As we have came cross a phenomenon where a picture get blurry and more blurry as we zoom in a image beyond a limit.
Why does it happen?
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Digital zooming is commonly done by using some form of interpolation, which commonly represents making up some form of smooth change between actual data samples. Assuming the data was low pass filtered (or roughly near bandlimited) before sampling, the information about where any sharp (non-smooth) changes were located between samples has been lost. Adding back any non-smoothly-changing details would be arbitrary (guessing from context, etc.), and thus possibly incorrect (non-corresponding to the optical field before sampling).
Smooth changes or interpolations across a large area of pixels looks blurry to humans, since this effect does not occur in real life vision often unless one's eye or optical instrument is out-of-focus or not focused on the feature of interest.
If it wouldn't get blurry, then where would the information in the image come from?
Since you can only store images of finite size, it's impossible to contain infinite detail in a picture (totally ignoring the physical impossibility of that).
The fact that images typically are displayed "blurry" when you enlarge them (and not mosaic-y) is really just due to the fact that the viewer software has been designed to make the picture look blurry when interpolating the pixels that are actually in the picture to the larger number of pixels that you're trying to show.
Lens structure can no longer focus the image to the image sensor. Every lens system has a depth of field and focus distance. Once you are outside the focus distance and depth of field, photons from the target area are no longer hitting the same pixels and start to hit adjacent pixels which you perceive as blurry. As you come closer impact will be more severe.
That's the optical explanation. If you enlarge an image more than its resolution the application you use will have to interpolate the missing information. It creates new pixels using some interpolation such as bicubic. If it doesn't interpolate, you will see pixelation. Either way the info is estimated therefore contrast is lost.