system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why when taking a photo on your smartphone is there a clear difference when you're using zoom, even though the quality hasn't visibly changed.
gpt
Smartphones generally^1 have only what is known as "digital zoom".

This is as opposed to "optical zoom". Optical zoom is when the lens moves or changes in some way, enabling you to get a close-up of the subject, still with the same high resolution (i.e. lots of megapixels).

Now, imagine that you take a photo without zooming at all. Then, you cut out the edges of that photo leaving only the middle bit (and obviously fewer megapixels), but you make it so that the remaining middle bit takes up the whole of the screen by zooming in on it with your computer.

This is "digital zoom". When you zoom on your smart phone, it literally just ignores the edge part of the picture, and gives you the middle part, and that obviously has fewer megapixels in it than the whole picture did.

Is this a problem? Almost certainly not if you're only going to show the picture on a screen (e.g. upload it to Facebook), but fewer megapixels makes it less suitable for printing, especially if you want to print at photo-quality.

^1 Note that some smartphones now have started to include a form of optical zoom, for example the iPhone 7+ uses two different lenses so that one can be adjusted in the factory for normal photos and the other for fully zoomed photos. Then, for something in between, it uses software to combine the two images. This is a relatively new feature, though, and doesn't apply to most smartphones.