exercise: depth

In this exercise we are asked to experiment with the depth of field in the camera, zooming in and out the same framing, where two objects are separated at different distances from the lens.
With the zoom out, or open, this is a short focal distance (18mm in the image below), the depth of field is big. The distant object can then be within the focus zone - although this didn't happen in my examples below - and the distance between the two objects seems larger. The best way of seeing this in the examples below is noticing the difference between the far side of the box in the 18mm and the 58mm images:

When you zoom in, increasing focal distance (and move away from the objects to have a similar frame) the objects seem closer between each other, and the distant object is blurrier as the depth decreases. Images are flatter with larger focal distances. 

The image below shows graphically how the focal distance affects the depth of field:

In the examples below the distance object seems always out of focus. This is because the distance to the objects also affects the depth of field, and I was taking the images close to the lenses. 

 
All graphics from http://www.blogdivvy.com/photog/depth-of-field.htm