How to Resize Images & Maintain Original Sharpness
By Laurel Storm
Finding the perfect image you need is already a challenge; finding it at precisely the right size is even harder. Regardless of what you actually need the image for, you will likely need to resize it to be smaller, or, worse, larger. Resizing without losing sharpness can be difficult -- or, depending on the quality of the original image, even impossible.
All image-editing software and most photo managers can resize images. If you want to keep your images sharp, however, basic resizing won't cut it. Instead, you'll need a full-fledged graphics software that includes multiple resizing methods and a full toolkit of image-enhancing functions, such as Adobe Photoshop or GIMP.
Digital images are made up of pixels -- building blocks that store color information. Seen together, form the image you see, similar to the way separate dots of ink form printed images. Larger images contain more pixels than smaller ones and because of that can store and display more information, from subtle color shifts to extremely intricate details. When you resize an image, you increase or decrease the number of pixels it contains, affecting the amount of information it can store.
Image Type and Compression
When it comes to resizing, the file format you choose for both your original image and the resized one is extremely important. If you are starting from a fairly low quality image saved in a lossy compression format such as JPG, you may be able to shrink it while maintaining its sharpness, but it's unlikely you'll be able to make it much bigger without compression artifacts becoming plainly visible. Similarly, if you save an image you've successfully shrunk and kept sharp as a highly compressed JPG, all the detail you try to preserve may be lost, since compression works by discarding even more information about the image.
When you shrink an image, you are effectively discarding some of the information contained in the original. Exactly how the software choose which information to discard depends on the software you are using and on the resizing method. In general, you should expect very fine details to be lost, especially if you're resizing a very large image to be very small. A gray suit with white pinstripes, for example, may simply appear uniformly gray when the image is shrunk. Try every resizing method included in the software you are using and choose the one that gives the best results for your particular image. Alternatively, try applying a sharpening filter to your image before resizing it.
If shrinking an image means discarding information, it logically follows that enlarging an image necessarily means adding information. The original image, however, doesn't contain the necessary information -- so the software you're using simply makes guesses about what to add depending on the pixels surrounding each spot that needs to be filled. The resulting image will, unavoidably, be at least a bit blurry, with the blurriness increasing the more you enlarge the image. Applying a sharpening filter may help a bit, but the image will never look as good as one that was originally that size or was resized down to it.
Laurel Storm has been writing since 2001, and helping people with technology for far longer than that. Some of her articles have been published in "Messaggero dei Ragazzi", an Italian magazine for teenagers. She holds a Master of Arts in writing for television and new media from the University of Turin.