DPP Home Technique (R)evolution Noise–Lose It

Tuesday, October 5, 2010

Noise–Lose It

Dark slides for hot pixels

This Article Features Photo Zoom

3) The images combined to eliminate noise
Steps to take to use a dark slide:
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
This technique resembles how in-camera noise reduction works. In fact, when making multiple exposures of very long duration, in-camera noise reduction actually can take longer than applying this technique in Photoshop; the camera makes a dark slide for each exposure, doubling the exposure times. What’s more, you can refine the effect for a more precise effect with fewer side effects, using 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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