How to achieve moving mask that follows animation?

[Willy35423185w6qt]

I have a magnifying loupe in the video that I want to animate. It zooms the underlying footage a bit more to give it that magnifying look. In the screenshot you can see the idea.

How i'm trying to do it now is: 

Footage with person (A) -> Duplicated it (B) -> Mask on clip B -> Lasso clip B to layer with magnifying loupe. But doing that keeps the mask in a fixed place but moves clip B when I want to animate the loupe.  

Is there a quick and easy way to link mask to loupe properly and without any (paying) plugins?

[Mylenium]

Create the circle as a track matte and animate its position. That would also facilitate actual parenting of the graphic and matte, so everything stays aligned and can be refined easily.

Mylenium

[ShiveringCactus]

I'd use an Adjustment Layer underneath the Loupe picture with the Magnify effect on it.  Then I'd link the Magnify's Center property to the Loupe's position, so that when I moved the loupe, the effect follows it.  If the centre of the glass is not the centre of the image, use the anchor property to adjust it.

This way you only need one layer of footage, and only have one property (loupe's position) to adjust.