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.

This topic is valid for:

Nokia Lumia 1320,Nokia Lumia 1020,Nokia Lumia 928,Nokia Lumia 925,Nokia Lumia 920,Nokia Lumia 822,Nokia Lumia 820,Nokia Lumia 810,Nokia Lumia 735,Nokia Lumia 635,Nokia Lumia 630,Nokia Lumia 620,Nokia Lumia 530,Nokia Lumia 521,Nokia Lumia 520