While it's always better to make sure your camera is in focus before taking a photo, this isn't always possible. Even professional photographers occasionally end up with a few blurry photos. Some DSLR cameras on the market today can actually refocus a photo after you've taken a picture. For the rest of us, there are photo editing programs that can sharpen blurry photos on a computer and there are even some helpful apps for mobile devices.
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Changeable Depth of Field
If you happen to have a Lytro camera, you need never worry about blurry photos. After taking a photo, just pick any place in the frame, and the software automatically adjusts the depth of field to focus the image on the point you clicked. You can do nearly the same thing if you take videos with a DSLR and change the depth of field while you're shooting. When you load the video into a Web app made by The Chaos Collective, you can pull a still image from the video selecting any focus point used at any time in the video.
Advanced graphics and photo editing programs like Photoshop and GIMP include sharpening filters that can be used to bring fuzzy photos into focus. While the filter is called an unsharpen mask, it actually does sharpen rather than unsharpen details. Using sharpening filters gives you mixed results. With some images, like trees and buildings that are just a little blurry, the filter often works well. Photoshop includes specialized filters for sharpening photos, including Smart Sharpen, which automatically sharpens areas that are blurry without affecting areas that are already in focus. Photoshop's Shake Reduction filter is designed to automatically sharpen blurry images caused by a shaking camera, often due to wind or hand trembles.
When an unsharpen mask doesn't help a photo, you may have better results with the sharpen tool, available on both GIMP and Photoshop. Instead of trying to unblur the entire photo, you can select specific areas and lines to sharpen. Dragging the sharpen tool over the outside edge of blurry objects can often make them appear more in focus than they were, without causing pixelization or other distortions on the rest of the object. For example, sharpening the outline of a blurry eye can make the eye look more in focus without distorting the rest of the face or the composition of the person's iris.
For those who take pictures on smartphones and tablets, there are a few apps you can use to sharpen a blurry photo before sharing it with your friends. As its name suggests, Fix Blur and Sharpen Images is a specialty photo editing app for Android devices that can help sharpen blurry photos. For Apple iOS devices, FocusTwist was reviewed in the New York Times as an app that "re-focuses photos like a $400 Lytro." Snapseed is another highly-rated photo editing app for Apple devices that can sharpen blurry photos.