is just one of the many techniques you can use to reduce noise in your images and significantly expand the range of conditions in which you find yourself making high-quality exposures.
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. Get over 100 downloads and his enews Insights free at, www.johnpaulcaponigro.com.
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