406 CHAPTER 10 ADVANCED COMPOSITING Note Even (Apache web server tutorial)
406 CHAPTER 10 ADVANCED COMPOSITING Note Even the best tripod will not give you perfectly registered photos when you take pictures of the night sky. Why? The earth is turning, which means, from the camera s point of view, that the sky is moving about a degree every four minutes. Once the images are approximately aligned, select the Move tool in the Toolbox and set the mode of the upper layer to Difference. This will show every place the two images differ. Two perfectly registered, identical layers will look totally black when the top layer is in Difference mode. Otherwise, move the upper layer around (remember the arrow keys for moving by one pixel at a time) to make the combined image as black as possible. Now it s time to make a stack, using one of the methods below. After you ve stacked one layer, make the next layer visible, register it, and stack it. Repeat until you re out of layers. Increasing Light by Additive Stacking There are times when a flash just isn t appropriate. Did you ever go out at night with your little digital camera and wish you had a fancy SLR that could take time exposures? With stacking, you can even if your camera can t. Just take a lot of short exposures, and then stack them in Addition mode. For instance, I shot some photos of a fence near a local reservoir. The camera I was using wouldn t do an exposure longer than four seconds. At that setting, the original images just look black. It s hard to tell that there s anything in the shot at all. If I take a single image and use Curves or Levels on it, I can see that I do indeed have a shot of the fence. But I have to enhance the image so much that it becomes extremely noisy and grainy. I can do a little better by making 10 or 12 copies of the image and compositing them all in Addition mode. But it s still very noisy. But what if I take 12 different images of the fence, and then stack those? Stacking different images allows you to increase the light level a lot with hardly any increase in the noise. You can even duplicate some layers: Figure 10-33 shows a stack of 24 layers total, two copies each of 12 different layers, all in Addition mode. This technique reduces noise so well that you can boost the brightness even more using Levels or Curves without hurting the image much. So grab a tripod (even one of the little plastic ones with legs a few inches long is fine if your camera is small enough), wander off to a dark place, and try taking some shots!
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