Digital Focus: Better Night Photos

Feature: Coaxing Better Night Photos From Your Digital Camera

For the most part, the transition from film to digital is painless. Stuff like shutter speeds, ISO ratings, aperture settings, and zoom controls all translate to the digital world pretty easily. But every once in a while, digital photography throws you a curve ball. Take night photography, for instance: Long exposures at night introduce new problems that film photographers would never encounter.

I'm referring to the noise generated by "stuck" pixels in the camera's image sensor. In any digital camera, at least a few of the light-sensitive pixels in the image sensor don't work. Consequently, they can register as stuck in the "on" position all the time. You can see this for yourself by taking a picture with the camera's lens cap on. You would expect to see a perfectly black image, but that's not quite what you get. You'll see a black image with a few seemingly random dots of light. Those are pixels that just don't work right. In normal daylight photography, you never notice such inconsequential glitches. But in a night photo, they stand out as scattered hot spots and can be quite annoying.

Subtraction Is the Answer

Thankfully, there's a very easy way to undo the effects of those stuck pixels in your night photos. Since we can take a picture with the lens cap on and see which pixels are causing problems, we have a map to the affected pixels. What, you're probably wondering, can we do with such a map?

It's simple: Using a mathematic operation built into most image editors, we can "subtract" the stuck pixels from the night photo and remove them from the final scene. What are they replaced with? Just black. But it's a night scene anyway, so the black pixel is far more aesthetic than flashes of color. So let's see this technique in action. We'll use Jasc's Paint Shop Pro 8 to remove the stuck pixels from a typical night shot.

Take a Control Shot

It all starts when you're out at night with your digital camera. Go ahead and shoot the way you normally would. But whenever it's convenient--such as at the very start or very end of your photo session--snap the lens cap onto your lens and take a photo.

You should set the exposure for about the same amount of time as the other images you've been taking to allow the pixels to burn into the scene properly. What if you don't know how long that was? You can take another picture and pay attention to the shutter speed that the camera automatically sets, then switch the camera to shutter priority and set the speed to match. This is your control shot, and we'll use it on the desktop in a moment.

What if your camera doesn't have a lens cap, or if closing the lens automatically shuts off the camera? You can follow these same steps, but take the control shot indoors, in a perfectly dark room like a darkroom or closet while covering the lens with a barrier like a towel or a jacket to be sure no light gets into the camera.

Digital Photo Math

Now that you have your night shots and your control photo, start by loading the control image into Paint Shop Pro. I have a night shot and a control shot that you can use to test this technique; be sure to examine both so you can see where the hot pixels are.

The dead pixels are hard to see in these images, but if you look closely at the control shot you'll see meandering lines of white pixels through the top and bottom of the image, as well as a cluster of red and blue pixels in the middle.

After you've loaded the control photo, choose Edit, Copy from the menu to save the mostly black control shot to the clipboard. You really don't need that file anymore, so you can close it.

Now load the night shot into Paint Shop Pro and immediately choose Edit, Paste, Paste As New Layer. You've layered the control shot on top of the night shot. The night shot is still there, but it's covered by the control shot.

Now for the magic moment. Find the Layer Palette on the right side of the screen. If you don't see it, choose View, Palettes, Layers and it should appear. The top layer--the control image--is called Raster 1 and should be listed above the night image, which is called Background. Double-click the Raster layer to open the Layer Properties dialog box. Change the Blend Mode to Difference and click OK to accept the changes. That's it!

You need only make one control photo per night photo session; not once after every picture you take. Don't take one control image and rely on it for weeks or months, though; the pixels in your sensor can change over time. It takes only a moment to take a control photo, but the results can be very satisfying.

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