Use the document camera or laptop to project a grid onto the projection screen.  Place the tall ringstand about 3 m in front of the screen.  Students are asked to note the change in position of the ringstand against the background grid with first one eye closed and then the other.

They can estimate their distance d from the ringstand by measuring its angular shift.  They are given the distance between the gridlines so they can express the shift x in meters.  Then, if h is the distance from the tall object to the screen and x is the distance that the object appears to shift against the background grid, their distance from the object is: