Digital zoom was used to make the middle photo from the top photo, while the lens was zoomed-in optically for the bottom photo. Typically, the digital zoom would not be available until after the optical zoom had been exhausted.
Digital zoom is a method of decreasing (narrowing) the apparent angle of view of a digital photographic or video image. Digital zoom is accomplished by cropping an image down to a centered area with the same aspect ratio as the original, and usually also interpolating the result back up to the pixel dimensions of the original. It is accomplished electronically, without any adjustment of the camera's optics, and no optical resolution is gained in the process.
When comparing the image quality achieved by digital zoom with image quality achieved by resizing the image in post-processing, one must differentiate between cameras that perform lossy image compression like JPEG, and those which save images in a lossless Raw image format. In the former case, digital zoom tends to be superior to post-processing because the camera may apply its interpolation before small details are lost to compression. In the latter case, resizing in post-production will yield results equal or superior to digital zoom.
Some digital cameras rely entirely on digital zoom, lacking a real zoom lens, as on most camera phones. Other cameras do have a real zoom lens, but apply digital zoom automatically once its longest focal length has been reached. Professional cameras generally do not feature digital zoom.
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-taken from Wikipedia-
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