It's depending in the phone model you are using. For example, the following technical info about zooming applies only to Nokia Camera in the Nokia Lumia 1020:
3x zoom for still photos 6x zoom for 720p video 4x zoom for 1080p video
If you’re using Nokia Camera with some other product, see the relevant technical specifications.
Nokia Camera’s zoom is a high-resolution zoom, which means that you don’t lose resolution or quality when you zoom.