When a portion of an image is masked out jagged edges can be left behind which will be apparent when zooming in. There is a way to refine the Layer Mask in Photoshop so that this is removed and two different images seem as one even while zoomed in.
This is a composite image. The model and the background are different images.
The two different images from which the composite image has been made using a Layer Mask.
The tutorial is here