Is there a way to scale down the movement of a parented object, while keeping it on the same path as the parent?

Question

I have three elements in my comp--a foreground element, a mediumground element (a tree), and a background element (the sky). I want to parent
the medium and background elements to the path of the foreground element--but since they are farther away form the camera, they would not move
as much as the foreground layer. Is there a way to parent these two layers to the foreground element, and then scale back the percentage of their movement?

Answer 1

I think you are making this much too difficult. If you have foreground, middle ground, and background layers make them all 3-D and arrange them in 3-D space then add a camera and move the camera. Your parallax shift will be automatic and perfect if you arrange the layers properly.
As you move the layers in z space you can scale them up so your composition looks right. You can't even use a simple expression to tie the distance of the layer to the camera to automatically fix the scale.
If you want to just move a layer on the x axis half as far as the foreground layer moves in x then the expression would look like this:
mlx = thisComp.layer("master").position[0];
sf  =  .5 // scale factor;
nx = mlx * sf;
[value[0] + nx, value[1]]
In English this expression says mlx equals the master layer x value
The scale factor (sf) is 50% experssed as a decimal
The new x value (nx) is equal to the master layer x position times the scale factor
Then you just write an array for the original x value plus the new x value  and take the existing y value from this layer.
This expression gives you the opportunity to keyframe both the X and Y position of the layer.  If I wasn't typing this post on my phone I would've taken the time to subtract the master layer X and Y position from Half of the comp height and width so the following layer would not have to be brought back into position after writing the expression.